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).



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.



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.



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 single observation, but only that part of the motion that is in the line of sight. For a complete knowledge of a star's motion the proper motion and parallax must also be known.

When Huggins first applied the Doppler principle to measure velocities in the line of sight,[7] the faintness of star spectra diminished the accuracy; but Vogel, in 1888, overcame this to a great extent by long exposures of photographic plates.

It has often been noticed that stars which seem to belong to a group of nearly uniform magnitude have the same proper motion. The spectroscope has shown that these have also often the same velocity in the line of sight. Thus in the Great Bear, beta, gamma, delta, epsilon, zeta, all agree as to angular proper motion. delta was too faint for a spectroscopic measurement, but all the others have been shown to be approaching us at a rate of twelve to twenty miles a second. The same has been proved for proper motion, and line of sight motion, in the case of Pleiades and other groups.

Maskelyne measured many proper motions of stars, from which W. Herschel[8] came to the conclusion that these apparent motions are for the most part due to a motion of the solar system in space towards a point in the constellation Hercules, R.A. 257 degrees; N. Decl. 25 degrees. This grand discovery has been amply confirmed, and, though opinions differ as to the exact direction, it happens that the point first indicated by Herschel, from totally insufficient data, agrees well with modern estimates.

Comparing the proper motions and parallaxes to get the actual velocity of each star relative to our system, C.L. Struve found the probable velocity of the solar system in space to be fifteen miles a second, or five astronomical units a year.

The work of Herschel in this matter has been checked by comparing spectroscopic velocities in the line of sight which, so far as the sun's motion is concerned, would give a maximum rate of approach for stars near Hercules, a maximum rate of recession for stars in the opposite part of the heavens, and no effect for stars half-way between. In this way the spectroscope has confirmed generally Herschel's view of the direction, and makes the velocity eleven miles a second, or nearly four astronomical units a year.

The average proper motion of a first magnitude star has been found to be 0".25 annually, and of a sixth magnitude star 0".04. But that all bright stars are nearer than all small stars, or that they show greater proper motion for that reason, is found to be far from the truth. Many statistical studies have been made in this connection, and interesting results may be expected from this treatment in the hands of Kapteyn of Groningen, and others.[9]

On analysis of the directions of proper motions of stars in all parts of the heavens, Kapteyn has shown[10] that these indicate, besides the solar motion towards Hercules, two general drifts of stars in nearly opposite directions, which can be detected in any part of the heavens. This result has been confirmed from independent data by Eddington (R.A.S., M.N.) and Dyson (R.S.E. Proc.).

Photography promises to assist in the measurement of parallax and proper motions. Herr Pulfrich, of the firm of Carl Zeiss, has vastly extended the applications of stereoscopic vision to astronomy—a subject which De la Rue took up in the early days of photography. He has made a stereo-comparator of great beauty and convenience for comparing stereoscopically two star photographs taken at different dates. Wolf of Heidelberg has used this for many purposes. His investigations depending on the solar motion in space are remarkable. He photographs stars in a direction at right angles to the line of the sun's motion. He has taken photographs of the same region fourteen years apart, the two positions of his camera being at the two ends of a base-line over 5,000,000,000 miles apart, or fifty-six astronomical units. On examining these stereoscopically, some of the stars rise out of the general plane of the stars, and seem to be much nearer. Many of the stars are thus seen to be suspended in space at different distances corresponding exactly to their real distances from our solar system, except when their proper motion interferes. The effect is most striking; the accuracy of measurement exceeds that of any other method of measuring such displacements, and it seems that with a long interval of time the advantage of the method increases.

Double Stars.—The large class of double stars has always been much studied by amateurs, partly for their beauty and colour, and partly as a test for telescopic definition. Among the many unexplained stellar problems there is one noticed in double stars that is thought by some to be likely to throw light on stellar evolution. It is this: There are many instances where one star of the pair is comparatively faint, and the two stars are contrasted in colour; and in every single case the general colour of the faint companion is invariably to be classed with colours more near to the blue end of the spectrum than that of the principal star.

Binary Stars.—Sir William Herschel began his observations of double stars in the hope of discovering an annual parallax of the stars. In this he was following a suggestion of Galileo's. The presumption is that, if there be no physical connection between the stars of a pair, the largest is the nearest, and has the greatest parallax. So, by noting the distance between the pair at different times of the year, a delicate test of parallax is provided, unaffected by major instrumental errors.

