Modern Astronomy

MODERN ASTRONOMY The Law of Gravitation.—The accumulation of facts does not in itself constitute science. Empirical knowledge scarcely de serves the name. Vere scire est per causas scire. Francis Bacon's prescient dream, however, of a living astronomy by which the physical laws governing terrestrial relations should be extended to the highest heavens, had long to wait for realization. Kepler divined its possibility; but his thoughts, derailed (so to speak) by the false analogy of magnetism, brought him no farther than to the rough draft of the scheme of vortices expounded in detail by Rene Descartes in his Principia Pliilosopjiiae (1644). And this was a cul-de-sac. The only practicable road struck aside from it. The true foundations of a mechanical theory of the heavens were laid by Kepler's discoveries, and by Galileo's dynamical demonstrations; its construction was facilitated by the develop ment of mathematical methods. The invention of logarithms, the rise of analytical geometry, and the evolution of B. Cavalieri's "indivisibles" into the infinitesimal calculus, all accomplished dur ing the 17th century, immeasurably widened the scope of exact astronomy. Gradually, too, the nature of the problem awaiting solution came to be apprehended. Jeremiah Horrocks had some intuition, previously to 1639, that the motion of the moon was controlled by the earth's gravity, and disturbed by the action of the sun. Ismael Bouillaud (1605-1694) stated in 1645 the fact of planetary circulation under the sway of a sun-force decreasing as the inverse square of the distance ; and the inevitableness of this same "duplicate ratio" was separately perceived by Robert Hooke, Edmund Halley and Sir Christopher Wren before Newton's dis covery had yet been made public. But Newton was the only man of his generation who both recognized the law, and had power to demonstrate its validity. And this was only a beginning. His complete achievement had a twofold aspect. It consisted, first, in the identification, by strict numerical comparisons, of terrestrial gravity with the mutual attraction of the heavenly bodies; sec ondly, in the following out of its mechanical consequences through out the solar system. Gravitation was thus shown to be the sole influence governing the movements of planets and satellites; the figure of the rotating earth was successfully explained by its ac tion on the minuter particles of matter; tides and the precession of the equinoxes proved amenable to reasonings based on the same principle ; and it satisfactorily accounted as well for some of the chief lunar and planetary inequalities.

Euler, Clairault and d'Alembert.—Newton's investigations, however, were very far from being exhaustive. Colossal though his powers were, they had limits; and his work could not but re main unterminated, since it was by its nature interminable. Nor was it possible to provide it with what could properly be called a sequel. The synthetic method employed by him was too unwieldy for common use. Yet no other was just then at hand. Mathe matical analysis needed half a century of cultivation before it was fully available for the arduous tasks reserved for it. They were accordingly taken up anew by a band of continental in quirers, primarily by three men of untiring energy and vivid genius, Leonhard Euler, Alexis Clairault, and Jean le Ronal d'Alembert. The first of the outstanding gravitational problems with which they grappled was the unaccountably rapid advance of the lunar perigee. But the apparent anomaly disappeared under Euler's powerful treatment in 1749, and his result was shortly afterwards still further assured by Clairault. The subject of planetary perturbations was next attacked. Euler devised in 1753 a new method, that of the "variation of parameters," for their investigation, and applied it to unravel some of the earth's ir regularities in a memoir crowned by the French Academy in 17 56 ; while in 1757, Clairault estimated the masses of the moon and Venus by their respective disturbing effects upon terrestrial move ments. But the most striking incident in the history of the verifi cation of Newton's law was the return of Halley's comet to peri helion, on the 12th of March 1759, in approximate accordance with Clairault's calculation of the delays due to the action of Jupiter and Saturn. Visual proof was thus, it might be said, af forded of the harmonious working of a single principle to the uttermost boundaries of the sun's dominion.

