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History of Astronomy by George Forbes

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Jupiter and Saturn by the theory of gravitation, reducing the errors
of the tables from 20' down to 12", thus abolishing the use of
empirical corrections to the planetary tables, and providing another
glorious triumph for the law of gravitation. As Laplace justly said:
"These inequalities appeared formerly to be inexplicable by the law of
gravitation--they now form one of its most striking proofs."

Let us take one more discovery of Halley, furnishing directly a new
triumph for the theory. He noticed that Newton ascribed parabolic
orbits to the comets which he studied, so that they come from
infinity, sweep round the sun, and go off to infinity for ever, after
having been visible a few weeks or months. He collected all the
reliable observations of comets he could find, to the number of
twenty-four, and computed their parabolic orbits by the rules laid
down by Newton. His object was to find out if any of them really
travelled in elongated ellipses, practically undistinguishable, in the
visible part of their paths, from parabolae, in which case they would
be seen more than once. He found two old comets whose orbits, in shape
and position, resembled the orbit of a comet observed by himself in
1682. Apian observed one in 1531; Kepler the other in 1607. The
intervals between these appearances is seventy-five or seventy-six
years. He then examined and found old records of similar appearance in
1456, 1380, and 1305. It is true, he noticed, that the intervals
varied by a year and a-half, and the inclination of the orbit to the
ecliptic diminished with successive apparitions. But he knew from
previous calculations that this might easily be due to planetary
perturbations. Finally, he arrived at the conclusion that all of these
comets were identical, travelling in an ellipse so elongated that the
part where the comet was seen seemed to be part of a parabolic
orbit. He then predicted its return at the end of 1758 or beginning of
1759, when he should be dead; but, as he said, "if it should return,
according to our prediction, about the year 1758, impartial posterity
will not refuse to acknowledge that this was first discovered by an
Englishman."[3] [_Synopsis Astronomiae Cometicae_, 1749.]

Once again Halley's suggestion became an inspiration for the
mathematical astronomer. Clairaut, assisted by Lalande, found that
Saturn would retard the comet 100 days, Jupiter 518 days, and
predicted its return to perihelion on April 13th, 1759. In his
communication to the French Academy, he said that a comet travelling
into such distant regions might be exposed to the influence of forces
totally unknown, and "even of some planet too far removed from the sun
to be ever perceived."

The excitement of astronomers towards the end of 1758 became intense;
and the honour of first catching sight of the traveller fell to an
amateur in Saxony, George Palitsch, on Christmas Day, 1758. It reached
perihelion on March 13th, 1759.

This fact was a startling confirmation of the Newtonian theory,
because it was a new kind of calculation of perturbations, and also it
added a new member to the solar system, and gave a prospect of adding
many more.

When Halley's comet reappeared in 1835, Pontecoulant's computations
for the date of perihelion passage were very exact, and afterwards he
showed that, with more exact values of the masses of Jupiter and
Saturn, his prediction was correct within two days, after an invisible
voyage of seventy-five years!

Hind afterwards searched out many old appearances of this comet, going
back to 11 B.C., and most of these have been identified as being
really Halley's comet by the calculations of Cowell and Cromellin[4]
(of Greenwich Observatory), who have also predicted its next
perihelion passage for April 8th to 16th, 1910, and have traced back
its history still farther, to 240 B.C.

Already, in November, 1907, the Astronomer Royal was trying to catch
it by the aid of photography.

FOOTNOTES:

[1] Born 1736; died 1813.

[2] Born 1749; died 1827.

[3] This sentence does not appear in the original memoir communicated
to the Royal Society, but was first published in a posthumous reprint.

[4] _R. A. S. Monthly Notices_, 1907-8.

9. DISCOVERY OF NEW PLANETS--HERSCHEL, PIAZZI, ADAMS, AND LE VERRIER.

It would be very interesting, but quite impossible in these pages, to
discuss all the exquisite researches of the mathematical astronomers,
and to inspire a reverence for the names connected with these
researches, which for two hundred years have been establishing the
universality of Newton's law. The lunar and planetary theories, the
beautiful theory of Jupiter's satellites, the figure of the earth, and
the tides, were mathematically treated by Maclaurin, D'Alembert,
Legendre, Clairaut, Euler, Lagrange, Laplace, Walmsley, Bailly,
Lalande, Delambre, Mayer, Hansen, Burchardt, Binet, Damoiseau, Plana,
Poisson, Gauss, Bessel, Bouvard, Airy, Ivory, Delaunay, Le Verrier,
Adams, and others of later date.

By passing over these important developments it is possible to trace
some of the steps in the crowning triumph of the Newtonian theory, by
which the planet Neptune was added to the known members of the solar
system by the independent researches of Professor J.C. Adams and of
M. Le Verrier, in 1846.

It will be best to introduce this subject by relating how the
eighteenth century increased the number of known planets, which was
then only six, including the earth.

On March 13th, 1781, Sir William Herschel was, as usual, engaged on
examining some small stars, and, noticing that one of them appeared to
be larger than the fixed stars, suspected that it might be a comet.
To test this he increased his magnifying power from 227 to 460 and
932, finding that, unlike the fixed stars near it, its definition was
impaired and its size increased. This convinced him that the object
was a comet, and he was not surprised to find on succeeding nights
that the position was changed, the motion being in the ecliptic. He
gave the observations of five weeks to the Royal Society without a
suspicion that the object was a new planet.

For a long time people could not compute a satisfactory orbit for the
supposed comet, because it seemed to be near the perihelion, and no
comet had ever been observed with a perihelion distance from the sun
greater than four times the earth's distance. Lexell was the first to
suspect that this was a new planet eighteen times as far from the sun
as the earth is. In January, 1783, Laplace published the elliptic
elements. The discoverer of a planet has a right to name it, so
Herschel called it Georgium Sidus, after the king. But Lalande urged
the adoption of the name Herschel. Bode suggested Uranus, and this
was adopted. The new planet was found to rank in size next to Jupiter
and Saturn, being 4.3 times the diameter of the earth.

In 1787 Herschel discovered two satellites, both revolving in nearly
the same plane, inclined 80 degrees to the ecliptic, and the motion of both
was retrograde.

In 1772, before Herschel's discovery, Bode[1] had discovered a curious
arbitrary law of planetary distances. Opposite each planet's name
write the figure 4; and, in succession, add the numbers 0, 3, 6, 12,
24, 48, 96, etc., to the 4, always doubling the last numbers. You
then get the planetary distances.

Mercury, dist.-- 4 4 + 0 = 4
Venus " 7 4 + 3 = 7
Earth " 10 4 + 6 = 10
Mars " 15 4 + 12 = 16
-- 4 + 24 = 28
Jupiter dist. 52 4 + 48 = 52
Saturn " 95 4 + 96 = 100
(Uranus) " 192 4 + 192 = 196
-- 4 + 384 = 388

All the five planets, and the earth, fitted this rule, except that
there was a blank between Mars and Jupiter. When Uranus was
discovered, also fitting the rule, the conclusion was irresistible
that there is probably a planet between Mars and Jupiter. An
association of twenty-four astronomers was now formed in Germany to
search for the planet. Almost immediately afterwards the planet was
discovered, not by any member of the association, but by Piazzi, when
engaged upon his great catalogue of stars. On January 1st, 1801, he
observed a star which had changed its place the next night. Its motion
was retrograde till January 11th, direct after the 13th. Piazzi fell
ill before he had enough observations for computing the orbit with
certainty, and the planet disappeared in the sun's rays. Gauss
published an approximate ephemeris of probable positions when the
planet should emerge from the sun's light. There was an exciting hunt,
and on December 31st (the day before its birthday) De Zach captured
the truant, and Piazzi christened it Ceres.

The mean distance from the sun was found to be 2.767, agreeing with
the 2.8 given by Bode's law. Its orbit was found to be inclined over
10 degrees to the ecliptic, and its diameter was only 161 miles.

On March 28th, 1802, Olbers discovered a new seventh magnitude star,
which turned out to be a planet resembling Ceres. It was called
Pallas. Gauss found its orbit to be inclined 35 degrees to the
ecliptic, and to cut the orbit of Ceres; whence Olbers considered that
these might be fragments of a broken-up planet. He then commenced a
search for other fragments. In 1804 Harding discovered Juno, and in
1807 Olbers found Vesta. The next one was not discovered until 1845,
from which date asteroids, or minor planets (as these small planets
are called), have been found almost every year. They now number about
700.

It is impossible to give any idea of the interest with which the first
additions since prehistoric times to the planetary system were
received. All of those who showered congratulations upon the
discoverers regarded these discoveries in the light of rewards for
patient and continuous labours, the very highest rewards that could be
desired. And yet there remained still the most brilliant triumph of
all, the addition of another planet like Uranus, before it had ever
been seen, when the analysis of Adams and Le Verrier gave a final
proof of the powers of Newton's great law to explain any planetary
irregularity.

After Sir William Herschel discovered Uranus, in 1781, it was found
that astronomers had observed it on many previous occasions, mistaking
it for a fixed star of the sixth or seventh magnitude. Altogether,
nineteen observations of Uranus's position, from the time of
Flamsteed, in 1690, had been recorded.

In 1790 Delambre, using all these observations, prepared tables for
computing its position. These worked well enough for a time, but at
last the differences between the calculated and observed longitudes of
the planet became serious. In 1821 Bouvard undertook a revision of the
tables, but found it impossible to reconcile all the observations of
130 years (the period of revolution of Uranus is eighty-four years).
So he deliberately rejected the old ones, expressing the opinion that
the discrepancies might depend upon "some foreign and unperceived
cause which may have been acting upon the planet." In a few years the
errors even of these tables became intolerable. In 1835 the error of
longitude was 30"; in 1838, 50"; in 1841, 70"; and, by comparing the
errors derived from observations made before and after opposition, a
serious error of the distance (radius vector) became apparent.

In 1843 John Couch Adams came out Senior Wrangler at Cambridge, and
was free to undertake the research which as an undergraduate he had
set himself--to see whether the disturbances of Uranus could be
explained by assuming a certain orbit, and position in that orbit, of
a hypothetical planet even more distant than Uranus. Such an
explanation had been suggested, but until 1843 no one had the boldness
to attack the problem. Bessel had intended to try, but a fatal
illness overtook him.

Adams first recalculated all known causes of disturbance, using the
latest determinations of the planetary masses. Still the errors were
nearly as great as ever. He could now, however, use these errors as
being actually due to the perturbations produced by the unknown
planet.

In 1844, assuming a circular orbit, and a mean distance agreeing with
Bode's law, he obtained a first approximation to the position of the
supposed planet. He then asked Professor Challis, of Cambridge, to
procure the latest observations of Uranus from Greenwich, which Airy
immediately supplied. Then the whole work was recalculated from the
beginning, with more exactness, and assuming a smaller mean distance.

In September, 1845, he handed to Challis the elements of the
hypothetical planet, its mass, and its apparent position for September
30th, 1845. On September 22nd Challis wrote to Airy explaining the
matter, and declaring his belief in Adams's capabilities. When Adams
called on him Airy was away from home, but at the end of October,
1845, he called again, and left a paper with full particulars of his
results, which had, for the most part, reduced the discrepancies to
about 1". As a matter of fact, it has since been found that the
heliocentric place of the new planet then given was correct within
about 2 degrees.

Airy wrote expressing his interest, and asked for particulars about
the radius vector. Adams did not then reply, as the answer to this
question could be seen to be satisfactory by looking at the data
already supplied. He was a most unassuming man, and would not push
himself forward. He may have felt, after all the work he had done,
that Airy's very natural inquiry showed no proportionate desire to
search for the planet. Anyway, the matter lay in embryo for nine
months.

