five planets, besides sharing the common movement, described variously conditioned orbits round tlie same centre. The body of doctrine it inculcated made part of the universal stock of knowledge until the sixteenth century. The formidable task of demonstrating its falsity, and of replacing it with a system corresponding to the true relations of the world, was imdertaken by an active and exemplary ecclesiastic, Nicholas Copernicus, Canon of Frauen- burg (1473-1543). The treatise in which it was accomplished, entitled "De Revolutionibus Orbium Ccelestium", saw the light only when its author lay dying; but a dedication to Pope Paul III bespoke the protection of the Holy See for the new anil philosophically subversive views which it propountled. Denounced as impious by Luther and Melanchthon, they were, in fact, favourably received at Rome until theological discredit was brought upon them by the wild speculations of Giordano Brimo (1548-1600), and the imprudent utterances of Galileo Galilei (1.564-1642).
Descriptive Astronomy. — Descriptive astronomy may be said to have originated with the invention of the telescope by Hans Lippershey in 1608, Its application to the scrutiny of the heavenly bodies, by Galileo and others, led at once to a crowd of striking discoveries. Jupiter's satellites, the phases of Venus, the mountains of the moon, the spots on the sun, Saturn's unique appendages, all descried with a little instrument resembling a imiocular opera-glass, formed, each in its way, a significant and surprising revelation; and the perception of the stellar composition of the Milky Way represented the first step in sidereal exploration. Johann Kepler (1571-1630) invented in 1611, and Father Scheiner of Ingolstadt (1575-1650) first employed, the modern refracting telescope; and the farther course of dis- covery corresponded closely to the development of its powers. Christian Huygens (1629-95) resolved, in 1656, the ansce of Saturn into a ring, divided into two by Giovanni Domenico Cassini (1625-1712) in 1675. Titan, the largest of Saturn's moons, was detected by Huygens in 1655, and four additional members of the family by Cassini before 1684. The Andromeda nebula w'as brought to notice by Simon Marius in 1612, the Orion nebula by J. B. Cysatus, a Swiss Jesuit, in 1618; and some few variable and multiple stars were recognized.
Theoretical Astronomy. — The theoretical, how- ever, far outweighed the practical achievements of the seventeenth century. Kepler published the first two of his "Three Laws" in 1609, the third in 1619. The import of these great generalizations is: (1) that the planets describe ellipses of which the sun occupies one focus; (2) that the straight line joining each planet with the sun (its radius vector) sweeps out equal areas in equal times; (3) that the squares of the planetary periods are severally pro- portional to the cubes of their mean distances from the sun. The geometrical plan of movement in the solar system was thus laid down with marvellous intuition. But it was reserved for Sir Isaac Newton (1643-1727) to expound its significance by showing that the same uniformly acting force regulates celestial revolutions, and compels heaN-y bodies to fall towards the earth's surface. The law of gravity, published in 1687 in "Philosophise Naturalis Principia Mathe- matica" is to the following effect: every particle of matter attracts every other with a force directly proportional to their masses, and inversely propor- tional to the squares of their distances apart. Its validity was tested by comparing the amount of the moon's orbital deflection in a second with the rate at which an apple (say) drops in an orchard. Allow- ance being made for the distance of the moon, the two velocities proved to tally perfectly; and the identity of terrestrial gravity with the force control-
ling tlie revolutions of the lieavenly bodies was defini- tively established. But this was only a beginning. The colossal work remained to be accomplished of calculating the consequences of the law, in the minute details of its working, and of comparing them with the heavens. It was carried forward, first by Newton himself, and in the ensuing century, by Elder, Clairaut, d'Alembert. Lagrange, and Laplace. Urbain Leverrier (1811-77) inherited from these men of genius a task never likely to be completed; and the intricacies of lunar theory have been shown, by the researches of John Couch Adams (1819-92). of Hansen and Delaunay, of Professors Hill and Newcomb, and many more, to be fraught with issues of unexpected and varied interest.
Discoveries in the Sol.\r System. — The ex- traorilinary improvement of reflecting telescopes by Sir William Herschel (1738-1822) opened a fresh epoch of discovery. His recognition of the planet Uranus (13 March, 1781) as a non-stellar object marked the first enlargement of the bounds assigned of old to the solar system; two Uranian moons, Oberon and Titania. were detected by him 11 Jan- uary, 1787, and the innermost Saturnian pair, I'^nce- ladus and Mimas, 28 August and 17 September of the ■same year. Saturn was. in 1906. known to possess ten satellites. Hyperion was descried by W. C. Bond at the observatory of Harvard College 16 Sep- tember, 1848, and Professor W. H. Pickering, of the same establishment, discovered by laborious photographic researches. Phoebe in 1898, and Themis in 1905. In point of fact, an indefinite number of satellites are agglomerated in the rings of Saturn. Their constitution by separately revolving, small bodies, theoretically demonstrated by J. Clerk Maxwell in 1857, was .spectroscopically confirmed by the late Professor Keeler in 1895. The system includes a dusky inner member, detected by Bond. 15 November, 1850. The discovery of the planet Neptune, 23 September, 1846, was a mathematical, not an observational feat. Leverrier and Adams independently divined the existence of a massive boily, revolving outside Uranus, and exercising over its movements disturbances the analysis of which led to its capture. Its solitary moon was noted by William Lassell of Liverpool in October, 1846; and he added, in 1851, two inner satellites to the re- markable system of Uranus. With the great Wash- ington refractor, 26 inches in aperture. Professor Asaph Hall discerned, 16 and 17 August, 1877, Deimos and Phobos, the swiftly circling moonlets of Mars; the Lick 36-inch enabled Professor Barnard to perceive, 9 September, 1892, the evasive inner satellite of Jupiter; and two exterior attendants on the same planet were photographically detected by Professor Perrlne in 1904-05. The distances of the planets are visibly regulated by a method. They increase by an ordered progression, announced by Titius of Wittenberg in 1772, and since designated as " Bode's Law". But their succession was quickly seen to be interrupted by a huge gap between the orbits of Mars and Jupiter; and the conjecture was hazarded that here a new planet might be found to revolve. It was verified by the discovery of an army of asteroids. Ceres, their leader, was captured at Palermo, 1 January, 1801, by Giuseppe Piazzi, a Theatine monk (1746-1826); Pallas, in 1802 by Olbers (1758-1840), and Juno and Vesta in 1804 and 1807, by Harding and Olbers respectively. The original quartette of minor planets began in 1845 to be reinforced with companions, the known number of which now approximates to 600, and may be indefinitely increased. Their discovery has been immensely facilitated by Professor Max Wolf's in- troduction, in 1891, of the photographic method of tliscriminating them from stars through the effects of tlieir motion on sensitive plates.