Lagrange and Laplace.—These successes paved the way for the higher triumphs of Joseph Louis Lagrange and of Pierre Simon Laplace. The subject of the lunar librations was treated by Lagrange with great originality in an essay crowned by the Paris Academy of Sciences in 1764; and he filled up the lacunae in his theory of them in a memoir communicated to the Berlin Academy in 1780. He again won the prize of the Paris Academy in 1766 with an analytical discussion of the movements of Jupi ter's satellites (Miscellanea, Turin Acad. t. iv.) ; and in the same year expanded Euler's adumbrated method of the variation of parameters into a highly effective engine of perturbational re search. It was especially adapted to the tracing out of "secular inequalities," or those depending upon changes in the orbital ele ments of the bodies affected by them, and hence progressing in definitely with time; and by its means, accordingly, the mechan ical stability of the solar system was splendidly demonstrated through the successive efforts of Lagrange and Laplace. The proper share of each in bringing about this memorable result is not easy to apportion, since they freely imparted and profited by one another's advances and improvements; it need only be said that the fundamental proposition of the invariability of the plan etary major axes laid down with restrictions by Laplace in 1773, was finally established by Lagrange in 1776; while Laplace in 1784 proved the subsistence of such a relation between the eccen tricities of the planetary orbits on the one hand, and their inclina tions on the other, that an increase of either element could, in any single case, proceed only to a very small extent. The system was thus shown, apart from unknown agencies of subversion, to be constructed for indefinite permanence. The prize of the Berlin Academy was, in 1780, adjudged to Lagrange for a treatise on the perturbations of comets; and he contributed to the Berlin Mem oirs, 1781-1784, a set of five elaborate papers, embodying and unifying his perfected methods and their results.

The crowning trophies of gravitational astronomy in the 18th century were Laplace's explanations of the "great inequality" of Jupiter and Saturn in 1784, and of the "secular acceleration" of the moon in 1787. Both irregularities had been noted, a century earlier, by Edmund Halley; both had, since that time, vainly exercised the ingenuity of the ablest mathematicians; both now almost simultaneously yielded their secret to the same fortunate inquirer. Johann Heinrich Lambert pointed out in 1773 that the motion of Saturn, from being retarded, had become accelerated. A periodic character was thus indicated for the disturbance; and Laplace assigned its true cause in the near approach to commen surability in the periods of the two planets, the cycle of disturb ance completing itself in about goo (more accurately 9292) years. The lunar acceleration, too, obtains ultimate compensation, though only after a vastly protracted term of years. The dis covery, just one hundred years after the publication of Newton's Principia, of its dependence upon the slowly varying eccentricity of the earth's orbit signalized the removal of the last conspicu ous obstacle to admitting the unqualified validity of the law of gravitation. Laplace's calculations, it is true, were inexact. An error, corrected by J. C. Adams in 1853, nearly doubled the value of the acceleration deducible from them ; and served to conceal a discrepancy with observation which has since given occasion to much profound research (see MooN) .

After Laplace.—The Mecanique celeste, in which Laplace welded into a whole the items of knowledge accumulated by the labours of a century, has been termed the "Almagest of the 18th century" (Fourier). But imposing and complete though the mon ument appeared, it did not long hold possession of the field. Further developments ensued. The "method of least squares," by which the most probable result can be educed from a body of observational data, was published by Adrien Marie Legendre in 1806, by Carl Friedrich Gauss in his Theoria Motus (1809), which described also a mode of calculating the orbit of a planet from three complete observations, afterwards turned to important account for the recapture of Ceres, the first discovered asteroid (see MINOR PLANETS). Researches into rotational movement were facilitated by S. D. Poisson's application to them in 1809 of Lagrange's theory of the variation of constants; Philippe de Pontecoulant successfully used in 1829, for the prediction of the impending return of Halley's comet, a system of "mechanical quadratures" published by Lagrange in the Berlin Memoirs for 1778; and in his Theorie analytique du systeme du monde (1846) he modified and refined general theories of the lunar and planetary revolutions. P. A. Hansen in 1829 (Astr. Nach. Nos. 166-168, 179) left the beaten track by choosing time as the sole variable, the orbital elements remaining constant. A. L. Cauchy published in 1842-1845 a method similarly conceived, though otherwise developed ; and the scope of analysis in determining the move ments of the heavenly bodies has since been perseveringly widened by the labours of Urbain J. J. Leverrier, J. C. Adams, S. New comb, G. W. Hill, E. W. Brown, H. Gylden, Charles Delaunay, F. Tisserand, H. Poincare and others too numerous to mention. Nor were these abstract investigations unaccompanied by con crete results. Sir George Airy detected in 1831 an inequality, periodic in 240 years, between Venus and the earth. Leverrier undertook in 1839, and concluded in 1876, the formidable task of revising all the planetary theories and constructing from them improved tables. Not less comprehensive has been the work car ried out by Professor Newcomb of raising to a higher grade of perfection, and reducing to a uniform standard, all the theories and constants of the solar system. The discovery of Neptune in 1846 by Adams and Leverrier marked the first solution of the "inverse problem" of perturbations. That is to say, ascertained or ascertainable effects were made the starting-point instead of the goal of research.