Meanwhile, one of the ablest French astronomers, Le Verrier,
experienced in computing perturbations, was independently at work,
knowing nothing about Adams. He applied to his calculations every
possible refinement, and, considering the novelty of the problem, his
calculation was one of the most brilliant in the records of
astronomy. In criticism it has been said that these were exhibitions
of skill rather than helps to a solution of the particular problem,
and that, in claiming to find the elements of the orbit within certain
limits, he was claiming what was, under the circumstances, impossible,
as the result proved.

In June, 1846, Le Verrier announced, in the _Comptes Rendus de
l'Academie des Sciences_, that the longitude of the disturbing planet,
for January 1st, 1847, was 325, and that the probable error did not
exceed 10 degrees.

This result agreed so well with Adams's (within 1 degrees) that Airy urged
Challis to apply the splendid Northumberland equatoreal, at Cambridge,
to the search. Challis, however, had already prepared an exhaustive
plan of attack which must in time settle the point. His first work
was to observe, and make a catalogue, or chart, of all stars near
Adams's position.

On August 31st, 1846, Le Verrier published the concluding
part of his labours.

On September 18th, 1846, Le Verrier communicated his results to the
Astronomers at Berlin, and asked them to assist in searching for the
planet. By good luck Dr. Bremiker had just completed a star-chart of
the very part of the heavens including Le Verrier's position; thus
eliminating all of Challis's preliminary work. The letter was received
in Berlin on September 23rd; and the same evening Galle found the new
planet, of the eighth magnitude, the size of its disc agreeing with Le
Verrier's prediction, and the heliocentric longitude agreeing within
57'. By this time Challis had recorded, without reduction, the
observations of 3,150 stars, as a commencement for his search. On
reducing these, he found a star, observed on August 12th, which was
not in the same place on July 30th. This was the planet, and he had
also observed it on August 4th.

The feeling of wonder, admiration, and enthusiasm aroused by this
intellectual triumph was overwhelming. In the world of astronomy
reminders are met every day of the terrible limitations of human
reasoning powers; and every success that enables the mind's eye to see
a little more clearly the meaning of things has always been heartily
welcomed by those who have themselves been engaged in like
researches. But, since the publication of the _Principia_, in 1687,
there is probably no analytical success which has raised among
astronomers such a feeling of admiration and gratitude as when Adams
and Le Verrier showed the inequalities in Uranus's motion to mean that
an unknown planet was in a certain place in the heavens, where it was
found.

At the time there was an unpleasant display of international jealousy.
The British people thought that the earlier date of Adams's work, and
of the observation by Challis, entitled him to at least an equal share
of credit with Le Verrier. The French, on the other hand, who, on the
announcement of the discovery by Galle, glowed with pride in the new
proof of the great powers of their astronomer, Le Verrier, whose life
had a long record of successes in calculation, were incredulous on
being told that it had all been already done by a young man whom they
had never heard of.

These displays of jealousy have long since passed away, and there is
now universally an _entente cordiale_ that to each of these great men
belongs equally the merit of having so thoroughly calculated this
inverse problem of perturbations as to lead to the immediate discovery
of the unknown planet, since called Neptune.

It was soon found that the planet had been observed, and its position
recorded as a fixed star by Lalande, on May 8th and 10th, 1795.

Mr. Lassel, in the same year, 1846, with his two-feet reflector,
discovered a satellite, with retrograde motion, which gave the mass of
the planet about a twentieth of that of Jupiter.

FOOTNOTES:

[1] Bode's law, or something like it, had already been fore-shadowed
by Kepler and others, especially Titius (see _Monatliche
Correspondenz_, vol. vii., p. 72).

BOOK III. OBSERVATION

10. INSTRUMENTS OF PRECISION--STATE OF THE SOLAR SYSTEM.

Having now traced the progress of physical astronomy up to the time
when very striking proofs of the universality of the law of
gravitation convinced the most sceptical, it must still be borne in
mind that, while gravitation is certainly the principal force
governing the motions of the heavenly bodies, there may yet be a
resisting medium in space, and there may be electric and magnetic
forces to deal with. There may, further, be cases where the effects of
luminous radiative repulsion become apparent, and also Crookes'
vacuum-effects described as "radiant matter." Nor is it quite certain
that Laplace's proofs of the instantaneous propagation of gravity are
final.

And in the future, as in the past, Tycho Brahe's dictum must be
maintained, that all theory shall be preceded by accurate
observations. It is the pride of astronomers that their science stands
above all others in the accuracy of the facts observed, as well as in
the rigid logic of the mathematics used for interpreting these facts.

It is interesting to trace historically the invention of those
instruments of precision which have led to this result, and, without
entering on the details required in a practical handbook, to note the
guiding principles of construction in different ages.

It is very probable that the Chaldeans may have made spheres, like the
armillary sphere, for representing the poles of the heavens; and with
rings to show the ecliptic and zodiac, as well as the equinoctial and
solstitial colures; but we have no record. We only know that the tower
of Belus, on an eminence, was their observatory. We have, however,
distinct records of two such spheres used by the Chinese about 2500
B.C. Gnomons, or some kind of sundial, were used by the Egyptians and
others; and many of the ancient nations measured the obliquity of the
ecliptic by the shadows of a vertical column in summer and winter. The
natural horizon was the only instrument of precision used by those who
determined star positions by the directions of their risings and
settings; while in those days the clepsydra, or waterclock, was the
best instrument for comparing their times of rising and setting.

About 300 B.C. an observatory fitted with circular instruments for
star positions was set up at Alexandria, the then centre of
civilisation. We know almost nothing about the instruments used by
Hipparchus in preparing his star catalogues and his lunar and solar
tables; but the invention of the astrolabe is attributed to him.[1]

In more modern times Nuremberg became a centre of astronomical
culture. Waltherus, of that town, made really accurate observations of
star altitudes, and of the distances between stars; and in 1484
A.D. he used a kind of clock. Tycho Brahe tried these, but discarded
them as being inaccurate.

Tycho Brahe (1546-1601 A.D.) made great improvements in armillary
spheres, quadrants, sextants, and large celestial globes. With these
he measured the positions of stars, or the distance of a comet from
several known stars. He has left us full descriptions of them,
illustrated by excellent engravings. Previous to his time such
instruments were made of wood. Tycho always used metal. He paid the
greatest attention to the stability of mounting, to the orientation of
his instruments, to the graduation of the arcs by the then new method
of transversals, and to the aperture sight used upon his
pointer. There were no telescopes in his day, and no pendulum
clocks. He recognised the fact that there must be instrumental
errors. He made these as small as was possible, measured their amount,
and corrected his observations. His table of refractions enabled him
to abolish the error due to our atmosphere so far as it could affect
naked-eye observations. The azimuth circle of Tycho's largest quadrant
had a diameter of nine feet, and the quadrant a radius of six feet. He
introduced the mural quadrant for meridian observations.[2]

[Illustration: ANCIENT CHINESE INSTRUMENTS, Including quadrant, celestial
globe, and two armillae, in the Observatory at Peking. Photographed in
Peking by the author in 1875, and stolen by the Germans when the
Embassies were relieved by the allies in 1900.]

The French Jesuits at Peking, in the seventeenth century, helped the
Chinese in their astronomy. In 1875 the writer saw and photographed,
on that part of the wall of Peking used by the Mandarins as an
observatory, the six instruments handsomely designed by Father
Verbiest, copied from the instruments of Tycho Brahe, and embellished
with Chinese dragons and emblems cast on the supports. He also saw
there two old instruments (which he was told were Arabic) of date
1279, by Ko Show-King, astronomer to Koblai Khan, the grandson of
Chenghis Khan. One of these last is nearly identical with the armillae
of Tycho; and the other with his "armillae aequatoriae maximae," with
which he observed the comet of 1585, besides fixed stars and
planets.[3]

The discovery by Galileo of the isochronism of the pendulum, followed
by Huyghens's adaptation of that principle to clocks, has been one of
the greatest aids to accurate observation. About the same time an
equally beneficial step was the employment of the telescope as a
pointer; not the Galilean with concave eye-piece, but with a
magnifying glass to examine the focal image, at which also a fixed
mark could be placed. Kepler was the first to suggest this. Gascoigne
was the first to use it. Huyghens used a metal strip of variable width
in the focus, as a micrometer to cover a planetary disc, and so to
measure the width covered by the planet. The Marquis Malvasia, in
1662, described the network of fine silver threads at right angles,
which he used in the focus, much as we do now.

In the hands of such a skilful man as Tycho Brahe, the old open
sights, even without clocks, served their purpose sufficiently well to
enable Kepler to discover the true theory of the solar system. But
telescopic sights and clocks were required for proving some of
Newton's theories of planetary perturbations. Picard's observations at
Paris from 1667 onwards seem to embody the first use of the telescope
as a pointer. He was also the first to introduce the use of Huyghens's
clocks for observing the right ascension of stars. Olaus Romer was
born at Copenhagen in 1644. In 1675, by careful study of the times of
eclipses of Jupiter's satellites, he discovered that light took time
to traverse space. Its velocity is 186,000 miles per second. In 1681
he took up his duties as astronomer at Copenhagen, and built the first
transit circle on a window-sill of his house. The iron axis was five
feet long and one and a-half inches thick, and the telescope was fixed
near one end with a counterpoise. The telescope-tube was a double
cone, to prevent flexure. Three horizontal and three vertical wires
were used in the focus. These were illuminated by a speculum, near the
object-glass, reflecting the light from a lantern placed over the
axis, the upper part of the telescope-tube being partly cut away to
admit the light. A divided circle, with pointer and reading
microscope, was provided for reading the declination. He realised the
superiority of a circle with graduations over a much larger
quadrant. The collimation error was found by reversing the instrument
and using a terrestrial mark, the azimuth error by star observations.
The time was expressed in fractions of a second. He also constructed a
telescope with equatoreal mounting, to follow a star by one axial
motion. In 1728 his instruments and observation records were destroyed
by fire.

Hevelius had introduced the vernier and tangent screw in his
measurement of arc graduations. His observatory and records were burnt
to the ground in 1679. Though an old man, he started afresh, and left
behind him a catalogue of 1,500 stars.

Flamsteed began his duties at Greenwich Observatory, as first
Astronomer Royal, in 1676, with very poor instruments. In 1683 he put
up a mural arc of 140 degrees, and in 1689 a better one, seventy-nine
inches radius. He conducted his measurements with great skill, and
introduced new methods to attain accuracy, using certain stars for
determining the errors of his instruments; and he always reduced his
observations to a form in which they could be readily used. He
introduced new methods for determining the position of the equinox and
the right ascension of a fundamental star. He produced a catalogue of
2,935 stars. He supplied Sir Isaac Newton with results of observation
required in his theoretical calculations. He died in 1719.

Halley succeeded Flamsteed to find that the whole place had been
gutted by the latter's executors. In 1721 he got a transit instrument,
and in 1726 a mural quadrant by Graham. His successor in 1742,
Bradley, replaced this by a fine brass quadrant, eight feet radius, by
Bird; and Bradley's zenith sector was purchased for the observatory.
An instrument like this, specially designed for zenith stars, is
capable of greater rigidity than a more universal instrument; and
there is no trouble with refraction in the zenith. For these reasons
Bradley had set up this instrument at Kew, to attempt the proof of the
earth's motion by observing the annual parallax of stars. He certainly
found an annual variation of zenith distance, but not at the times of
year required by the parallax. This led him to the discovery of the
"aberration" of light and of nutation. Bradley has been described as
the founder of the modern system of accurate observation. He died in
1762, leaving behind him thirteen folio volumes of valuable but
unreduced observations. Those relating to the stars were reduced by
Bessel and published in 1818, at Konigsberg, in his well-known
standard work, _Fundamenta Astronomiae_. In it are results showing the
laws of refraction, with tables of its amount, the maximum value of
aberration, and other constants.

