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