Practical Astronomy.— Observational astronomy, meanwhile, was advancing to some extent independently. The descriptive branch found its principle of development in the growing powers of the telescope, and had little to do with mathematical theory; which, on the contrary, was closely allied, by relations of mutual helpfulness, with practical astronomy. Meanwhile, the elemen tary requirement of making visual acquaintance with the stellar heavens was met, as regards the unknown southern skies, when Johann Bayer published at Nuremberg in 1603 a celestial atlas depicting twelve new constellations formedfrom the rude obser vations of navigators across the line. In the same work, the cur rent mode of star-nomenclature by the letters of the Greek alpha bet made its appearance. On the 7th of November 1631 Pierre Gassendi watched at Paris the passage of Mercury across the sun. This was the first planetary transit observed. The next was that of Venus on the 24th of November (0.S.) 1639, of which Jere miah Horrocks and William Crabtree were the sole spectators. The improvement of telescopes was prosecuted by Christiaan Huygens from 1655, and promptly led to his discoveries of the sixth Saturnian moon, of the true shape of the Saturnian ap pendages, and of the multiple character of the "trapezium" of stars in the Orion nebula. William Gascoigne's invention of the filar micrometer and of the adaptation of telescopes to graduated instruments remained submerged for a quarter of a century in consequence of his untimely death at Marston Moor (1644). The latter combination had also been ineffectually proposed in by Jean Baptiste Morin (1583-1656) ; and both devices were re contrived at Paris about 1667, the micrometer by Adrien Auzout (d. 1691), telescopic sights (so-called) by Jean Picard (162o 1682), who simultaneously introduced the astronomical use of pendulum-clocks, constructed by Huygens eleven years previously. These improvements were ignored or rejected by Johann Hevelius of Danzig, the author of the last important star-catalogue based solely upon naked-eye determinations. He, nevertheless, used tele scopes to good purpose in his studies of lunar topography, and his designations for the chief mountain-chains and "seas" of the moon have never been superseded. He, moreover, threw out the suggestion (in his Cometographia, i668) that comets move round the sun in orbits of a parabolic form.

Paris Observatory.

The establishment, in 1671 and 1676 respectively, of the French and English national observatories at once typified and stimulated progress. The Paris institution, it is true, lacked unity of direction. No authoritative chief was assigned to it until 1771. G. D. Cassini, his son and his grand son were only primi inter pares. Claude Perrault's stately edifice was equally accessible to all the more eminent members of the Academy of Sciences; and researches were, more or less inde pendently, carried on there by (among others) Philippe de la Hire (164o-1718), G. F. Maraldi (1665-1729), and his nephew, J. D. Maraldi, Jean Picard, Huygens, Olaus Romer and Nicolas de Lacaille. Some of the best instruments then extant were mounted at the Paris observatory. G. D. Cassini brought from Rome a i7-ft. telescope by G. Campani, with which he discovered in 1671 Iapetus, the eighth in distance of Saturn's family of satellites; Rhea was detected in 1672 with a glass by the same maker of 34-ft. focus; the duplicity of the ring showed in 1675; and in 1684, two additional satellites were disclosed by a Campani tele scope of ioo ft. Cassini, moreover, set up an altazimuth in 1678, and employed from about 1682 a "parallactic machine," pro vided with clockwork to enable it to follow the diurnal motion. Both inventions have been ascribed to Olaus Romer, who used but did not claim them, and must have become familiar with their principles during the nine years (1672-1681) spent by him at the Paris observatory. Romer, on the other hand, deserves full credit for originating the transit-circle and the prime vertical instrument; and he earned undying fame by his discovery of the finite velocity of light, made at Paris in 1675 by comparing his observations of the eclipses of Jupiter's satellites at the conjunctions and oppo sitions of the planet.

Work at Greenwich.