Bradley was succeeded by Bliss, and he by Maskelyne (1765), who
carried on excellent work, and laid the foundations of the Nautical
Almanac (1767). Just before his death he induced the Government to
replace Bird's quadrant by a fine new mural _circle_, six feet in
diameter, by Troughton, the divisions being read off by microscopes
fixed on piers opposite to the divided circle. In this instrument the
micrometer screw, with a divided circle for turning it, was applied
for bringing the micrometer wire actually in line with a division on
the circle--a plan which is still always adopted.

Pond succeeded Maskelyne in 1811, and was the first to use this
instrument. From now onwards the places of stars were referred to the
pole, not to the zenith; the zero being obtained from measures on
circumpolar stars. Standard stars were used for giving the clock
error. In 1816 a new transit instrument, by Troughton, was added, and
from this date the Greenwich star places have maintained the very
highest accuracy.

George Biddell Airy, Seventh Astronomer Royal,[4] commenced his
Greenwich labours in 1835. His first and greatest reformation in the
work of the observatory was one he had already established at
Cambridge, and is now universally adopted. He held that an observation
is not completed until it has been reduced to a useful form; and in
the case of the sun, moon, and planets these results were, in every
case, compared with the tables, and the tabular error printed.

Airy was firmly impressed with the object for which Charles II. had
wisely founded the observatory in connection with navigation, and for
observations of the moon. Whenever a meridian transit of the moon
could be observed this was done. But, even so, there are periods in
the month when the moon is too near the sun for a transit to be well
observed. Also weather interferes with many meridian observations. To
render the lunar observations more continuous, Airy employed
Troughton's successor, James Simms, in conjunction with the engineers,
Ransome and May, to construct an altazimuth with three-foot circles,
and a five-foot telescope, in 1847. The result was that the number of
lunar observations was immediately increased threefold, many of them
being in a part of the moon's orbit which had previously been bare of
observations. From that date the Greenwich lunar observations have
been a model and a standard for the whole world.

Airy also undertook to superintend the reduction of all Greenwich
lunar observations from 1750 to 1830. The value of this laborious
work, which was completed in 1848, cannot be over-estimated.

The demands of astronomy, especially in regard to small minor planets,
required a transit instrument and mural circle with a more powerful
telescope. Airy combined the functions of both, and employed the same
constructors as before to make a _transit-circle_ with a telescope of
eleven and a-half feet focus and a circle of six-feet diameter, the
object-glass being eight inches in diameter.

Airy, like Bradley, was impressed with the advantage of employing
stars in the zenith for determining the fundamental constants of
astronomy. He devised a _reflex zenith tube_, in which the zenith
point was determined by reflection from a surface of mercury. The
design was so simple, and seemed so perfect, that great expectations
were entertained. But unaccountable variations comparable with those
of the transit circle appeared, and the instrument was put out of use
until 1903, when the present Astronomer Royal noticed that the
irregularities could be allowed for, being due to that remarkable
variation in the position of the earth's axis included in circles of
about six yards diameter at the north and south poles, discovered at
the end of the nineteenth century. The instrument is now being used
for investigating these variations; and in the year 1907 as many as
1,545 observations of stars were made with the reflex zenith tube.

In connection with zenith telescopes it must be stated that Respighi,
at the Capitol Observatory at Rome, made use of a deep well with a
level mercury surface at the bottom and a telescope at the top
pointing downwards, which the writer saw in 1871. The reflection of
the micrometer wires and of a star very near the zenith (but not quite
in the zenith) can be observed together. His mercury trough was a
circular plane surface with a shallow edge to retain the mercury. The
surface quickly came to rest after disturbance by street traffic.

Sir W. M. H. Christie, Eighth Astronomer Royal, took up his duties in
that capacity in 1881. Besides a larger altazimuth that he erected in
1898, he has widened the field of operations at Greenwich by the
extensive use of photography and the establishment of large
equatoreals. From the point of view of instruments of precision, one
of the most important new features is the astrographic equatoreal, set
up in 1892 and used for the Greenwich section of the great
astrographic chart just completed. Photography has come to be of use,
not only for depicting the sun and moon, comets and nebulae, but also
to obtain accurate relative positions of neighbouring stars; to pick
up objects that are invisible in any telescope; and, most of all
perhaps, in fixing the positions of faint satellites. Thus Saturn's
distant satellite, Phoebe, and the sixth and seventh satellites of
Jupiter, have been followed regularly in their courses at Greenwich
ever since their discovery with the thirty-inch reflector (erected in
1897); and while doing so Mr. Melotte made, in 1908, the splendid
discovery on some of the photographic plates of an eighth satellite of
Jupiter, at an enormous distance from the planet. From observations in
the early part of 1908, over a limited arc of its orbit, before
Jupiter approached the sun, Mr. Cowell computed a retrograde orbit and
calculated the future positions of this satellite, which enabled
Mr. Melotte to find it again in the autumn--a great triumph both of
calculation and of photographic observation. This satellite has never
been seen, and has been photographed only at Greenwich, Heidelberg,
and the Lick Observatory.

Greenwich Observatory has been here selected for tracing the progress
of accurate measurement. But there is one instrument of great value,
the heliometer, which is not used at Greenwich. This serves the
purpose of a double image micrometer, and is made by dividing the
object-glass of a telescope along a diameter. Each half is mounted so
as to slide a distance of several inches each way on an arc whose
centre is the focus. The amount of the movement can be accurately
read. Thus two fields of view overlap, and the adjustment is made to
bring an image of one star over that of another star, and then to do
the same by a displacement in the opposite direction. The total
movement of the half-object glass is double the distance between the
star images in the focal plane. Such an instrument has long been
established at Oxford, and German astronomers have made great use of
it. But in the hands of Sir David Gill (late His Majesty's Astronomer
at the Cape of Good Hope), and especially in his great researches on
Solar and on Stellar parallax, it has been recognised as an instrument
of the very highest accuracy, measuring the distance between stars
correctly to less than a tenth of a second of arc.

The superiority of the heliometer over all other devices (except
photography) for measuring small angles has been specially brought
into prominence by Sir David Gill's researches on the distance of the
sun--_i.e.,_ the scale of the solar system. A measurement of the
distance of any planet fixes the scale, and, as Venus approaches the
earth most nearly of all the planets, it used to be supposed that a
Transit of Venus offered the best opportunity for such measurement,
especially as it was thought that, as Venus entered on the solar disc,
the sweep of light round the dark disc of Venus would enable a very
precise observation to be made. The Transit of Venus in 1874, in
which the present writer assisted, overthrew this delusion.

In 1877 Sir David Gill used Lord Crawford's heliometer at the Island
of Ascension to measure the parallax of Mars in opposition, and found
the sun's distance 93,080,000 miles. He considered that, while the
superiority of the heliometer had been proved, the results would be
still better with the points of light shown by minor planets rather
than with the disc of Mars.

In 1888-9, at the Cape, he observed the minor planets Iris, Victoria,
and Sappho, and secured the co-operation of four other heliometers.
His final result was 92,870,000 miles, the parallax being 8",802
(_Cape Obs_., Vol. VI.).

So delicate were these measures that Gill detected a minute periodic
error of theory of twenty-seven days, owing to a periodically
erroneous position of the centre of gravity of the earth and moon to
which the position of the observer was referred. This led him to
correct the mass of the moon, and to fix its ratio to the earth's mass
= 0.012240.

Another method of getting the distance from the sun is to measure the
velocity of the earth's orbital motion, giving the circumference
traversed in a year, and so the radius of the orbit. This has been
done by comparing observation and experiment. The aberration of light
is an angle 20" 48, giving the ratio of the earth's velocity to the
velocity of light. The velocity of light is 186,000 miles a second;
whence the distance to the sun is 92,780,000 miles. There seems,
however, to be some uncertainty about the true value of the
aberration, any determination of which is subject to irregularities
due to the "seasonal errors." The velocity of light was experimentally
found, in 1862, by Fizeau and Foucault, each using an independent
method. These methods have been developed, and new values found, by
Cornu, Michaelson, Newcomb, and the present writer.

Quite lately Halm, at the Cape of Good Hope, measured
spectroscopically the velocity of the earth to and from a star by
observations taken six months apart. Thence he obtained an accurate
value of the sun's distance.[5]

But the remarkably erratic minor planet, Eros, discovered by Witte in
1898, approaches the earth within 15,000,000 miles at rare intervals,
and, with the aid of photography, will certainly give us the best
result. A large number of observatories combined to observe the
opposition of 1900. Their results are not yet completely reduced, but
the best value deduced so far for the parallax[6] is 8".807 +/-
0".0028.[7]

FOOTNOTES:

[1] In 1480 Martin Behaim, of Nuremberg, produced his _astrolabe_ for
measuring the latitude, by observation of the sun, at sea. It
consisted of a graduated metal circle, suspended by a ring which was
passed over the thumb, and hung vertically. A pointer was fixed to a
pin at the centre. This arm, called the _alhidada_, worked round the
graduated circle, and was pointed to the sun. The altitude of the sun
was thus determined, and, by help of solar tables, the latitude could
be found from observations made at apparent noon.

[2] See illustration on p. 76.

[3] See Dreyer's article on these instruments in _Copernicus_,
Vol. I. They were stolen by the Germans after the relief of the
Embassies, in 1900. The best description of these instruments is
probably that contained in an interesting volume, which may be seen in
the library of the R. A. S., entitled _Chinese Researches_, by
Alexander Wyllie (Shanghai, 1897).

[4] Sir George Airy was very jealous of this honourable title. He
rightly held that there is only one Astronomer Royal at a time, as
there is only one Mikado, one Dalai Lama. He said that His Majesty's
Astronomer at the Cape of Good Hope, His Majesty's Astronomer for
Scotland, and His Majesty's Astronomer for Ireland are not called
Astronomers Royal.

[5] _Annals of the Cape Observatory_, vol. x., part 3.

[6] The parallax of the sun is the angle subtended by the earth's
radius at the sun's distance.

[7] A. R. Hinks, R.A.S.; _Monthly Notices_, June, 1909.

11. HISTORY OF THE TELESCOPE

Accounts of wonderful optical experiments by Roger Bacon (who died in
1292), and in the sixteenth century by Digges, Baptista Porta, and
Antonio de Dominis (Grant, _Hist. Ph. Ast_.), have led some to
suppose that they invented the telescope. The writer considers that it
is more likely that these notes refer to a kind of _camera
obscura_, in which a lens throws an inverted image of a landscape
on the wall.

The first telescopes were made in Holland, the originator being either
Henry Lipperhey,[1] Zacharias Jansen, or James Metius, and the date
1608 or earlier.

In 1609 Galileo, being in Venice, heard of the invention, went home
and worked out the theory, and made a similar telescope. These
telescopes were all made with a convex object-glass and a concave
eye-lens, and this type is spoken of as the Galilean telescope. Its
defects are that it has no real focus where cross-wires can be placed,
and that the field of view is very small. Kepler suggested the convex
eye-lens in 1611, and Scheiner claimed to have used one in 1617. But
it was Huyghens who really introduced them. In the seventeenth century
telescopes were made of great length, going up to 300 feet. Huyghens
also invented the compound eye-piece that bears his name, made of two
convex lenses to diminish spherical aberration.