Herschel did, indeed, discover changes of distance, but not of the character to indicate parallax. Following this by further observation, he found that the motions were not uniform nor rectilinear, and by a clear analysis of the movements he established the remarkable and wholly unexpected fact that in all these cases the motion is due to a revolution about their common centre of gravity.[11] He gave the approximate period of revolution of some of these: Castor, 342 years; delta Serpentis, 375 years; gamma Leonis, 1,200 years; epsilon Bootis, 1,681 years.

Twenty years later Sir John Herschel and Sir James South, after re-examination of these stars, confirmed[12] and extended the results, one pair of Coronae having in the interval completed more than a whole revolution.

It is, then, to Sir William Herschel that we owe the extension of the law of gravitation, beyond the limits of the solar system, to the whole universe. His observations were confirmed by F.G.W. Struve (born 1793, died 1864), who carried on the work at Dorpat. But it was first to Savary,[13] and later to Encke and Sir John Herschel, that we owe the computation of the elliptic elements of these stars; also the resulting identification of their law of force with Newton's force of gravitation applied to the solar system, and the force that makes an apple fall to the ground. As Grant well says in his History: "This may be justly asserted to be one of the most sublime truths which astronomical science has hitherto disclosed to the researches of the human mind."

Latterly the best work on double stars has been done by S. W. Burnham,[14] at the Lick Observatory. The shortest period he found was eleven years (kappa Pegasi). In the case of some of these binaries the parallax has been measured, from which it appears that in four of the surest cases the orbits are about the size of the orbit of Uranus, these being probably among the smallest stellar orbits.

The law of gravitation having been proved to extend to the stars, a discovery (like that of Neptune in its origin, though unlike it in the labour and originality involved in the calculation) that entrances the imagination became possible, and was realised by Bessel—the discovery of an unknown body by its gravitational disturbance on one that was visible. In 1834 and 1840 he began to suspect a want of uniformity in the proper motion of Sirius and Procyon respectively. In 1844, in a letter to Sir John Herschel,[15] he attributed these irregularities in each case to the attraction of an invisible companion, the period of revolution of Sirius being about half a century. Later he said: "I adhere to the conviction that Procyon and Sirius form real binary systems, consisting of a visible and an invisible star. There is no reason to suppose luminosity an essential quality of cosmical bodies. The visibility of countless stars is no argument against the invisibility of countless others." This grand conception led Peters to compute more accurately the orbit, and to assign the place of the invisible companion of Sirius. In 1862 Alvan G. Clark was testing a new 18-inch object-glass (now at Chicago) upon Sirius, and, knowing nothing of these predictions, actually found the companion in the very place assigned to it. In 1896 the companion of Procyon was discovered by Professor Schaeberle at the Lick Observatory.

Now, by the refined parallax determinations of Gill at the Cape, we know that of Sirius to be 0".38. From this it has been calculated that the mass of Sirius equals two of our suns, and its intrinsic brightness equals twenty suns; but the companion, having a mass equal to our sun, has only a five-hundredth part of the sun's brightness.

Spectroscopic Binaries.—On measuring the velocity of a star in the line of sight at frequent intervals, periodic variations have been found, leading to a belief in motion round an invisible companion. Vogel, in 1889, discovered this in the case of Spica (alpha Virginis), whose period is 4d. 0h. 19m., and the diameter of whose orbit is six million miles. Great numbers of binaries of this type have since then been discovered, all of short period.

Also, in 1889, Pickering found that at regular intervals of fifty-two days the lines in the spectrum of zeta of the Great Bear are duplicated, indicating a relative velocity, equal to one hundred miles a second, of two components revolving round each other, of which that apparently single star must be composed.

It would be interesting, no doubt, to follow in detail the accumulating knowledge about the distances, proper motions, and orbits of the stars; but this must be done elsewhere. Enough has been said to show how results are accumulating which must in time unfold to us the various stellar systems and their mutual relationships.

Variable Stars.—It has often happened in the history of different branches of physical science that observation and experiment were so far ahead of theory that hopeless confusion appeared to reign; and then one chance result has given a clue, and from that time all differences and difficulties in the previous researches have stood forth as natural consequences, explaining one another in a rational sequence. So we find parallax, proper motion, double stars, binary systems, variable stars, and new stars all bound together.