The organization of the Greenwich ob servatory differed widely from that adopted at Paris. There a fundamental scheme of practical amelioration was initiated by John Flamsteed, the first astronomer royal, and has never since been lost sight of. Its purpose is the attainment of so complete a power of prediction that the places of the sun, moon and planets may be assigned without noticeable error for an indefinite future time. Sidereal inquiries, as such, made no part of the original programme in which the stars figured merely as points of ref erence. But these points are not stationary. They have an ap parent precessional movement, the exact amount of which can be arrived at only by prolonged and toilsome enquiries. They have besides "proper motions," detected in 1718 by E. Halley in a few cases, and since found to prevail universally. Further, James Bradley discovered in 1728 the annual shifting of the stars due to the aberration of light (see ABERRATION), and in 1748, the complicating effects upon precession of the "nutation" of the earth's axis. Hence, the preparation of a catalogue recording the "mean" positions of a number of stars for a given epoch involves considerable preliminary labour; nor do those positions long con tinue to satisfy observation. They need, after a time, to be cor rected, not only systematically for precession, but also empirically for proper motion. Before the stars can safely be employed as route-marks in the sky, their movements must accordingly be tab ulated, and research into the method of such movements inevit ably follows. We perceive then that the fundamental problems of sidereal science are closely linked up with the elementary and indispensable procedures of celestial measurement.

The history of the Greenwich observatory is one of strenuous efforts for refinement, stimulated by the growing stringency of theoretical necessities. Improved practice, again, reacted upon theory by bringing to notice residual errors, demanding the cor rection of formulae, or intimating neglected disturbances. Each increase of mechanical skill claims a corresponding gain in the subtlety of analysis; and vice versa. And this kind of interaction has gone on ever since Flamsteed reluctantly furnished the "places of the moon," which enabled Newton to lay the foundations of lunar theory.

Edmund Halley, the second astronomer royal, devoted most of his official attention to the moon. Lut his plan of attack was not happily chosen; he carried it out with deficient instrumental means; and his administration (172o–1742) remained compara tively barren. That of his successor, though shorter, was vastly more productive. James Bradley chose the most appropriate tasks, and executed them supremely well, with the indispensable aid of John Bird (17o9-1776), who constructed for him an 8-ft. quadrant of unsurpassed quality. Bradley's store of observations has accordingly proved invaluable. Those of 3,222 stars, reduced by F. W. Bessel in 1818, and again with masterly insight by Dr. A. Auwers in 1882, form the true basis of exact astronomy, and of our knowledge of proper motions. Those relating to the moon and planets, corrected by Sir George Airy, 1840-1846, form part of the standard materials for discussing theories of move ment in the solar system. The fourth astronomer royal, Na thaniel Bliss, provided in two years a sequel of some value to Bradley's performance. Nevil Maskelyne, who succeeded him in 1764, set on foot, in 1767, the publication of the Nautical Alma nac, and about the same time had an achromatic telescope fitted to the Greenwich mural quadrant. The invention, perfected by John Dollond in 1757, was long debarred from becoming effective by difficulties in the manufacture of glass, aggravated in England by a heavy excise duty levied until 1845. More immediately efficacious was the innovation made by John Pond (astronomer royal, 1811-1836) of substituting entire circles for quadrants. He further introduced in 1821, the method of duplicate obser vations by direct vision and by reflection, and by these means obtained results of very high precision. During Sir George Airy's long term of office (1836-1881) exact astronomy and the tradi tional purposes of the royal observatory were promoted with in creased vigour, while the scope of research was at the same time memorably widened.

Advances Elsewhere.

Meanwhile, advances were being made in various parts of the continent of Europe. Peter Wargentin (1717-1783), secretary to the Swedish Academy of Sciences, made a special study of the Jovian system. James Bradley had described to the Royal Society on July 2, 1719, the curious cycli cal relations of the three inner satellites; and their period of 437 days was independently discovered by Wargentin, who based upon it in 1746 a set of tables, superseded only by those of J. B. J. Delambre in 1792. Among the fruits of the strenuous career of Nicolas Louis de Lacaille were tables of the sun, in which terms depending upon planetary perturbations were, for the first time, introduced (1758) ; an extended acquaintance with the southern heavens; and a determination of the moon's parallax from observa tions made at opposite extremities of an arc of the meridian 85° in length. Tobias Mayer of Gottingen (1723-1762) originated the mode of adjusting transit-instruments still in vogue ; drew up a catalogue of nearly a thousand zodiacal stars (published post humously in ; and deduced the proper motions of eighty stars from a comparison of their places as given by Olaus Romer in 1706 with those obtained by himself in 1756. He executed besides a chart and forty drawings of the moon (published at Gottingen in 1881), and calculated lunar tables from a skilful de velopment of Euler's theory, for which a reward of £3,000 was in 1765 paid to his widow by the British government. They were published by the Board of Longitude, together with his solar tables, in I 7 70. The material interests of navigation were in these works primarily regarded ; but the imaginative side of knowledge had also potent representatives during the latter half of the 18th century. In France, especially, the versatile activity of J. J. Lalande popularized the acquisitions of astronomy, and enforced its demands; and he had a German counterpart in J. E. Bode.