But the defects of colour remained, although their cause was unknown
until Newton carried out his experiments on dispersion and the solar
spectrum. To overcome the spherical aberration James Gregory,[2] of
Aberdeen and Edinburgh, in 1663, in his _Optica Promota_,
proposed a reflecting speculum of parabolic form. But it was Newton,
about 1666, who first made a reflecting telescope; and he did it with
the object of avoiding colour dispersion.

Some time elapsed before reflectors were much used. Pound and Bradley
used one presented to the Royal Society by Hadley in 1723. Hawksbee,
Bradley, and Molyneaux made some. But James Short, of Edinburgh, made
many excellent Gregorian reflectors from 1732 till his death in 1768.

Newton's trouble with refractors, chromatic aberration, remained
insurmountable until John Dollond (born 1706, died 1761), after many
experiments, found out how to make an achromatic lens out of two
lenses--one of crown glass, the other of flint glass--to destroy the
colour, in a way originally suggested by Euler. He soon acquired a
great reputation for his telescopes of moderate size; but there was a
difficulty in making flint-glass lenses of large size. The first
actual inventor and constructor of an achromatic telescope was Chester
Moor Hall, who was not in trade, and did not patent it. Towards the
close of the eighteenth century a Swiss named Guinand at last
succeeded in producing larger flint-glass discs free from
striae. Frauenhofer, of Munich, took him up in 1805, and soon
produced, among others, Struve's Dorpat refractor of 9.9 inches
diameter and 13.5 feet focal length, and another, of 12 inches
diameter and 18 feet focal length, for Lamont, of Munich.

In the nineteenth century gigantic _reflectors_ have been
made. Lassel's 2-foot reflector, made by himself, did much good work,
and discovered four new satellites. But Lord Rosse's 6-foot
reflector, 54 feet focal length, constructed in 1845, is still the
largest ever made. The imperfections of our atmosphere are against
the use of such large apertures, unless it be on high mountains.
During the last half century excellent specula have been made of
silvered glass, and Dr. Common's 5-foot speculum (removed, since his
death, to Harvard) has done excellent work. Then there are the 5-foot
Yerkes reflector at Chicago, and the 4-foot by Grubb at Melbourne.

Passing now from these large reflectors to refractors, further
improvements have been made in the manufacture of glass by Chance, of
Birmingham, Feil and Mantois, of Paris, and Schott, of Jena; while
specialists in grinding lenses, like Alvan Clark, of the U.S.A., and
others, have produced many large refractors.

Cooke, of York, made an object-glass, 25-inch diameter, for Newall, of
Gateshead, which has done splendid work at Cambridge. We have the
Washington 26-inch by Clark, the Vienna 27-inch by Grubb, the Nice
29-1/2-inch by Gautier, the Pulkowa 30-inch by Clark. Then there was
the sensation of Clark's 36-inch for the Lick Observatory in
California, and finally his _tour de force_, the Yerkes 40-inch
refractor, for Chicago.

At Greenwich there is the 28-inch photographic refractor, and the
Thompson equatoreal by Grubb, carrying both the 26-inch photographic
refractor and the 30-inch reflector. At the Cape of Good Hope we find
Mr. Frank McClean's 24-inch refractor, with an object-glass prism for
spectroscopic work.

It would be out of place to describe here the practical adjuncts of a
modern equatoreal--the adjustments for pointing it, the clock for
driving it, the position-micrometer and various eye-pieces, the
photographic and spectroscopic attachments, the revolving domes,
observing seats, and rising floors and different forms of mounting,
the siderostats and coelostats, and other convenient adjuncts, besides
the registering chronograph and numerous facilities for aiding
observation. On each of these a chapter might be written; but the
most important part of the whole outfit is the man behind the
telescope, and it is with him that a history is more especially
concerned.

SPECTROSCOPE.

Since the invention of the telescope no discovery has given so great
an impetus to astronomical physics as the spectroscope; and in giving
us information about the systems of stars and their proper motions it
rivals the telescope.

Frauenhofer, at the beginning of the nineteenth century, while
applying Dollond's discovery to make large achromatic telescopes,
studied the dispersion of light by a prism. Admitting the light of the
sun through a narrow slit in a window-shutter, an inverted image of
the slit can be thrown, by a lens of suitable focal length, on the
wall opposite. If a wedge or prism of glass be interposed, the image
is deflected to one side; but, as Newton had shown, the images formed
by the different colours of which white light is composed are
deflected to different extents--the violet most, the red least. The
number of colours forming images is so numerous as to form a
continuous spectrum on the wall with all the colours--red, orange,
yellow, green, blue, indigo, and violet. But Frauenhofer found with a
narrow slit, well focussed by the lens, that some colours were missing
in the white light of the sun, and these were shown by dark lines
across the spectrum. These are the Frauenhofer lines, some of which
he named by the letters of the alphabet. The D line is a very marked
one in the yellow. These dark lines in the solar spectrum had already
been observed by Wollaston. [3]

On examining artificial lights it was found that incandescent solids
and liquids (including the carbon glowing in a white gas flame) give
continuous spectra; gases, except under enormous pressure, give bright
lines. If sodium or common salt be thrown on the colourless flame of a
spirit lamp, it gives it a yellow colour, and its spectrum is a bright
yellow line agreeing in position with line D of the solar spectrum.

In 1832 Sir David Brewster found some of the solar black lines
increased in strength towards sunset, and attributed them to
absorption in the earth's atmosphere. He suggested that the others
were due to absorption in the sun's atmosphere. Thereupon Professor
J. D. Forbes pointed out that during a nearly total eclipse the lines
ought to be strengthened in the same way; as that part of the sun's
light, coming from its edge, passes through a great distance in the
sun's atmosphere. He tried this with the annular eclipse of 1836,
with a negative result which has never been accounted for, and which
seemed to condemn Brewster's view.

In 1859 Kirchoff, on repeating Frauenhofer's experiment, found that,
if a spirit lamp with salt in the flame were placed in the path of the
light, the black D line is intensified. He also found that, if he used
a limelight instead of the sunlight and passed it through the flame
with salt, the spectrum showed the D line black; or the vapour of
sodium absorbs the same light that it radiates. This proved to him the
existence of sodium in the sun's atmosphere.[4] Iron, calcium, and
other elements were soon detected in the same way.

Extensive laboratory researches (still incomplete) have been carried
out to catalogue (according to their wave-length on the undulatory
theory of light) all the lines of each chemical element, under all
conditions of temperature and pressure. At the same time, all the
lines have been catalogued in the light of the sun and the brighter of
the stars.

Another method of obtaining spectra had long been known, by
transmission through, or reflection from, a grating of equidistant
lines ruled upon glass or metal. H. A. Rowland developed the art of
constructing these gratings, which requires great technical skill, and
for this astronomers owe him a debt of gratitude.

In 1842 Doppler[5] proved that the colour of a luminous body, like the
pitch or note of a sounding body, must be changed by velocity of
approach or recession. Everyone has noticed on a railway that, on
meeting a locomotive whistling, the note is lowered after the engine
has passed. The pitch of a sound or the colour of a light depends on
the number of waves striking the ear or eye in a second. This number
is increased by approach and lowered by recession.

Thus, by comparing the spectrum of a star alongside a spectrum of
hydrogen, we may see all the lines, and be sure that there is hydrogen
in the star; yet the lines in the star-spectrum may be all slightly
displaced to one side of the lines of the comparison spectrum. If
towards the violet end, it means mutual approach of the star and
earth; if to the red end, it means recession. The displacement of
lines does not tell us whether the motion is in the star, the earth,
or both. The displacement of the lines being measured, we can
calculate the rate of approach or recession in miles per second.

In 1868 Huggins[6] succeeded in thus measuring the velocities of stars
in the direction of the line of sight.

In 1873 Vogel[7] compared the spectra of the sun's East (approaching)
limb and West (receding) limb, and the displacement of lines endorsed
the theory. This last observation was suggested by Zollner.

FOOTNOTES:

[1] In the _Encyclopaedia Britannica_, article "Telescope," and in
Grant's _Physical Astronomy_, good reasons are given for awarding the
honour to Lipperhey.

[2] Will the indulgent reader excuse an anecdote which may encourage
some workers who may have found their mathematics defective through
want of use? James Gregory's nephew David had a heap of MS. notes by
Newton. These descended to a Miss Gregory, of Edinburgh, who handed
them to the present writer, when an undergraduate at Cambridge, to
examine. After perusal, he lent them to his kindest of friends,
J. C. Adams (the discoverer of Neptune), for his opinion. Adams's
final verdict was: "I fear they are of no value. It is pretty evident
that, when he wrote these notes, _Newton's mathematics were a little
rusty_."

[3] _R. S. Phil. Trans_.

[4] The experiment had been made before by one who did not understand
its meaning;. But Sir George G. Stokes had already given verbally the
true explanation of Frauenhofer lines.

[5] _Abh. d. Kon. Bohm. d. Wiss_., Bd. ii., 1841-42, p. 467. See
also Fizeau in the _Ann. de Chem. et de Phys_., 1870, p. 211.

[6] _R. S. Phil. Trans_., 1868.

[7] _Ast. Nach_., No. 1, 864.

BOOK IV. THE PHYSICAL PERIOD

We have seen how the theory of the solar system was slowly developed
by the constant efforts of the human mind to find out what are the
rules of cause and effect by which our conception of the present
universe and its development seems to be bound. In the primitive ages
a mere record of events in the heavens and on the earth gave the only
hope of detecting those uniform sequences from which to derive rules
or laws of cause and effect upon which to rely. Then came the
geometrical age, in which rules were sought by which to predict the
movements of heavenly bodies. Later, when the relation of the sun to
the courses of the planets was established, the sun came to be looked
upon as a cause; and finally, early in the seventeenth century, for
the first time in history, it began to be recognised that the laws of
dynamics, exactly as they had been established for our own terrestrial
world, hold good, with the same rigid invariability, at least as far
as the limits of the solar system.

Throughout this evolution of thought and conjecture there were two
types of astronomers--those who supplied the facts, and those who
supplied the interpretation through the logic of mathematics. So
Ptolemy was dependent upon Hipparchus, Kepler on Tycho Brahe, and
Newton in much of his work upon Flamsteed.

When Galileo directed his telescope to the heavens, when Secchi and
Huggins studied the chemistry of the stars by means of the
spectroscope, and when Warren De la Rue set up a photoheliograph at
Kew, we see that a progress in the same direction as before, in the
evolution of our conception of the universe, was being made. Without
definite expression at any particular date, it came to be an accepted
fact that not only do earthly dynamics apply to the heavenly bodies,
but that the laws we find established here, in geology, in chemistry,
and in the laws of heat, may be extended with confidence to the
heavenly bodies. Hence arose the branch of astronomy called
astronomical physics, a science which claims a large portion of the
work of the telescope, spectroscope, and photography. In this new
development it is more than ever essential to follow the dictum of
Tycho Brahe--not to make theories until all the necessary facts are
obtained. The great astronomers of to-day still hold to Sir Isaac
Newton's declaration, "Hypotheses non fingo." Each one may have his
suspicions of a theory to guide him in a course of observation, and
may call it a working hypothesis. But the cautious astronomer does
not proclaim these to the world; and the historian is certainly not
justified in including in his record those vague speculations founded
on incomplete data which may be demolished to-morrow, and which,
however attractive they may be, often do more harm than good to the
progress of true science. Meanwhile the accumulation of facts has
been prodigious, and the revelations of the telescope and spectroscope
entrancing.