The logical and necessary explanation given of the cause of ordinary spectroscopic binaries, and of irregular proper motions of Sirius and Procyon, leads to the inference that if ever the plane of such a binary orbit were edge-on to us there ought to be an eclipse of the luminous partner whenever the non-luminous one is interposed between us. This should give rise either to intermittence in the star's light or else to variability. It was by supposing the existence of a dark companion to Algol that its discoverer, Goodricke of York,[16] in 1783, explained variable stars of this type. Algol (beta Persei) completes the period of variable brightness in 68.8 hours. It loses three-fifths of its light, and regains it in twelve hours. In 1889 Vogel,[17] with the Potsdam spectrograph, actually found that the luminous star is receding before each eclipse, and approaching us after each eclipse; thus entirely supporting Goodricke's opinion. There are many variables of the Algol type, and information is steadily accumulating. But all variable stars do not suffer the sudden variations of Algol. There are many types, and the explanations of others have not proved so easy.

The Harvard College photographs have disclosed the very great prevalence of variability, and this is certainly one of the lines in which modern discovery must progress.

Roberts, in South Africa, has done splendid work on the periods of variables of the Algol type.

New Stars.—Extreme instances of variable stars are the new stars such as those detected by Hipparchus, Tycho Brahe, and Kepler, of which many have been found in the last half-century. One of the latest great "Novae" was discovered in Auriga by a Scotsman, Dr. Anderson, on February 1st, 1892, and, with the modesty of his race, he communicated the fact to His Majesty's Astronomer for Scotland on an unsigned post-card.[18] Its spectrum was observed and photographed by Huggins and many others. It was full of bright lines of hydrogen, calcium, helium, and others not identified. The astounding fact was that lines were shown in pairs, bright and dark, on a faint continuous spectrum, indicating apparently that a dark body approaching us at the rate of 550 miles a second[19] was traversing a cold nebulous atmosphere, and was heated to incandescence by friction, like a meteor in our atmosphere, leaving a luminous train behind it. It almost disappeared, and on April 26th it was of the sixteenth magnitude; but on August 17th it brightened to the tenth, showing the principal nebular band in its spectrum, and no sign of approach or recession. It was as if it emerged from one part of the nebula, cooled down, and rushed through another part of the nebula, rendering the nebular gas more luminous than itself.[20]

Since 1892 one Nova after another has shown a spectrum as described above, like a meteor rushing towards us and leaving a train behind, for this seems to be the obvious meaning of the spectra.

The same may be said of the brilliant Nova Persei, brighter at its best than Capella, and discovered also by Dr. Anderson on February 22nd, 1901. It increased in brightness as it reached the densest part of the nebula, then it varied for some weeks by a couple of magnitudes, up and down, as if passing through separate nebular condensations. In February, 1902, it could still be seen with an opera-glass. As with the other Novae, when it first dashed into the nebula it was vaporised and gave a continuous spectrum with dark lines of hydrogen and helium. It showed no bright lines paired with the dark ones to indicate a train left behind; but in the end its own luminosity died out, and the nebular spectrum predominated.

The nebular illumination as seen in photographs, taken from August to November, seemed to spread out slowly in a gradually increasing circle at the rate of 90" in forty-eight days. Kapteyn put this down to the velocity of light, the original outburst sending its illumination to the nebulous gas and illuminating a spherical shell whose radius increased at the velocity of light. This supposition seems correct, in which case it can easily be shown from the above figures that the distance of this Nova was 300 light years.

Star Catalogues.—Since the days of very accurate observations numerous star-catalogues have been produced by individuals or by observatories. Bradley's monumental work may be said to head the list. Lacaille's, in the Southern hemisphere, was complementary. Then Piazzi, Lalande, Groombridge, and Bessel were followed by Argelander with his 324,000 stars, Rumker's Paramatta catalogue of the southern hemisphere, and the frequent catalogues of national observatories. Later the Astronomische Gesellschaft started their great catalogue, the combined work of many observatories. Other southern ones were Gould's at Cordova and Stone's at the Cape.

After this we have a new departure. Gill at the Cape, having the comet 1882.ii. all to himself in those latitudes, wished his friends in Europe to see it, and employed a local photographer to strap his camera to the observatory equatoreal, driven by clockwork, and adjusted on the comet by the eye. The result with half-an-hour's exposure was good, so he tried three hours. The result was such a display of sharp star images that he resolved on the Cape Photographic Durchmusterung, which after fourteen years, with Kapteyn's aid in reducing, was completed. Meanwhile the brothers Henry, of Paris, were engaged in going over Chacornac's zodiacal stars, and were about to catalogue the Milky Way portion, a serious labour, when they saw Gill's Comet photograph and conceived the idea of doing the rest of their work by photography. Gill had previously written to Admiral Mouchez, of the Paris Observatory, and explained to him his project for charting the heavens photographically, by combining the work of many observatories. This led Admiral Mouchez to support the brothers Henry in their scheme.[21] Gill, having got his own photographic work underway, suggested an international astrographic chart, the materials for different zones to be supplied by observatories of all nations, each equipped with similar photographic telescopes. At a conference in Paris, 1887, this was decided on, the stars on the charts going down to the fourteenth magnitude, and the catalogues to the eleventh.