Between the time of Aristarchus and the opposition of Mars in 1672, no serious attempt was made to solve the problem of the sun's distance. In that year, however, Jean Richer at Cayenne and G. D. Cassini at Paris made combined observations of the planet, which yielded a parallax for the sun of 9.5", correspond ing to a mean radius for the terrestrial orbit of 87,000,000m. This result, though widely inaccurate, came much nearer to the truth than any previously obtained; and it instructively illustrated the feasibility of concerted astronomical operations at distant parts of the earth. The way was thus prepared for availing to the full of the opportunities for a celestial survey offered by the transits of Venus in 1761 and 1769. They had been signalized by E. Halley in 1716; they were later insisted upon by Lalande ; an enthusiasm for co-operation was evoked, and the globe, from Siberia to Otaheite, was studded with observing parties. The out come, nevertheless,. disappointed expectation. The instants of contact between the limbs of the sun and planet defied precise determination. Optical complications fatally impeded sharpness of vision, and the phenomena took place in a debatable border land of uncertainty. J. F. Encke, it is true, derived from them in 1822-1824 what seemed an authentic parallax of 8.57", imply ing a distance of 95,370,000 m.; but the confidence it inspired was finally overthrown in 1854 by P. A. Hansen's announcement of its incompatibility with lunar theory. An appeal then lay to the 19th century pair of transits in 1874 and 1882; but no peremp tory decision ensued ; observations were marred by the same optical evils as before. Their upshot, however, had lost its essen tial importance ; for a fresh series of investigations based on a variety of principles had already been started. Leverrier, in 1858, calculated a value of 8.95" for the solar parallax (equivalent to a distance of 91,000,000m.) from the "parallactic inequality" of the moon ; Professor Newcomb, using other forms of the gravi tational method, derived in 1895 a parallax of 8.76". For more recent researches on this problem see PARALLAX.

Improvements in Telescopes.

The first specimen of a re flecting telescope was constructed by Isaac Newton in 1668. It was of what is still called "Newtonian" design, and had a specu lum 2in. in diameter. Through the skill of John Hadley (1682-1743) and James Short of Edinburgh (1710-1768) the in strument unfolded, in the ensuing century, some of its capabili ties, which the labours of William Herschel enormously en hanced. Between 1774 and 1789 he built scores of specula of continually augmented size, up to a diameter of 4f t., the optical excellence of which approved itself by a crowd of discoveries. Uranus (q.v.) was recognized by its disc on March 13, 1781; two of its satellites, Oberon and Titania, disclosed themselves on Jan. I I, 1787; while with the giant 48-in. mirror, used on the "front view" plan, Mimas and Enceladus, the innermost Saturnian moons, were brought to view on Aug. 28 and Sept. 17, 1789. These were incidental trophies; Herschel's main object was the exploration of the sidereal heavens. The task, though novel and formidable, was executed with almost incredible success. Messier (173o-1817) catalogued 103 nebulae and star clusters; Her schel discovered 2,500, laid down the lines of their classification, divined the laws of their distribution, and assigned their place in a scheme of development. The proof supplied by him in 1802 that double stars are mutually revolving threw open a boundless field of research ; and he originated experimental inquiries into the construction of the heavens by systematically collecting and sifting stellar statistics. He, moreover, definitely established, in 1783, the fact and general direction of the sun's movement in space, and thus introduced an element of order into the maze of stellar proper motions. Sir John Herschel continued in the north ern, and extended to the southern hemisphere, his father's work. The third earl of Rosse mounted, at Parsonstown in 1845, a speculum 6ft. in diameter, which afforded the first indications of the spiral structure shown in recent photographs to be a very prev alent characteristic of many nebulae. Down to near the close of the 19th century, both the use and the improvement of reflectors were left mainly in British hands; but the gift of the "Crossley" instrument in 1895, to the Lick observatory, and its splendid sub sequent performances in nebular photography, brought similar tools of research into extensive use among American astronomers; and they are now, for many of the various purposes of Astro physics, strongly preferred to refractors. At present the largest instruments are the ioo-in. reflector at Mount Wilson, California, and the 72-in. reflector at Victoria, British Columbia.