12. THE SUN.

One of Galileo's most striking discoveries, when he pointed his
telescope to the heavenly bodies, was that of the irregularly shaped
spots on the sun, with the dark central _umbra_ and the less
dark, but more extensive, _penumbra_ surrounding it, sometimes
with several umbrae in one penumbra. He has left us many drawings of
these spots, and he fixed their period of rotation as a lunar month.

[Illustration: SOLAR SURFACE, As Photographed at the Royal
Observatory, Greenwich, showing sun-spots with umbrae, penumbrae, and
faculae.]

It is not certain whether Galileo, Fabricius, or Schemer was the first
to see the spots. They all did good work. The spots were found to be
ever varying in size and shape. Sometimes, when a spot disappears at
the western limb of the sun, it is never seen again. In other cases,
after a fortnight, it reappears at the eastern limb. The faculae, or
bright areas, which are seen all over the sun's surface, but specially
in the neighbourhood of spots, and most distinctly near the sun's
edge, were discovered by Galileo. A high telescopic power resolves
their structure into an appearance like willow-leaves, or rice-grains,
fairly uniform in size, and more marked than on other parts of the
sun's surface.

Speculations as to the cause of sun-spots have never ceased from
Galileo's time to ours. He supposed them to be clouds. Scheiner[1]
said they were the indications of tumultuous movements occasionally
agitating the ocean of liquid fire of which he supposed the sun to be
composed.

A. Wilson, of Glasgow, in 1769,[2] noticed a movement of the umbra
relative to the penumbra in the transit of the spot over the sun's
surface; exactly as if the spot were a hollow, with a black base and
grey shelving sides. This was generally accepted, but later
investigations have contradicted its universality. Regarding the cause
of these hollows, Wilson said:--

Whether their first production and subsequent numberless changes
depend upon the eructation of elastic vapours from below, or upon
eddies or whirlpools commencing at the surface, or upon the
dissolving of the luminous matter in the solar atmosphere, as clouds
are melted and again given out by our air; or, if the reader
pleases, upon the annihilation and reproduction of parts of this
resplendent covering, is left for theory to guess at.[3]

Ever since that date theory has been guessing at it. The solar
astronomer is still applying all the instruments of modern research to
find out which of these suppositions, or what modification of any of
them, is nearest the truth. The obstacle--one that is perhaps fatal to
a real theory--lies in the impossibility of reproducing comparative
experiments in our laboratories or in our atmosphere.

Sir William Herschel propounded an explanation of Wilson's observation
which received much notice, but which, out of respect for his memory,
is not now described, as it violated the elementary laws of heat.

Sir John Herschel noticed that the spots are mostly confined to two
zones extending to about 35 degrees on each side of the equator, and that a
zone of equatoreal calms is free from spots. But it was
R. C. Carrington[4] who, by his continuous observations at Redhill, in
Surrey, established the remarkable fact that, while the rotation
period in the highest latitudes, 50 degrees, where spots are seen, is
twenty-seven-and-a-half days, near the equator the period is only
twenty-five days. His splendid volume of observations of the sun led
to much new information about the average distribution of spots at
different epochs.

Schwabe, of Dessau, began in 1826 to study the solar surface, and,
after many years of work, arrived at a law of frequency which has been
more fruitful of results than any discovery in solar physics.[5] In
1843 he announced a decennial period of maxima and minima of sun-spot
displays. In 1851 it was generally accepted, and, although a period of
eleven years has been found to be more exact, all later observations,
besides the earlier ones which have been hunted up for the purpose, go
to establish a true periodicity in the number of sun-spots. But quite
lately Schuster[6] has given reasons for admitting a number of
co-existent periods, of which the eleven-year period was predominant
in the nineteenth century.

In 1851 Lament, a Scotchman at Munich, found a decennial period in the
daily range of magnetic declination. In 1852 Sir Edward Sabine
announced a similar period in the number of "magnetic storms"
affecting all of the three magnetic elements--declination, dip, and
intensity. Australian and Canadian observations both showed the
decennial period in all three elements. Wolf, of Zurich, and Gauthier,
of Geneva, each independently arrived at the same conclusion.

It took many years before this coincidence was accepted as certainly
more than an accident by the old-fashioned astronomers, who want rigid
proof for every new theory. But the last doubts have long vanished,
and a connection has been further traced between violent outbursts of
solar activity and simultaneous magnetic storms.

The frequency of the Aurora Borealis was found by Wolf to follow the
same period. In fact, it is closely allied in its cause to terrestrial
magnetism. Wolf also collected old observations tracing the
periodicity of sun-spots back to about 1700 A.D.

Spoerer deduced a law of dependence of the average latitude of
sun-spots on the phase of the sun-spot period.

All modern total solar eclipse observations seem to show that the
shape of the luminous corona surrounding the moon at the moment of
totality has a special distinct character during the time of a
sun-spot maximum, and another, totally different, during a sun-spot
minimum.

A suspicion is entertained that the total quantity of heat received by
the earth from the sun is subject to the same period. This would have
far-reaching effects on storms, harvests, vintages, floods, and
droughts; but it is not safe to draw conclusions of this kind except
from a very long period of observations.

Solar photography has deprived astronomers of the type of Carrington
of the delight in devoting a life's work to collecting data. It has
now become part of the routine work of an observatory.

In 1845 Foucault and Fizeau took a daguerreotype photograph of the
sun. In 1850 Bond produced one of the moon of great beauty, Draper
having made some attempts at an even earlier date. But astronomical
photography really owes its beginning to De la Rue, who used the
collodion process for the moon in 1853, and constructed the Kew
photoheliograph in 1857, from which date these instruments have been
multiplied, and have given us an accurate record of the sun's surface.
Gelatine dry plates were first used by Huggins in 1876.

It is noteworthy that from the outset De la Rue recognised the value
of stereoscopic vision, which is now known to be of supreme
accuracy. In 1853 he combined pairs of photographs of the moon in the
same phase, but under different conditions regarding libration,
showing the moon from slightly different points of view. These in the
stereoscope exhibited all the relief resulting from binocular vision,
and looked like a solid globe. In 1860 he used successive photographs
of the total solar eclipse stereoscopically, to prove that the red
prominences belong to the sun, and not to the moon. In 1861 he
similarly combined two photographs of a sun-spot, the perspective
effect showing the umbra like a floor at the bottom of a hollow
penumbra; and in one case the faculae were discovered to be sailing
over a spot apparently at some considerable height. These appearances
may be partly due to a proper motion; but, so far as it went, this was
a beautiful confirmation of Wilson's discovery. Hewlett, however, in
1894, after thirty years of work, showed that the spots are not always
depressions, being very subject to disturbance.

The Kew photographs [7] contributed a vast amount of information about
sun-spots, and they showed that the faculae generally follow the spots
in their rotation round the sun.

The constitution of the sun's photosphere, the layer which is the
principal light-source on the sun, has always been a subject of great
interest; and much was done by men with exceptionally keen eyesight,
like Mr. Dawes. But it was a difficult subject, owing to the rapidity
of the changes in appearance of the so-called rice-grains, about 1" in
diameter. The rapid transformations and circulations of these
rice-grains, if thoroughly studied, might lead to a much better
knowledge of solar physics. This seemed almost hopeless, as it was
found impossible to identify any "rice-grain" in the turmoil after a
few minutes. But M. Hansky, of Pulkowa (whose recent death is
deplored), introduced successfully a scheme of photography, which
might almost be called a solar cinematograph. He took photographs of
the sun at intervals of fifteen or thirty seconds, and then enlarged
selected portions of these two hundred times, giving a picture
corresponding to a solar disc of six metres diameter. In these
enlarged pictures he was able to trace the movements, and changes of
shape and brightness, of individual rice-grains. Some granules become
larger or smaller. Some seem to rise out of a mist, as it were, and to
become clearer. Others grow feebler. Some are split in two. Some are
rotated through a right angle in a minute or less, although each of
the grains may be the size of Great Britain. Generally they move
together in groups of very various velocities, up to forty kilometres
a second. These movements seem to have definite relation to any
sun-spots in the neighbourhood. From the results already obtained it
seems certain that, if this method of observation be continued, it
cannot fail to supply facts of the greatest importance.

It is quite impossible to do justice here to the work of all those who
are engaged on astronomical physics. The utmost that can be attempted
is to give a fair idea of the directions of human thought and
endeavour. During the last half-century America has made splendid
progress, and an entirely new process of studying the photosphere has
been independently perfected by Professor Hale at Chicago, and
Deslandres at Paris.[8] They have succeeded in photographing the sun's
surface in monochromatic light, such as the light given off as one of
the bright lines of hydrogen or of calcium, by means of the
"Spectroheliograph." The spectroscope is placed with its slit in the
focus of an equatoreal telescope, pointed to the sun, so that the
circular image of the sun falls on the slit. At the other end of the
spectroscope is the photographic plate. Just in front of this plate
there is another slit parallel to the first, in the position where the
image of the first slit formed by the K line of calcium falls. Thus is
obtained a photograph of the section of the sun, made by the first
slit, only in K light. As the image of the sun passes over the first
slit the photographic plate is moved at the same rate and in the same
direction behind the second slit; and as successive sections of the
sun's image in the equatoreal enter the apparatus, so are these
sections successively thrown in their proper place on the photographic
plate, always in K light. By using a high dispersion the faculae which
give off K light can be correctly photographed, not only at the sun's
edge, but all over his surface. The actual mechanical method of
carrying out the observation is not quite so simple as what is here
described.

By choosing another line of the spectrum instead of calcium K--for
example, the hydrogen line H(3)--we obtain two photographs, one
showing the appearance of the calcium floculi, and the other of the
hydrogen floculi, on the same part of the solar surface; and nothing
is more astonishing than to note the total want of resemblance in the
forms shown on the two. This mode of research promises to afford many
new and useful data.

The spectroscope has revealed the fact that, broadly speaking, the sun
is composed of the same materials as the earth. Angstrom was the first
to map out all of the lines to be found in the solar spectrum. But
Rowland, of Baltimore, after having perfected the art of making true
gratings with equidistant lines ruled on metal for producing spectra,
then proceeded to make a map of the solar spectrum on a large scale.

In 1866 Lockyer[9] threw an image of the sun upon the slit of a
spectroscope, and was thus enabled to compare the spectrum of a spot
with that of the general solar surface. The observation proved the
darkness of a spot to be caused by increased absorption of light, not
only in the dark lines, which are widened, but over the entire
spectrum. In 1883 Young resolved this continuous obscurity into an
infinite number of fine lines, which have all been traced in a shadowy
way on to the general solar surface. Lockyer also detected
displacements of the spectrum lines in the spots, such as would be
produced by a rapid motion in the line of sight. It has been found
that both uprushes and downrushes occur, but there is no marked
predominance of either in a sun-spot. The velocity of motion thus
indicated in the line of sight sometimes appears to amount to 320
miles a second. But it must be remembered that pressure of a gas has
some effect in displacing the spectral lines. So we must go on,
collecting data, until a time comes when the meaning of all the facts
can be made clear.