This monumental work is nearing completion. The labour involved was immense, and the highest skill was required for devising instruments and methods to read off the star positions from the plates.

Then we have the Harvard College collection of photographic plates, always being automatically added to; and their annex at Arequipa in Peru.

Such catalogues vary in their degree of accuracy; and fundamental catalogues of standard stars have been compiled. These require extension, because the differential methods of the heliometer and the camera cannot otherwise be made absolute.

The number of stars down to the fourteenth magnitude may be taken at about 30,000,000; and that of all the stars visible in the greatest modern telescopes is probably about 100,000,000.

Nebulae and Star-clusters.—Our knowledge of nebulae really dates from the time of W. Herschel. In his great sweeps of the heavens with his giant telescopes he opened in this direction a new branch of astronomy. At one time he held that all nebulae might be clusters of innumerable minute stars at a great distance. Then he recognised the different classes of nebulae, and became convinced that there is a widely-diffused "shining fluid" in space, though many so-called nebulae could be resolved by large telescopes into stars. He considered that the Milky Way is a great star cluster, whose form may be conjectured from numerous star-gaugings. He supposed that the compact "planetary nebulae" might show a stage of evolution from the diffuse nebulae, and that his classifications actually indicate various stages of development. Such speculations, like those of the ancients about the solar system, are apt to be harmful to true progress of knowledge unless in the hands of the ablest mathematical physicists; and Herschel violated their principles in other directions. But here his speculations have attracted a great deal of attention, and, with modifications, are accepted, at least as a working hypothesis, by a fair number of people.

When Sir John Herschel had extended his father's researches into the Southern Hemisphere he was also led to the belief that some nebulae were a phosphorescent material spread through space like fog or mist.

Then his views were changed by the revelations due to the great discoveries of Lord Rosse with his gigantic refractor,[22] when one nebula after another was resolved into a cluster of minute stars. At that time the opinion gained ground that with increase of telescopic power this would prove to be the case with all nebulae.

In 1864 all doubt was dispelled by Huggins[23] in his first examination of the spectrum of a nebula, and the subsequent extension of this observation to other nebulae; thus providing a certain test which increase in the size of telescopes could never have given. In 1864 Huggins found that all true nebulae give a spectrum of bright lines. Three are due to hydrogen; two (discovered by Copeland) are helium lines; others are unknown. Fifty-five lines have been photographed in the spectrum of the Orion nebula. It seems to be pretty certain that all true nebulae are gaseous, and show almost exactly the same spectrum.

Other nebulae, and especially the white ones like that in Andromeda, which have not yet been resolved into stars, show a continuous spectrum; others are greenish and give no lines.

A great deal has to be done by the chemist before the astronomer can be on sure ground in drawing conclusions from certain portions of his spectroscopic evidence.

The light of the nebulas is remarkably actinic, so that photography has a specially fine field in revealing details imperceptible in the telescope. In 1885 the brothers Henry photographed, round the star Maia in the Pleiades, a spiral nebula 3' long, as bright on the plate as that star itself, but quite invisible in the telescope; and an exposure of four hours revealed other new nebula in the same district. That painstaking and most careful observer, Barnard, with 10-1/4 hours' exposure, extended this nebulosity for several degrees, and discovered to the north of the Pleiades a huge diffuse nebulosity, in a region almost destitute of stars. By establishing a 10-inch instrument at an altitude of 6,000 feet, Barnard has revealed the wide distribution of nebular matter in the constellation Scorpio over a space of 4 degrees or 5 degrees square. Barnard asserts that the "nebular hypothesis" would have been killed at its birth by a knowledge of these photographs. Later he has used still more powerful instruments, and extended his discoveries.

The association of stars with planetary nebulae, and the distribution of nebulae in the heavens, especially in relation to the Milky Way, are striking facts, which will certainly bear fruit when the time arrives for discarding vague speculations, and learning to read the true physical structure and history of the starry universe.