Sidereal Astronomy.

The progress of science during the century had no more distinctive feature than the rapid growth of sidereal astronomy (see STAR). Its scope, wide as the universe, can be compassed no otherwise than by statistical means, and the collection of materials for this purpose involves most arduous preliminary labour. The multitudinous enrollment of stars was the first requisite. Only one "catalogue of precision"—Nevil Maskelyne's of 36 fundamental stars—was available in I800. J. J. Lalande, however, published in 1801, in his Histoire celeste, the approximate places of 47,390 stars. A valuable catalogue of about 7,600 stars was issued by Giuseppe Piazzi in 1814; Stephen Groombridge determined 4,239 at Blackheath in 18o6-16; while through the joint and successive work of F. W. Bessel and W. A. Argelander, 324,00o stars were recorded in the Bonn Durchmus terung (1859-62). The southern hemisphere was subsequently reviewed on a similar duplicate plan by E. Schonfeld (1828-1891) at Bonn, by B. A. Gould and J. M. Thome at Cordoba. Moreover, the imposing catalogue set pn foot in 1865 at thirteen observa tories by the Astronomische Gesellschaft was duly completed; and adjuncts to it have, from time to time, been provided in the publications of the royal observatories at Greenwich and the Cape of Good Hope, and of national, imperial and private establish ments in the United States and on the continent of Europe. But in the execution of these protracted undertakings, the human eye has been, to a large and increasing extent, supplemented by the camera. Photographic star-charting was begun by Sir David Gill in 1885, and the third and concluding volume of the Cape Photo graphic Durchmusterung appeared in 1900. It gives the co-or dinates of above 450,000 stars, measured by Professor J. C. Kap teyn at Groningen on plates taken by C. Ray Woods at the Cape observatory. And this comprehensive work was merely prepara tory to the International Catalogue and Chart, the production of which was initiated by the resolutions of the Paris Photographic Congress of 1887. Eighteen observatories scattered north and south of the equator divided the sky among them ; and the out come of their combined operations aimed at the production of a catalogue of at least 2,000,000 strictly determined stars, together with a colossal map in 22,000 sheets, showing stars to the four teenth magnitude, in numbers difficult to estimate. (See PHO TOGRAPHY : CELESTIAL.) The investigation of double stars was carried on from 1819 to 185o with singular persistence and ability at Dorpat and Pulkowa by F. G. W. Struve, and by his son and successor, O. W. Struve. The high excellence of the data collected by them was a combined result of their skill, and of the vast improvement in refracting telescopes due to the genius of Joseph Fraunhofer (1787-1826). Among the inheritors of his renown were Alvan Clark and Alvan G. Clark of Cambridgeport, Massachusetts ; and the superb defini tion of their great achromatics rendered practicable the division of what might have been deemed impossibly close star-pairs. These facilities were remarkably illustrated by Professor S. W. Burnham's record of discovery, which roused fresh enthusiasm for this line of inquiry by compelling recognition of the extraor dinary profusion throughout the heavens of compound objects. Discoveries with the spectroscope have ratified and extended this conclusion.

Stellar Proper Motions.

Only spurious star-parallaxes had claimed the attention of astronomers until F. W. Bessel announced, in December, 1838, the perspective yearly shifting of 61 Cygni in an ellipse with a mean radius of about one-third of a second. Thomas Henderson (1798-1844) had indeed measured the larger displacements of a Centauri at the Cape in 1832-33, but delayed until 1839 to publish his result.

The exhaustive ascertainment of stellar parallaxes, combined with the visible facts of stellar distribution, would enable us to build a perfect plan of the universe in three dimensions. Its per fection would, nevertheless, be undermined by the mobility of all its constituent parts. Their configuration at a given instant sup plies no information as to their configuration hereafter unless the mode and laws of their movements have been determined. Hence, one of the leading inducements to the construction of exact and comprehensive catalogues has been to elicit, by comparisons of those for widely separated epochs, the proper motions of the stars enumerated in them. Little was known on the subject at the beginning of the 19th century. William Herschel founded his determination in 1783 of the sun's route in space upon the move ments of thirteen stars; and he took into account those of only six in his second solution of the problem in 1805. But in 1837 Argelander employed 390 proper motions as materials for the treatment of the same subject ; and L. Struve had at his disposal, in 1887, no fewer than 2,80o.