_Total Solar Eclipses_.--During total solar eclipses the time is so
short, and the circumstances so impressive, that drawings of the
appearance could not always be trusted. The red prominences of jagged
form that are seen round the moon's edge, and the corona with its
streamers radiating or interlacing, have much detail that can hardly
be recorded in a sketch. By the aid of photography a number of records
can be taken during the progress of totality. From a study of these
the extent of the corona is demonstrated in one case to extend to at
least six diameters of the moon, though the eye has traced it
farther. This corona is still one of the wonders of astronomy, and
leads to many questions. What is its consistency, if it extends many
million miles from the sun's surface? How is it that it opposed no
resistance to the motion of comets which have almost grazed the sun's
surface? Is this the origin of the zodiacal light? The character of
the corona in photographic records has been shown to depend upon the
phase of the sun-spot period. During the sun-spot maximum the corona
seems most developed over the spot-zones--i.e., neither at the
equator nor the poles. The four great sheaves of light give it a
square appearance, and are made up of rays or plumes, delicate like
the petals of a flower. During a minimum the nebulous ring seems to
be made of tufts of fine hairs with aigrettes or radiations from both
poles, and streamers from the equator.

[Illustration: SOLAR ECLIPSE, 1882. From drawing by W. H. Wesley,
Secretary R.A.S.; showing the prominences, the corona, and an unknown
comet.]

On September 19th, 1868, eclipse spectroscopy began with the Indian
eclipse, in which all observers found that the red prominences showed
a bright line spectrum, indicating the presence of hydrogen and other
gases. So bright was it that Jansen exclaimed: "_Je verrai ces
lignes-la en dehors des eclipses_." And the next day he observed the
lines at the edge of the uneclipsed sun. Huggins had suggested this
observation in February, 1868, his idea being to use prisms of such
great dispersive power that the continuous spectrum reflected by our
atmosphere should be greatly weakened, while a bright line would
suffer no diminution by the high dispersion. On October 20th
Lockyer,[10] having news of the eclipse, but not of Jansen's
observations the day after, was able to see these lines. This was a
splendid performance, for it enabled the prominences to be observed,
not only during eclipses, but every day. Moreover, the next year
Huggins was able, by using a wide slit, to see the whole of a
prominence and note its shape. Prominences are classified, according
to their form, into "flame" and "cloud" prominences, the spectrum of
the latter showing calcium, hydrogen, and helium; that of the former
including a number of metals.

The D line of sodium is a double line, and in the same eclipse (1868)
an orange line was noticed which was afterwards found to lie close to
the two components of the D line. It did not correspond with any known
terrestrial element, and the unknown element was called "helium." It
was not until 1895 that Sir William Ramsay found this element as a gas
in the mineral cleavite.

The spectrum of the corona is partly continuous, indicating light
reflected from the sun's body. But it also shows a green line
corresponding with no known terrestrial element, and the name
"coronium" has been given to the substance causing it.

A vast number of facts have been added to our knowledge about the sun
by photography and the spectroscope. Speculations and hypotheses in
plenty have been offered, but it may be long before we have a complete
theory evolved to explain all the phenomena of the storm-swept
metallic atmosphere of the sun.

The proceedings of scientific societies teem with such facts and
"working hypotheses," and the best of them have been collected by Miss
Clerke in her _History of Astronomy during the Nineteenth Century_. As
to established facts, we learn from the spectroscopic researches (1)
that the continuous spectrum is derived from the _photosphere_ or
solar gaseous material compressed almost to liquid consistency; (2)
that the _reversing layer_ surrounds it and gives rise to black
lines in the spectrum; that the _chromosphere_ surrounds this, is
composed mainly of hydrogen, and is the cause of the red prominences
in eclipses; and that the gaseous _corona_ surrounds all of
these, and extends to vast distances outside the sun's visible
surface.

FOOTNOTES:

[1] _Rosa Ursina_, by C. Scheiner, _fol_.; Bracciani, 1630.

[2] _R. S. Phil. Trans_., 1774.

[3] _Ibid_, 1783.

[4] _Observations on the Spots on the Sun, etc.,_ 4 degrees; London and
Edinburgh, 1863.

[5] _Periodicitat der Sonnenflecken. Astron. Nach. XXI._, 1844,
P. 234.

[6] _R.S. Phil. Trans._ (ser. A), 1906, p. 69-100.

[7] "Researches on Solar Physics," by De la Rue, Stewart and Loewy;
_R. S. Phil. Trans_., 1869, 1870.

[8] "The Sun as Photographed on the K line"; _Knowledge_, London,
1903, p. 229.

[9] _R. S. Proc._, xv., 1867, p. 256.

[10] _Acad. des Sc._, Paris; _C. R._, lxvii., 1868, p. 121.

13. THE MOON AND PLANETS.

_The Moon_.--Telescopic discoveries about the moon commence with
Galileo's discovery that her surface has mountains and valleys, like
the earth. He also found that, while she always turns the same face to
us, there is periodically a slight twist to let us see a little round
the eastern or western edge. This was called _libration_, and the
explanation was clear when it was understood that in showing always
the same face to us she makes one revolution a month on her axis
_uniformly_, and that her revolution round the earth is not
uniform.

Galileo said that the mountains on the moon showed greater differences
of level than those on the earth. Shroter supported this
opinion. W. Herschel opposed it. But Beer and Madler measured the
heights of lunar mountains by their shadows, and found four of them
over 20,000 feet above the surrounding plains.

Langrenus [1] was the first to do serious work on selenography, and
named the lunar features after eminent men. Riccioli also made lunar
charts. In 1692 Cassini made a chart of the full moon. Since then we
have the charts of Schroter, Beer and Madler (1837), and of Schmidt,
of Athens (1878); and, above all, the photographic atlas by Loewy and
Puiseux.

The details of the moon's surface require for their discussion a whole
book, like that of Neison or the one by Nasmyth and Carpenter. Here a
few words must suffice. Mountain ranges like our Andes or Himalayas
are rare. Instead of that, we see an immense number of circular
cavities, with rugged edges and flat interior, often with a cone in
the centre, reminding one of instantaneous photographs of the splash
of a drop of water falling into a pool. Many of these are fifty or
sixty miles across, some more. They are generally spoken of as
resembling craters of volcanoes, active or extinct, on the earth. But
some of those who have most fully studied the shapes of craters deny
altogether their resemblance to the circular objects on the moon.
These so-called craters, in many parts, are seen to be closely
grouped, especially in the snow-white parts of the moon. But there are
great smooth dark spaces, like the clear black ice on a pond, more
free from craters, to which the equally inappropriate name of seas has
been given. The most conspicuous crater, _Tycho_, is near the south
pole. At full moon there are seen to radiate from Tycho numerous
streaks of light, or "rays," cutting through all the mountain
formations, and extending over fully half the lunar disc, like the
star-shaped cracks made on a sheet of ice by a blow. Similar cracks
radiate from other large craters. It must be mentioned that these
white rays are well seen only in full light of the sun at full moon,
just as the white snow in the crevasses of a glacier is seen bright
from a distance only when the sun is high, and disappears at
sunset. Then there are deep, narrow, crooked "rills" which may have
been water-courses; also "clefts" about half a mile wide, and often
hundreds of miles long, like deep cracks in the surface going straight
through mountain and valley.

The moon shares with the sun the advantage of being a good subject for
photography, though the planets are not. This is owing to her larger
apparent size, and the abundance of illumination. The consequence is
that the finest details of the moon, as seen in the largest telescope
in the world, may be reproduced at a cost within the reach of all.

No certain changes have ever been observed; but several suspicions
have been expressed, especially as to the small crater _Linne_, in the
_Mare Serenitatis_. It is now generally agreed that no certainty can
be expected from drawings, and that for real evidence we must await
the verdict of photography.

No trace of water or of an atmosphere has been found on the moon. It
is possible that the temperature is too low. In any case, no
displacement of a star by atmospheric refraction at occultation has
been surely recorded. The moon seems to be dead.

The distance of the moon from the earth is just now the subject of
re-measurement. The base line is from Greenwich to Cape of Good Hope,
and the new feature introduced is the selection of a definite point on
a crater (Mosting A), instead of the moon's edge, as the point whose
distance is to be measured.

_The Inferior Planets_.--When the telescope was invented, the phases
of Venus attracted much attention; but the brightness of this planet,
and her proximity to the sun, as with Mercury also, seemed to be a bar
to the discovery of markings by which the axis and period of rotation
could be fixed. Cassini gave the rotation as twenty-three hours, by
observing a bright spot on her surface. Shroter made it 23h. 21m. 19s.
This value was supported by others. In 1890 Schiaparelli[2] announced
that Venus rotates, like our moon, once in one of her revolutions, and
always directs the same face to the sun. This property has also been
ascribed to Mercury; but in neither case has the evidence been
generally accepted. Twenty-four hours is probably about the period of
rotation for each of these planets.

Several observers have claimed to have seen a planet within the orbit
of Mercury, either in transit over the sun's surface or during an
eclipse. It has even been named _Vulcan_. These announcements would
have received little attention but for the fact that the motion of
Mercury has irregularities which have not been accounted for by known
planets; and Le Verrier[3] has stated that an intra-Mercurial planet
or ring of asteroids would account for the unexplained part of the
motion of the line of apses of Mercury's orbit amounting to 38" per
century.

_Mars_.--The first study of the appearance of Mars by Miraldi led him
to believe that there were changes proceeding in the two white caps
which are seen at the planet's poles. W. Herschel attributed these
caps to ice and snow, and the dates of his observations indicated a
melting of these ice-caps in the Martian summer.

Schroter attributed the other markings on Mars to drifting clouds. But
Beer and Madler, in 1830-39, identified the same dark spots as being
always in the same place, though sometimes blurred by mist in the
local winter. A spot sketched by Huyghens in 1672, one frequently seen
by W. Herschel in 1783, another by Arago in 1813, and nearly all the
markings recorded by Beer and Madler in 1830, were seen and drawn by
F. Kaiser in Leyden during seventeen nights of the opposition of 1862
(_Ast. Nacht._, No. 1,468), whence he deduced the period of rotation
to be 24h. 37m. 22s.,62--or one-tenth of a second less than the period
deduced by R. A. Proctor from a drawing by Hooke in 1666.

It must be noted that, if the periods of rotation both of Mercury and
Venus be about twenty-four hours, as seems probable, all the four
planets nearest to the sun rotate in the same period, while the great
planets rotate in about ten hours (Uranus and Neptune being still
indeterminate).

The general surface of Mars is a deep yellow; but there are dark grey
or greenish patches. Sir John Herschel was the first to attribute the
ruddy colour of Mars to its soil rather than to its atmosphere.

The observations of that keen-sighted observer Dawes led to the first
good map of Mars, in 1869. In the 1877 opposition Schiaparelli revived
interest in the planet by the discovery of canals, uniformly about
sixty miles wide, running generally on great circles, some of them
being three or four thousand miles long. During the opposition of
1881-2 the same observer re-observed the canals, and in twenty of them
he found the canals duplicated,[4] the second canal being always 200
to 400 miles distant from its fellow.

The existence of these canals has been doubted. Mr. Lowell has now
devoted years to the subject, has drawn them over and over again, and
has photographed them; and accepts the explanation that they are
artificial, and that vegetation grows on their banks. Thus is revived
the old controversy between Whewell and Brewster as to the
habitability of the planets. The new arguments are not yet generally
accepted. Lowell believes he has, with the spectroscope, proved the
existence of water on Mars.