Stellar Spectra.—When the spectroscope was first available for stellar research, the leaders in this branch of astronomy were Huggins and Father Secchi,[24] of Rome. The former began by devoting years of work principally to the most accurate study of a few stars. The latter devoted the years from 1863 to 1867 to a general survey of the whole heavens, including 4,000 stars. He divided these into four principal classes, which have been of the greatest service. Half of his stars belonged to the first class, including Sirius, Vega, Regulus, Altair. The characteristic feature of their spectra is the strength and breadth of the hydrogen lines and the extreme faintness of the metallic lines. This class of star is white to the eye, and rich in ultra violet light.

The second class includes about three-eighths of his stars, including Capella, Pollux, and Arcturus. These stars give a spectrum like that of our sun, and appear yellowish to the eye.

The third class includes alpha Herculis, alpha Orionis (Betelgeux), Mira Ceti, and about 500 red and variable stars. The spectrum has fluted bands shaded from blue to red, and sharply defined at the more refrangible edge.

The fourth class is a small one, containing no stars over fifth magnitude, of which 152 Schjellerup, in Canes Venatici, is a good example. This spectrum also has bands, but these are shaded on the violet side and sharp on the red side. They are due to carbon in some form. These stars are ruby red in the telescope.

It would appear, then, that all stars are suns with continuous spectra, and the classes are differentiated by the character of the absorbent vapours of their atmospheres.

It is very likely that, after the chemists have taught us how to interpret all the varieties of spectrum, it will be possible to ascribe the different spectrum-classes to different stages in the life-history of every star. Already there are plenty of people ready to lay down arbitrary assumptions about the lessons to be drawn from stellar spectra. Some say that they know with certainty that each star begins by being a nebula, and is condensed and heated by condensation until it begins to shine as a star; that it attains a climax of temperature, then cools down, and eventually becomes extinct. They go so far as to declare that they know what class of spectrum belongs to each stage of a star's life, and how to distinguish between one that is increasing and another that is decreasing in temperature.

The more cautious astronomers believe that chemistry is not sufficiently advanced to justify all of these deductions; that, until chemists have settled the lately raised question of the transmutation of elements, no theory can be sure. It is also held that until they have explained, without room for doubt, the reasons for the presence of some lines, and the absence of others, of any element in a stellar spectrum; why the arc-spectrum of each element differs from its spark spectrum; what are all the various changes produced in the spectrum of a gas by all possible concomitant variations of pressure and temperature; also the meanings of all the flutings in the spectra of metalloids and compounds; and other equally pertinent matters—until that time arrives the part to be played by the astronomer is one of observation. By all means, they say, make use of "working hypotheses" to add an interest to years of laborious research, and to serve as a guide to the direction of further labours; but be sure not to fall into the error of calling any mere hypothesis a theory.

Nebular Hypothesis.—The Nebular Hypothesis, which was first, as it were, tentatively put forward by Laplace as a note in his Systeme du Monde, supposes the solar system to have been a flat, disk-shaped nebula at a high temperature in rapid rotation. In cooling it condensed, leaving revolving rings at different distances from the centre. These themselves were supposed to condense into the nucleus for a rotating planet, which might, in contracting, again throw off rings to form satellites. The speculation can be put in a really attractive form, but is in direct opposition to many of the actual facts; and so long as it is not favoured by those who wish to maintain the position of astronomy as the most exact of the sciences—exact in its facts, exact in its logic—this speculation must be recorded by the historian, only as he records the guesses of the ancient Greeks—as an interesting phase in the history of human thought.

Other hypotheses, having the same end in view, are the meteoritic hypothesis of Lockyer and the planetesimal hypothesis that has been largely developed in the United States. These can best be read in the original papers to various journals, references to which may be found in the footnotes of Miss Clerke's History of Astronomy during the Nineteenth Century. The same can be said of Bredichin's hypothesis of comets' tails, Arrhenius's book on the applications of the theory of light repulsion, the speculations on radium, the origin of the sun's heat and the age of the earth, the electron hypothesis of terrestrial magnetism, and a host of similar speculations, all combining to throw an interesting light on the evolution of a modern train of thought that seems to delight in conjecture, while rebelling against that strict mathematical logic which has crowned astronomy as the queen of the sciences.

FOOTNOTES:

[1] R. S. Phil Trans., 1810 and 1817-24.

[2] One of the most valuable contributions to our knowledge of stellar parallaxes is the result of Gill's work (Cape Results, vol. iii., part ii., 1900).

[3] Taking the velocity of light at 186,000 miles a second, and the earth's mean distance at 93,000,000 miles, 1 light year=5,865,696,000,000 miles or 63,072 astronomical units; 1 astronomical unit a year=2.94 miles a second; and the earth's orbital velocity=18.5 miles a second.