Spectroscopy.

A beam of sunlight admitted into a darkened room through a narrow aperture; and there dispersed into a vario tinted band by the interposition of a prism, is not absolutely con tinuous. Dr. W. H. Wollaston made the experiment in 1802, and perceived the spaces of colour to be interrupted by seven obscure gaps, which took the shape of lines owing to his use of a rec tangular slit. He thus caught a preliminary glimpse of the "Fraun hofer lines," so called because Joseph Fraunhofer brought them into prominent notice by the diligence and insight of his labours upon them in 1814-15. He mapped 324, chose out nine, which he designated by the letters of the alphabet, to be standards of measurement for the rest, and ascertained the coincidence in po sition between the double yellow ray derived from the flame of burning sodium and the pair of dark lines named by him "D" in the solar spectrum. There ensued 45 years of groping for a law which should clear up the enigma of the solar reversals. Partial anticipations abounded. The vital heart of the matter was barely missed by W. A. Miller in 1845, by L. Foucault in 1849, by A. J. Angstrom in 1853, by Balfour Stewart in 1858; while Sir George Stokes held the solution of the problem in the hollow of his hand from 1852 onward. But it was the synthetic genius of Gustav Kirchhoff which first gave unity to the scattered phenomena, and finally reconciled what was elicited in the laboratory with what was observed in the sun. On Dec. 15, 1859, he communicated to the Berlin Academy of Sciences the principle which bears his name. Its purport is that glowing vapours similarly circumstanced absorb the identical radiations which they emit. That is to say, they stop out just those sections of white light transmitted through them which form their own special luminous badges. Moreover, if the white light come from a source at a higher tem perature than theirs, the sections, or lines, absorbed by them show dark against a continuous background. And this is precisely the case with the sun. Kirchhoff's principle, accordingly, not only afforded a simple explanation of the Fraunhofer lines, but availed to found a far-reaching science of celestial chemistry. Thousands of the dark lines in the solar spectrum agree absolutely in wave length with the bright rays artificially obtained from known sub stances, and appertaining to them individually. These substances must then exist near the sun. They are in fact suspended in a state of vapour between our eyes and the photosphere, the dazzling prismatic radiance of which they, to a minute extent, intercept, thus writing their signatures on the coloured scroll of dispersed sunshine. Research has been powerfully aided by the photo graphic camera and by the concave gratings invented by H. A. Rowland (1848-19o1) in 1882.

Solar Research.

Solar physics has profited enormously by the abolition of glare during total eclipses. That of July 8, 1842, was the first to be efficiently observed; and the luminous appen dages to the sun disclosed by it were such as to excite startled attention. Their investigation has since been diligently prose cuted. The corona was photographed at Konigsberg during the totality of July 28, 1851 ; similar records of the red prominences, successively obtained by Father Angelo Secchi and Warren de la Rue, as the shadow-track crossed Spain on July 18, 186o, finally demonstrated their solar status. The Indian eclipse of Aug. 18, 1868, supplied knowledge of their spectrum, found to include the yellow ray of an exotic gas named by Sir Norman Lockyer "helium." It further suggested, to Lockyer and P. Janssen sepa rately, the spectroscopic method of observing these objects in day light. Under cover of an eclipse visible in North America on Aug. 7, 1869, the bright green line of the corona was discerned; and Professor C. A. Young caught the "flash spectrum" of the reversing layer, at the moment of second contact, at Xerez de la Frontera in Spain, on Dec. 22, 187o. This significant but eva nescent phenomenon, which represents the direct emissions of a low-lying solar envelope, was photographed by William Shackle ton- on the occasion of an eclipse in Novaya Zemlya on Aug. 9, 1896; and it has been abundantly registered by exposures made during subsequent eclipses.

The photography of prominences in full sunlight was, after some preliminary trials by C. A. Young and others, fully realized in 1891 by Professor George E. Hale at Chicago, and independ ently by Henri Deslandres at Paris. The pictures were taken, in both cases, with only one quality of light, the violet ray of cal cium, the remaining superfluous beams being eliminated by the agency of a double slit. The last-named expedient had been de scribed by Janssen in 1867. Hale devised on the same principle the spectroheliograph (q.v.) an instrument by which the sun's disc can be photographed in calcium-light by imparting a rapid movement to its image relatively to the sensitive plate ; and the method has proved in many ways fruitful.

Stellar Spectroscopy.