One of the most unexpected and interesting of all telescopic
discoveries took place in the opposition of 1877, when Mars was
unusually near to the earth. The Washington Observatory had acquired
the fine 26-inch refractor, and Asaph Hall searched for satellites,
concealing the planet's disc to avoid the glare. On August 11th he had
a suspicion of a satellite. This was confirmed on the 16th, and on the
following night a second one was added. They are exceedingly faint,
and can be seen only by the most powerful telescopes, and only at the
times of opposition. Their diameters are estimated at six or seven
miles. It was soon found that the first, Deimos, completes its orbit
in 30h. 18m. But the other, Phobos, at first was a puzzle, owing to
its incredible velocity being unsuspected. Later it was found that the
period of revolution was only 7h. 39m. 22s. Since the Martian day is
twenty-four and a half hours, this leads to remarkable results.
Obviously the easterly motion of the satellite overwhelms the diurnal
rotation of the planet, and Phobos must appear to the inhabitants, if
they exist, to rise in the west and set in the east, showing two or
even three full moons in a day, so that, sufficiently well for the
ordinary purposes of life, the hour of the day can be told by its
phases.

The discovery of these two satellites is, perhaps, the most
interesting telescopic visual discovery made with the large telescopes
of the last half century; photography having been the means of
discovering all the other new satellites except Jupiter's fifth (in
order of discovery).

[Illustration: JUPITER. From a drawing by E. M. Antoniadi, showing
transit of a satellite's shadow, the belts, and the "great red spot"
(_Monthly Notices_, R. A. S., vol. lix., pl. x.).]

_Jupiter._--Galileo's discovery of Jupiter's satellites was followed
by the discovery of his belts. Zucchi and Torricelli seem to have seen
them. Fontana, in 1633, reported three belts. In 1648 Grimaldi saw but
two, and noticed that they lay parallel to the ecliptic. Dusky spots
were also noticed as transient. Hooke[5] measured the motion of one in
1664. In 1665 Cassini, with a fine telescope, 35-feet focal length,
observed many spots moving from east to west, whence he concluded that
Jupiter rotates on an axis like the earth. He watched an unusually
permanent spot during twenty-nine rotations, and fixed the period at
9h. 56m. Later he inferred that spots near the equator rotate quicker
than those in higher latitudes (the same as Carrington found for the
sun); and W. Herschel confirmed this in 1778-9.

Jupiter's rapid rotation ought, according to Newton's theory, to be
accompanied by a great flattening at the poles. Cassini had noted an
oval form in 1691. This was confirmed by La Hire, Romer, and
Picard. Pound measured the ellipticity = 1/(13.25).

W. Herschel supposed the spots to be masses of cloud in the
atmosphere--an opinion still accepted. Many of them were very
permanent. Cassini's great spot vanished and reappeared nine times
between 1665 and 1713. It was close to the northern margin of the
southern belt. Herschel supposed the belts to be the body of the
planet, and the lighter parts to be clouds confined to certain
latitudes.

In 1665 Cassini observed transits of the four satellites, and also saw
their shadows on the planet, and worked out a lunar theory for
Jupiter. Mathematical astronomers have taken great interest in the
perturbations of the satellites, because their relative periods
introduce peculiar effects. Airy, in his delightful book,
_Gravitation_, has reduced these investigations to simple
geometrical explanations.

In 1707 and 1713 Miraldi noticed that the fourth satellite varies much
in brightness. W. Herschel found this variation to depend upon its
position in its orbit, and concluded that in the positions of
feebleness it is always presenting to us a portion of its surface,
which does not well reflect the sun's light; proving that it always
turns the same face to Jupiter, as is the case with our moon. This
fact had also been established for Saturn's fifth satellite, and may
be true for all satellites.

In 1826 Struve measured the diameters of the four satellites, and
found them to be 2,429, 2,180, 3,561, and 3,046 miles.

In modern times much interest has been taken in watching a rival to
Cassini's famous spot. The "great red spot" was first observed by
Niesten, Pritchett, and Tempel, in 1878, as a rosy cloud attached to a
whitish zone beneath the dark southern equatorial band, shaped like
the new war balloons, 30,000 miles long and 7,000 miles across. The
next year it was brick-red. A white spot beside it completed a
rotation in less time by 5-1/2 minutes than the red spot--a difference
of 260 miles an hour. Thus they came together again every six weeks,
but the motions did not continue uniform. The spot was feeble in
1882-4, brightened in 1886, and, after many changes, is still visible.

Galileo's great discovery of Jupiter's four moons was the last word in
this connection until September 9th, 1892, when Barnard, using the
36-inch refractor of the Lick Observatory, detected a tiny spot of
light closely following the planet. This proved to be a new satellite
(fifth), nearer to the planet than any other, and revolving round it
in 11h. 57m. 23s. Between its rising and setting there must be an
interval of 2-1/2 Jovian days, and two or three full moons. The sixth
and seventh satellites were found by the examination of photographic
plates at the Lick Observatory in 1905, since which time they have
been continuously photographed, and their orbits traced, at Greenwich.
On examining these plates in 1908 Mr. Melotte detected the eighth
satellite, which seems to be revolving in a retrograde orbit three
times as far from its planet as the next one (seventh), in these two
points agreeing with the outermost of Saturn's satellites (Phoebe).

_Saturn._--This planet, with its marvellous ring, was perhaps the most
wonderful object of those first examined by Galileo's telescope. He
was followed by Dominique Cassini, who detected bands like Jupiter's
belts. Herschel established the rotation of the planet in 1775-94.
From observations during one hundred rotations he found the period to
be 10h. 16m. 0s., 44. Herschel also measured the ratio of the polar to
the equatoreal diameter as 10:11.

The ring was a complete puzzle to Galileo, most of all when the planet
reached a position where the plane of the ring was in line with the
earth, and the ring disappeared (December 4th, 1612). It was not until
1656 that Huyghens, in his small pamphlet _De Saturni Luna Observatio
Nova_, was able to suggest in a cypher the ring form; and in 1659, in
his Systema Saturnium, he gave his reasons and translated the cypher:
"The planet is surrounded by a slender flat ring, everywhere distinct
from its surface, and inclined to the ecliptic." This theory explained
all the phases of the ring which had puzzled others. This ring was
then, and has remained ever since, a unique structure. We in this age
have got accustomed to it. But Huyghens's discovery was received with
amazement.

In 1675 Cassini found the ring to be double, the concentric rings
being separated by a black band--a fact which was placed beyond
dispute by Herschel, who also found that the thickness of the ring
subtends an angle less than 0".3. Shroter estimated its thickness at
500 miles.

Many speculations have been advanced to explain the origin and
constitution of the ring. De Sejour said [6] that it was thrown off
from Saturn's equator as a liquid ring, and afterwards solidified. He
noticed that the outside would have a greater velocity, and be less
attracted to the planet, than the inner parts, and that equilibrium
would be impossible; so he supposed it to have solidified into a
number of concentric rings, the exterior ones having the least
velocity.

Clerk Maxwell, in the Adams prize essay, gave a physico-mathematical
demonstration that the rings must be composed of meteoritic matter
like gravel. Even so, there must be collisions absorbing the energy of
rotation, and tending to make the rings eventually fall into the
planet. The slower motion of the external parts has been proved by the
spectroscope in Keeler's hands, 1895.

Saturn has perhaps received more than its share of attention owing to
these rings. This led to other discoveries. Huyghens in 1655, and
J. D. Cassini in 1671, discovered the sixth and eighth satellites
(Titan and Japetus). Cassini lost his satellite, and in searching for
it found Rhea (the fifth) in 1672, besides his old friend, whom he
lost again. He added the third and fourth in 1684 (Tethys and
Dione). The first and second (Mimas and Encelades) were added by
Herschel in 1789, and the seventh (Hyperion) simultaneously by Lassel
and Bond in 1848. The ninth (Phoebe) was found on photographs, by
Pickering in 1898, with retrograde motion; and he has lately added a
tenth.

The occasional disappearance of Cassini's Japetus was found on
investigation to be due to the same causes as that of Jupiter's fourth
satellite, and proves that it always turns the same face to the
planet.

_Uranus and Neptune_.--The splendid discoveries of Uranus and two
satellites by Sir William Herschel in 1787, and of Neptune by Adams
and Le Verrier in 1846, have been already described. Lassel added two
more satellites to Uranus in 1851, and found Neptune's satellite in
1846. All of the satellites of Uranus have retrograde motion, and
their orbits are inclined about 80 degrees to the ecliptic.

The spectroscope has shown the existence of an absorbing atmosphere on
Jupiter and Saturn, and there are suspicions that they partake
something of the character of the sun, and emit some light besides
reflecting solar light. On both planets some absorption lines seem to
agree with the aqueous vapour lines of our own atmosphere; while one,
which is a strong band in the red common to both planets, seems to
agree with a line in the spectrum of some reddish stars.

Uranus and Neptune are difficult to observe spectroscopically, but
appear to have peculiar spectra agreeing together. Sometimes Uranus
shows Frauenhofer lines, indicating reflected solar light. But
generally these are not seen, and six broad bands of absorption
appear. One is the F. of hydrogen; another is the red-star line of
Jupiter and Saturn. Neptune is a very difficult object for the
spectroscope.

Quite lately [7] P. Lowell has announced that V. M. Slipher, at
Flagstaff Observatory, succeeded in 1907 in rendering some plates
sensitive far into the red. A reproduction is given of photographed
spectra of the four outermost planets, showing (1) a great number of
new lines and bands; (2) intensification of hydrogen F. and C. lines;
(3) a steady increase of effects (1) and (2) as we pass from Jupiter
and Saturn to Uranus, and a still greater increase in Neptune.

_Asteroids_.--The discovery of these new planets has been
described. At the beginning of the last century it was an immense
triumph to catch a new one. Since photography was called into the
service by Wolf, they have been caught every year in shoals. It is
like the difference between sea fishing with the line and using a
steam trawler. In the 1908 almanacs nearly seven hundred asteroids are
included. The computation of their perturbations and ephemerides by
Euler's and Lagrange's method of variable elements became so laborious
that Encke devised a special process for these, which can be applied
to many other disturbed orbits. [8]

When a photograph is taken of a region of the heavens including an
asteroid, the stars are photographed as points because the telescope
is made to follow their motion; but the asteroids, by their proper
motion, appear as short lines.

The discovery of Eros and the photographic attack upon its path have
been described in their relation to finding the sun's distance.

A group of four asteroids has lately been found, with a mean distance
and period equal to that of Jupiter. To three of these masculine names
have been given--Hector, Patroclus, Achilles; the other has not yet
been named.

FOOTNOTES:

[1] Langrenus (van Langren), F. Selenographia sive lumina austriae
philippica; Bruxelles, 1645.

[2] _Astr. Nach._, 2,944.

[3] _Acad. des Sc._, Paris; _C.R._, lxxxiii., 1876.

[4] _Mem. Spettr. Ital._, xi., p. 28.

[5] _R. S. Phil. Trans_., No. 1.

[6] Grant's _Hist. Ph. Ast_., p. 267.

[7] _Nature_, November 12th, 1908.

[8] _Ast. Nach_., Nos. 791, 792, 814, translated by G. B. Airy.
_Naut. Alm_., Appendix, 1856.

14. COMETS AND METEORS.

Ever since Halley discovered that the comet of 1682 was a member of
the solar system, these wonderful objects have had a new interest for
astronomers; and a comparison of orbits has often identified the
return of a comet, and led to the detection of an elliptic orbit where
the difference from a parabola was imperceptible in the small portion
of the orbit visible to us. A remarkable case in point was the comet
of 1556, of whose identity with the comet of 1264 there could be
little doubt. Hind wanted to compute the orbit more exactly than
Halley had done. He knew that observations had been made, but they
were lost. Having expressed his desire for a search, all the
observations of Fabricius and of Heller, and also a map of the comet's
path among the stars, were eventually unearthed in the most unlikely
manner, after being lost nearly three hundred years. Hind and others
were certain that this comet would return between 1844 and 1848, but
it never appeared.