[4] Ast. Nacht., 1889.

[5] R. S. Phil. Trans., 1718.

[6] Mem. Acad. des Sciences, 1738, p. 337.

[7] R. S Phil. Trans., 1868.

[8] R.S. Phil Trans., 1783.

[9] See Kapteyn's address to the Royal Institution, 1908. Also Gill's presidential address to the British Association, 1907.

[10] Brit. Assoc. Rep., 1905.

[11] R. S. Phil. Trans., 1803, 1804.

[12] Ibid, 1824.

[13] Connaisance des Temps, 1830.

[14] R. A. S. Mem., vol. xlvii., p. 178; Ast. Nach., No. 3,142; Catalogue published by Lick Observatory, 1901.

[15] R. A. S., M. N., vol. vi.

[16] R. S. Phil. Trans., vol. lxxiii., p. 484.

[17] Astr. Nach., No. 2,947.

[18] R. S. E. Trans., vol. xxvii. In 1901 Dr. Anderson discovered Nova Persei.

[19] Astr. Nach., No. 3,079.

[20] For a different explanation see Sir W. Huggins's lecture, Royal Institution, May 13th, 1892.

[21] For the early history of the proposals for photographic cataloguing of stars, see the Cape Photographic Durchmusterung, 3 vols. (Ann. of the Cape Observatory, vols. in., iv., and v., Introduction.)

[22] R. S. Phil. Trans., 1850, p. 499 et seq.

[23] Ibid, vol. cliv., p. 437.

[24] Brit. Assoc. Rep., 1868, p. 165.



INDEX

Abul Wefa, 24 Acceleration of moon's mean motion, 60 Achromatic lens invented, 88 Adams, J. C., 61, 65, 68, 69, 70, 87, 118, 124 Airy, G. B., 13, 30, 37, 65, 69, 70, 80, 81, 114, 119 Albetegnius, 24 Alphonso, 24 Altazimuth, 81 Anaxagoras, 14, 16 Anaximander, 14 Anaximenes, 14 Anderson, T. D., 137, 138 Angstrom, A. J., 102 Antoniadi, 113 Apian, P., 63 Apollonius, 22, 23 Arago, 111 Argelander, F. W. A., 139 Aristarchus, 18, 29 Aristillus, 17, 19 Aristotle, 16, 30, 47 Arrhenius, 146 Arzachel, 24 Asshurbanapal, 12 Asteroids, discovery of, 67, 119 Astrology, ancient and modern, 1-7, 38

Backlund, 122 Bacon, R., 86 Bailly, 8, 65 Barnard, E. E., 115, 143 Beer and Madler, 107, 110, 111 Behaim, 74 Bessel, F.W., 65, 79, 128, 134, 139 Biela, 123 Binet, 65 Biot, 10 Bird, 79, 80 Bliss, 80 Bode, 66, 69 Bond, G. P., 99, 117, 122 Bouvard, A., 65, 68 Bradley, J., 79, 80, 81, 87, 127, 128, 139 Bredechin, 146 Bremiker, 71 Brewster, D., 52, 91, 112 Brinkley, 128 Bruno, G., 49 Burchardt, 65, 123 Burnham, S. W., 134

Callippus, 15, 16, 31 Carrington, R. C., 97, 99, 114 Cassini, G. D., 107, 114, 115, 116, 117, 118 Cassini, J., 109, 129 Chacornac, 139 Chaldaean astronomy, 11-13 Challis, J., 69, 70, 71, 72 Chance, 88 Charles, II., 50, 81 Chinese astronomy, 8-11 Christie, W. M. H. (Ast. Roy.), 64, 82, 125 Chueni, 9 Clairaut, A. C., 56, 63, 65 Clark, A. G., 89, 135 Clerke, Miss, 106, 146 Comets, 120 Common, A. A., 88 Cooke, 89 Copeland, R., 142 Copernicus, N., 14, 24-31, 37, 38, 41, 42, 49, 128 Cornu, 85 Cowell, P. H., 3, 5, 64, 83 Crawford, Earl of, 84 Cromellin, A. C., 5, 64