The likeness of the sun to the stars has been shown by the spectroscope to be profound and inherent. Yet the general agreement of solar and stellar chemistry does not exclude important diversities of detail. Fraunhofer was the pio neer in this branch. He observed, in 1823, dark lines in stellar spectra which Kirchhoff's discovery supplied the means of inter preting. The task, attempted by G. B. Donati in 186o, was effec tively taken in hand, two years later, by Angelo Secchi, William Huggins and Lewis M. Rutherfurd. There ensued a general classi fication of the stars by Secchi into four leading types, distin guished by diversities of spectral pattern; and the recognition by Huggins of a considerable number of terrestrial elements as pres ent in stellar atmospheres. Nebular chemistry was initiated by the same investigator when, on Aug. 29, 1864, he observed the bright-line spectrum of a planetary nebula in Draco. About sev enty analogous objects, including that in the Sword of Orion, were found by him to give light of the same quality ; and thus after seventy-three years, verification was brought to William Her schel's hypothesis of a "shining fluid" diffused through space, the possible raw material of stars. In 1874, Dr. H. C. Vogel pub lished a modification of Secchi's scheme of stellar diversities, and gave it organic meaning by connecting spectral differences with advance in "age." And in 1895, he set apart, as in the earliest stage of growth, a new class of "helium stars," supposed to de velop successively into Sirian, solar, Antarian, or alternatively into carbon stars. The classification which survives at the present time is that of the Draper Catalogue of stellar spectra observed at Harvard (i 890) comprising 10,3 51 stars.

On Aug. 5, 1864, G. B. Donati analysed the light of a small comet into three bright bands. Sir William Huggins repeated the experiment on Winnecke's comet in 1868, obtained the same bands, and traced them to their origin from glowing carbon vapour. A photograph of the spectrum of Tebbutt's comet, taken by him on June 24, 1881, showed radiations of shorter wave lengths but identical source, and in addition, a percentage of reflected solar light marked as such by the presence of some well-known Fraunhofer lines. Further experience has generalized these earlier results. The rule that comets yield carbon-spectra has scarcely any exceptions. The usual bands were, however, temporarily effaced in the two brilliant apparitions of 1882 by vivid rays of sodium and iron, emitted during the excitement of perihelion-passage.

An important contribution of the spectroscope to astronomy is the determination of velocities in the line of sight by measure ment of the Doppler displacement of spectral lines. In 1868 William Huggins attempted these measurements; but no trust worthy results were obtained till much later. Probably the ear liest results that can be counted successful were those of H. C. Vogel who in 1888 substituted photographic for eye observation. The first extensive catalogue of radial velocities of stars was published by W. W. Campbell in 1911.

Miscellaneous.

The first evening of the nineteenth century saw the discovery of the minor planet Ceres by Giuseppe Piazzi at Palermo. This was the forerunner of a host of similar dis coveries now numbering more than a thousand. Progress was greatly accelerated when Max Wolf of Heidelberg in 1891 introduced the photographic method of searching for minor planets. Discovery of the satellites of planets continued during the nineteenth century. Between 1846 and 1851 William Lassell added Neptune's satellite, Hyperion attending Saturn, Ariel and Umbriel attending Uranus. The two satellites of Mars were found by Asaph Hall at Washington in 1877. The fifth (innermost) satellite of Jupiter was found by E. E. Barnard in 1892; and four more faint and remote Jovian satellites have been added in the present century. Saturn's outermost satellite Phoebe was found by W. H. Pickering in 1898.

In regard to the progress of astronomy since the latter part of the last century we can only refer here to the general tend encies; fuller information is given in the separate articles on celestial objects and astronomical methods. One feature has been the development of statistical studies of the distribution, motions, and other characteristics of stars. Important work on the exten sion of the sidereal universe was done by H. von Seeliger who must be counted the pioneer of modern statistical astronomy; but the subject received most impetus from the researches of J. C. Kapteyn. This was the main line of stellar investigation from about 1902-1912, but since then there has been something like a reaction to intensive study of individual stars. More re cently the feature of stellar astronomy has been the application of atomic physics and the quantum theory to the conditions in the stars and nebulae. This closer linking of astronomy with physics (and in particular with thermodynamics) may be said to have originated in important pioneer investigations of the flow of radiation through a star's atmosphere by Arthur Schuster (1905) and Karl Schwarzschild (1908). The great possibilities in the interpretation of spectra were first made manifest by M. N.

Saha (1920).