When the spectroscope was first applied to finding the composition of
the heavenly bodies, there was a great desire to find out what comets
are made of. The first opportunity came in 1864, when Donati observed
the spectrum of a comet, and saw three bright bands, thus proving that
it was a gas and at least partly self-luminous. In 1868 Huggins
compared the spectrum of Winnecke's comet with that of a Geissler tube
containing olefiant gas, and found exact agreement. Nearly all comets
have shown the same spectrum.[1] A very few comets have given bright
band spectra differing from the normal type. Also a certain kind of
continuous spectrum, as well as reflected solar light showing
Frauenhofer lines, have been seen.

[Illustration: COPY OF THE DRAWING MADE BY PAUL FABRICIUS. To define
the path of comet 1556. After being lost for 300 years, this drawing
was recovered by the prolonged efforts of Mr. Hind and Professor
Littrow in 1856.]

When Wells's comet, in 1882, approached very close indeed to the sun,
the spectrum changed to a mono-chromatic yellow colour, due to sodium.

For a full account of the wonders of the cometary world the reader is
referred to books on descriptive astronomy, or to monographs on
comets.[2] Nor can the very uncertain speculations about the structure
of comets' tails be given here. A new explanation has been proposed
almost every time that a great discovery has been made in the theory
of light, heat, chemistry, or electricity.

Halley's comet remained the only one of which a prediction of the
return had been confirmed, until the orbit of the small, ill-defined
comet found by Pons in 1819 was computed by Encke, and found to have a
period of 3 1/3 years. It was predicted to return in 1822, and was
recognised by him as identical with many previous comets. This comet,
called after Encke, has showed in each of its returns an inexplicable
reduction of mean distance, which led to the assertion of a resisting
medium in space until a better explanation could be found.[3]

Since that date fourteen comets have been found with elliptic orbits,
whose aphelion distances are all about the same as Jupiter's mean
distance; and six have an aphelion distance about ten per cent,
greater than Neptune's mean distance. Other comets are similarly
associated with the planets Saturn and Uranus.

The physical transformations of comets are among the most wonderful of
unexplained phenomena in the heavens. But, for physical astronomers,
the greatest interest attaches to the reduction of radius vector of
Encke's comet, the splitting of Biela's comet into two comets in 1846,
and the somewhat similar behaviour of other comets. It must be noted,
however, that comets have a sensible size, that all their parts cannot
travel in exactly the same orbit under the sun's gravitation, and that
their mass is not sufficient to retain the parts together very
forcibly; also that the inevitable collision of particles, or else
fluid friction, is absorbing energy, and so reducing the comet's
velocity.

In 1770 Lexell discovered a comet which, as was afterwards proved by
investigations of Lexell, Burchardt, and Laplace, had in 1767 been
deflected by Jupiter out of an orbit in which it was invisible from
the earth into an orbit with a period of 5-1/2 years, enabling it to be
seen. In 1779 it again approached Jupiter closer than some of his
satellites, and was sent off in another orbit, never to be again
recognised.

But our interest in cometary orbits has been added to by the discovery
that, owing to the causes just cited, a comet, if it does not separate
into discrete parts like Biela's, must in time have its parts spread
out so as to cover a sensible part of the orbit, and that, when the
earth passes through such part of a comet's orbit, a meteor shower is
the result.

A magnificent meteor shower was seen in America on November 12th-13th,
1833, when the paths of the meteors all seemed to radiate from a point
in the constellation Leo. A similar display had been witnessed in
Mexico by Humboldt and Bonpland on November 12th, 1799. H. A. Newton
traced such records back to October 13th, A.D. 902. The orbital motion
of a cloud or stream of small particles was indicated. The period
favoured by H. A. Newton was 354-1/2 days; another suggestion was 375-1/2
days, and another 33-1/4 years. He noticed that the advance of the date
of the shower between 902 and 1833, at the rate of one day in seventy
years, meant a progression of the node of the orbit. Adams undertook
to calculate what the amount would be on all the five suppositions
that had been made about the period. After a laborious work, he found
that none gave one day in seventy years except the 33-1/4-year period,
which did so exactly. H. A. Newton predicted a return of the shower on
the night of November 13th-14th, 1866. He is now dead; but many of us
are alive to recall the wonder and enthusiasm with which we saw this
prediction being fulfilled by the grandest display of meteors ever
seen by anyone now alive.

The _progression_ of the nodes proved the path of the meteor
stream to be retrograde. The _radiant_ had almost the exact
longitude of the point towards which the earth was moving. This proved
that the meteor cluster was at perihelion. The period being known, the
eccentricity of the orbit was obtainable, also the orbital velocity of
the meteors in perihelion; and, by comparing this with the earth's
velocity, the latitude of the radiant enabled the inclination to be
determined, while the longitude of the earth that night was the
longitude of the node. In such a way Schiaparelli was able to find
first the elements of the orbit of the August meteor shower
(Perseids), and to show its identity with the orbit of Tuttle's comet
1862.iii. Then, in January 1867, Le Verrier gave the elements of the
November meteor shower (Leonids); and Peters, of Altona, identified
these with Oppolzer's elements for Tempel's comet 1866--Schiaparelli
having independently attained both of these results. Subsequently
Weiss, of Vienna, identified the meteor shower of April 20th (Lyrids)
with comet 1861. Finally, that indefatigable worker on meteors,
A. S. Herschel, added to the number, and in 1878 gave a list of
seventy-six coincidences between cometary and meteoric orbits.

Cometary astronomy is now largely indebted to photography, not merely
for accurate delineations of shape, but actually for the discovery of
most of them. The art has also been applied to the observation of
comets at distances from their perihelia so great as to prevent their
visual observation. Thus has Wolf, of Heidelburg, found upon old
plates the position of comet 1905.v., as a star of the 15.5 magnitude,
783 days before the date of its discovery. From the point of view of
the importance of finding out the divergence of a cometary orbit from
a parabola, its period, and its aphelion distance, this increase of
range attains the very highest value.

The present Astronomer Royal, appreciating this possibility, has been
searching by photography for Halley's comet since November, 1907,
although its perihelion passage will not take place until April, 1910.

FOOTNOTES:

[1] In 1874, when the writer was crossing the Pacific Ocean in
H.M.S. "Scout," Coggia's comet unexpectedly appeared, and (while
Colonel Tupman got its positions with the sextant) he tried to use the
prism out of a portable direct-vision spectroscope, without success
until it was put in front of the object-glass of a binocular, when, to
his great joy, the three band images were clearly seen.

[2] Such as _The World of Comets_, by A. Guillemin; _History of
Comets_, by G. R. Hind, London, 1859; _Theatrum Cometicum_, by S. de
Lubienietz, 1667; _Cometographie_, by Pingre, Paris, 1783; _Donati's
Comet_, by Bond.

[3] The investigations by Von Asten (of St. Petersburg) seem to
support, and later ones, especially those by Backlund (also of
St. Petersburg), seem to discredit, the idea of a resisting medium.

15. THE FIXED STARS AND NEBULAE.

Passing now from our solar system, which appears to be subject to the
action of the same forces as those we experience on our globe, there
remains an innumerable host of fixed stars, nebulas, and nebulous
clusters of stars. To these the attention of astronomers has been more
earnestly directed since telescopes have been so much enlarged.
Photography also has enabled a vast amount of work to be covered in a
comparatively short period, and the spectroscope has given them the
means, not only of studying the chemistry of the heavens, but also of
detecting any motion in the line of sight from less than a mile a
second and upwards in any star, however distant, provided it be bright
enough.

[Illustration: SIR WILLIAM HERSCHEL, F.R.S.--1738-1822. Painted by
Lemuel F. Abbott; National Portrait Gallery, Room XX.]

In the field of telescopic discovery beyond our solar system there is
no one who has enlarged our knowledge so much as Sir William Herschel,
to whom we owe the greatest discovery in dynamical astronomy among the
stars--viz., that the law of gravitation extends to the most distant
stars, and that many of them describe elliptic orbits about each
other. W. Herschel was born at Hanover in 1738, came to England in
1758 as a trained musician, and died in 1822. He studied science when
he could, and hired a telescope, until he learnt to make his own
specula and telescopes. He made 430 parabolic specula in twenty-one
years. He discovered 2,500 nebulae and 806 double stars, counted the
stars in 3,400 guage-fields, and compared the principal stars
photometrically.

Some of the things for which he is best known were results of those
accidents that happen only to the indefatigable enthusiast. Such was
the discovery of Uranus, which led to funds being provided for
constructing his 40-feet telescope, after which, in 1786, he settled
at Slough. In the same way, while trying to detect the annual parallax
of the stars, he failed in that quest, but discovered binary systems
of stars revolving in ellipses round each other; just as Bradley's
attack on stellar parallax failed, but led to the discovery of
aberration, nutation, and the true velocity of light.

_Parallax_.--The absence of stellar parallax was the great
objection to any theory of the earth's motion prior to Kepler's
time. It is true that Kepler's theory itself could have been
geometrically expressed equally well with the earth or any other point
fixed. But in Kepler's case the obviously implied physical theory of
the planetary motions, even before Newton explained the simplicity of
conception involved, made astronomers quite ready to waive the claim
for a rigid proof of the earth's motion by measurement of an annual
parallax of stars, which they had insisted on in respect of
Copernicus's revival of the idea of the earth's orbital motion.

Still, the desire to measure this parallax was only intensified by the
practical certainty of its existence, and by repeated failures. The
attempts of Bradley failed. The attempts of Piazzi and Brinkley,[1]
early in the nineteenth century, also failed. The first successes,
afterwards confirmed, were by Bessel and Henderson. Both used stars
whose proper motion had been found to be large, as this argued
proximity. Henderson, at the Cape of Good Hope, observed alpha
Centauri, whose annual proper motion he found to amount to 3".6, in
1832-3; and a few years later deduced its parallax 1".16. His
successor at the Cape, Maclear, reduced this to 0".92.

In 1835 Struve assigned a doubtful parallax of 0".261 to Vega (alpha
Lyrae). But Bessel's observations, between 1837 and 1840, of 61 Cygni,
a star with the large proper motion of over 5", established its annual
parallax to be 0".3483; and this was confirmed by Peters, who found
the value 0".349.

Later determinations for alpha2 Centauri, by Gill,[2] make its parallax
0".75--This is the nearest known fixed star; and its light takes 4 1/3
years to reach us. The light year is taken as the unit of measurement
in the starry heavens, as the earth's mean distance is "the
astronomical unit" for the solar system.[3] The proper motions and
parallaxes combined tell us the velocity of the motion of these stars
across the line of sight: alpha Centauri 14.4 miles a second=4.2
astronomical units a year; 61 Cygni 37.9 miles a second=11.2
astronomical units a year. These successes led to renewed zeal, and
now the distances of many stars are known more or less accurately.

Several of the brightest stars, which might be expected to be the
nearest, have not shown a parallax amounting to a twentieth of a
second of arc. Among these are Canopus, alpha Orionis, alpha Cygni, beta
Centauri, and gamma Cassiopeia. Oudemans has published a list of
parallaxes observed.[4]

_Proper Motion._--In 1718 Halley[5] detected the proper motions
of Arcturus and Sirius. In 1738 J. Cassinis[6] showed that the former
had moved five minutes of arc since Tycho Brahe fixed its position. In
1792 Piazzi noted the motion of 61 Cygni as given above. For a long
time the greatest observed proper motion was that of a small star 1830
Groombridge, nearly 7" a year; but others have since been found
reaching as much as 10".

Now the spectroscope enables the motion of stars to be detected at a

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