D'Alembert, 65 Damoiseau, 65 D'Arrest, H. L., 34 Dawes, W. R., 100, 111 Delambre, J. B. J., 8, 27, 51, 65, 68 De la Rue, W., 2, 94, 99, 100, 131 Delaunay, 65 Democritus, 16 Descartes, 51 De Sejour, 117 Deslandres, II., 101 Desvignolles, 9 De Zach, 67 Digges, L., 86 Dollond, J., 87, 90 Dominis, A. di., 86 Donati, 120 Doppler, 92, 129 Draper, 99 Dreyer, J. L. E., 29,77 Dunthorne, 60 Dyson, 131

Eclipses, total solar, 103 Ecphantes, 16 Eddington, 131 Ellipse, 41 Empedocles, 16 Encke, J. F., 119, 122, 123, 133 Epicycles, 22 Eratosthenes, 18 Euclid, 17 Eudoxus, 15, 31 Euler, L., 60, 61, 62, 65, 88, 119

Fabricius, D.,95, 120, 121 Feil and Mantois, 88 Fizeau, H. L., 85, 92, 99 Flamsteed, J., 50, 58, 68, 78, 79, 93 Fohi, 8 Forbes, J. D., 52, 91 Foucault, L., 85, 99 Frauenhofer, J., 88, 90, 91

Galilei, G., 38, 46-49, 77, 93, 94, 95, 96, 107, 113, 115, 116, 133 Galle, 71, 72 Gascoigne, W., 45, 77 Gauss, C. F., 65, 67 Gauthier, 98 Gautier, 89 Gilbert, 44 Gill, D., 84, 85, 128, 135, 139, 140 Goodricke, J., 136 Gould, B. A., 139 Grant, R., 27, 47, 51, 86, 134 Graham, 79 Greek astronomy, 8-11 Gregory, J. and D., 87 Grimaldi, 113 Groombridge, S., 139 Grubb, 88, 89 Guillemin, 122 Guinand, 88

Hale, G. E., 101 Hall, A., 112 Hall, C. M., 88 Halley, E., 19, 51, 58, 60, 61, 62, 63, 64, 79, 120, 122, 125, 129 Halley's comet, 62-64 Halm, 85 Hansen, P. A., 3, 65 Hansky, A. P., 100 Harding, C. L., 67 Heliometer, 83 Heller, 120 Helmholtz, H. L. F., 35 Henderson, T., 128 Henry, P. and P., 139, 140, 143 Heraclides, 16 Heraclitus, 14 Herodotus, 13 Herschel, W., 65, 68, 97, 107, 110, 114, 115, 116, 117, 118, 126, 127, 130, 131, 132, 141, 142 Herschel, J., 97, 111, 133, 134, 142 Herschel, A. S., 125 Hevelius, J., 178 Hind, J. R., 5, 64, 120, 121, 122 Hipparchus, 3, 18, 19, 20, 22, 23, 24, 26, 36, 55, 60, 74, 93, 137 Hooke, R., 51, 111, 114 Horrocks, J., 50, 56 Howlett, 100 Huggins, W., 92, 93, 99, 106, 120, 129, 137, 138, 142, 144 Humboldt and Bonpland, 124 Huyghens, C., 47, 77, 87, 110, 116, 117

Ivory, 65

Jansen, P. J. C., 105, 106 Jansen, Z., 86

Kaiser, F., 111 Kapteyn, J. C., 131, 138, 139 Keeler, 117 Kepler, J., 17, 23, 26, 29, 30, 36, 37, 38-46, 48, 49, 50, 52, 53, 63, 66, 77, 87, 93, 127, 137 Kepler's laws, 42 Kirchoff, G.R., 91 Kirsch, 9 Knobel, E.B., 12, 13 Ko-Show-King, 76

Lacaile, N.L., 139 Lagrange, J.L., 61, 62, 65, 119 La Hire, 114 Lalande, J.J.L., 60, 63, 65, 66, 72, 139 Lamont, J., 98 Langrenus, 107 Laplace, P.S. de, 50, 58, 61, 62, 65,66, 123, 146 Lassel, 72, 88, 117, 118 Law of universal gravitation, 53 Legendre, 65 Leonardo da Vinci, 46 Lewis, G.C., 17 Le Verrier, U.J.J., 65, 68, 70, 71,72, 110, 118, 125 Lexell, 66, 123 Light year, 128 Lipperhey, H., 86 Littrow, 121 Lockyer, J.N., 103, 105, 146 Logarithms invented, 50 Loewy, 2, 100 Long inequality of Jupiter and Saturn, 50, 62 Lowell, P., 111, 112, 118 Lubienietz, S. de, 122 Luther, M., 38 Lunar theory, 37, 50, 56, 64

Maclaurin, 65 Maclear, T., 128 Malvasia, 77 Martin, 9 Maxwell, J. Clerk, 117 Maskelyne, N., 80, 130 McLean, F., 89 Medici, Cosmo di, 48 Melancthon, 38 Melotte, 83, 116 Meteors, 123 Meton, 15 Meyer, 57, 65 Michaelson, 85 Miraldi, 110, 114 Molyneux, 87 Moon, physical observations, 107 Mouchez, 139 Moyriac de Mailla, 8

Napier, Lord, 50 Nasmyth and Carpenter, 108 Nebulae, 141, 146 Neison, E., 108 Neptune, discovery of, 68-72 Newall, 89 Newcomb, 85 Newton, H.A., 124 Newton, I., 5, 19, 43, 49, 51-60, 62, 64, 68, 77, 79, 87, 90, 93, 94, 114, 127, 133 Nicetas, 16, 25 Niesten, 115 Nunez, P., 35

Olbers, H.W.M., 67 Omar, 11, 24 Oppolzer, 13, 125 Oudemans, 129

Palitsch, G., 64 Parallax, solar, 85, 86 Parmenides, 14 Paul III., 30 Paul V., 48 Pemberton, 51 Peters, C.A.F., 125, 128, 135 Photography, 99 Piazzi, G., 67, 128, 129, 139 Picard, 54, 77, 114 Pickering, E.C., 118, 135 Pingre, 13, 122 Plana, 65 Planets and satellites, physical observations, 109-119 Plato, 17, 23, 26, 40 Poisson, 65 Pond, J., 80 Pons, 122 Porta, B., 86 Pound, 87, 114 Pontecoulant, 64 Precession of the equinoxes, 19-21, 55, 57 Proctor, R.A., 111 Pritchett, 115 Ptolemy, 11, 13, 21, 22, 23, 24, 93 Puiseux and Loewy, 108 Pulfrich, 131 Purbach, G., 24 Pythagoras, 14, 17, 25, 29

Ramsay, W., 106 Ransome and May, 81 Reflecting telescopes invented, 87 Regiomontanus (Muller), 24 Respighi, 82 Retrograde motion of planets, 22 Riccioli, 107 Roberts, 137 Romer, O.,78, 114 Rosse, Earl of, 88, 142 Rowland, H. A., 92, 102 Rudolph H.,37, 39 Rumker, C., 139

Sabine, E., 98 Savary, 133 Schaeberle, J. M., 135 Schiaparelli, G. V., 110, 111, 124, 125 Scheiner, C., 87, 95, 96 Schmidt, 108 Schott, 88 Schroter, J. H., 107, 110, 111, 124, 125 Schuster, 98 Schwabe, G. H., 97 Secchi, A., 93, 144 Short, 87 Simms, J., 81 Slipher, V. M., 119 Socrates, 17 Solon, 15 Souciet, 8 South, J., 133 Spectroscope, 89-92 Spectroheliograph, 101 Spoerer, G. F. W., 98 Spots on the sun, 84; periodicity of, 97 Stars, Parallax, 127; proper motion, 129; double, 132; binaries, 132, 135; new, 19, 36, 137; catalogues of, 19, 36, 139; spectra of, 143 Stewart, B., 2, 100 Stokes, G. G., 91 Stone, E. J., 139 Struve, C. L., 130 Struve, F. G. W,, 88, 115, 128, 133

Telescopes invented, 47, 86; large, 88 Temple, 115, 125 Thales, 13, 16 Theon, 60 Transit circle of Romer, 78 Timocharis, 17, 19 Titius, 66 Torricelli, 113 Troughton, E., 80 Tupman, G. L., 120 Tuttle, 125 Tycho Brahe, 23, 25, 30, 33-38, 39, 40, 44, 50, 75, 77, 93, 94, 129, 137

Ulugh Begh, 24 Uranus, discovery of, 65

Velocity of light, 86, 128; of earth in orbit, 128 Verbiest, 75 Vogel, H. C., 92, 129, 135, 136 Von Asten, 122

Walmsley, 65 Walterus, B., 24, 74 Weiss, E., 125 Wells, 122 Wesley, 104 Whewell, 112 Williams, 10 Wilson, A., 96, 100 Winnecke, 120 Witte, 86 Wollaston, 90 Wolf, M., 119, 125, 132 Wolf, R., 98 Wren, C., 51 Wyllie, A., 77

Yao, 9 Young, C. A., 103 Yu-Chi, 8

Zenith telescopes, 79, 82 Zollner, 92 Zucchi, 113

THE END