The solar system, as at present kBown, consists of four interior planets, Mercurj', Venus, the Earth, and Mars; four exterior, and relatively colossal planets, Jupiter, Saturn, Uranus, and Neptune, the diffuse crowd of pygniy globes called asteroids, or minor planets, and an outlying array of comets with their attendant meteor-systems. All the planets rotate on their axes, though in very different periods. That of Mercury was determined by Signor Scliia- parelli of Milan in 1889 to be 88 days, the identical time of his revolution round the sun, and Venus was, in the following year, shown by him to be, in all likelihood, similarly conditioned, the common period of rotation and circulation being, in her case, 225 days. This implies that both planets keep the same hemisphere always turned towards the sun, as the moon does towards the earth; nor can we doubt that the friction of tidal waves was, on the three bodies, the agency by which the observed sjmchro- nism was brouglit about. All the planets travel round the sun from west to east, or coimter clock- wise, and most of the satellites move in the same direction round their primaries. But there are e-xceptions. Phcebe. Saturn's remotest moon, cir- culates oppositely to the other members of the system; the four moons of Uranus are retrograde, their plane of movement being inclined at more than a right angle to the ecliptic; and the satellite of Neptune travels quite definitely backward. These anomalies are of profound import to theories of planetary origin. The "canals" of Mars were recognized by Schiaparelli in .\ugust. 1877, and he caught sight of some of them duplicated two years later. Their photographic registration at the Lowell observatory in 1905 proves them to be no optical illusion, but their nature remains enigmatical.
Co.METS AND Meteoks. — The predicted return of Halley's comet in 1759 afforded the first proof that bodies of the kind are permanently attached to the smi. They accompany its march through space, traversing, in either ilirection indifferently, higlily eccentric orbits inclined at all possible angles to the ecliptic. They are accordingly subject to violent, even subversive disturbances from the great planets. Jupiter, in particular, sways the movements of a group of over thirty " captured " comets, which have had their periods curtailed, and their primitive velocities reduced by his influence. Schiaparelli announced in 1866 that the August .shooting-stars, or Perseids, pursue the same orbit with a bright comet visible in 1862; and equally striking accord- ances of movement between three other comets and the Leonid, Lyraid, and Andromede meteor-swarms were soon afterwards established by Leverrier and Weiss. The obvious inference is that meteors are the disintegration-products of their cometary fellow- travellers. -\ theorj' of comets' tails, based upon the varying efficacy of electrical repulsion upon chemically different kinds of matter, was announced by Theodor Bredikhine of Moscow in 1882, and gave a satisfactory account of the appearances it was invented to explain. Latterly, however, the author- ity of Arrhenius of Stockholm has lent vogue to a "light-pressure" hypothesis, according to which, cometary appendages are formed of particles driven from the sun by the mechanical stress of liis radia- tions. But the singular and rapid changes pho- tographically disclosed as taking place in the tails of comets, remain unassociated with any known cause.
SiDERE.VL Astronomy. — Sir William Herschel's discovery, in 1802, of binary stars, imperfectly antici- pated by leather Christian Maj'er in 1778, was one of far-reaching scope. It virtually proved the realm of gravity to include sidereal regions; and the relations it intimated liave since proved to be much more widely prevalent than could have been miagined
beforehand. Mutually circling stars exist in such profusion as probably to amoimt to one in three or four of those unaccompanied. The}' are of limit- less variety, some of the systems formed by tliem being exceedingly close and rapid, while others describe, in millennial periods , vastly extended orbits. Many, too, comprise three or more members; and the multiple stars thus constituted merge, by pro- gressive increments of complexity, into actual clus- ters, globular and irregular. The latter class is exemplified by the Pleiades and the Hyades, by the Beehive cluster in Cancer, just visible to the naked eye, and by the double cluster in Perseus, which makes a splendid show with an opera-glass. Globu- lar clusters are compressed "balls" of minute stars, of which more than one hundred have been cata- logued. The scale on which these marvellous sys- tems are constructed remains conjectural, since their distances from the earth are entirely unknown. Variable stars are met with in the utmost diversity. Some are temporary apparitions, which spring up from invisibility often to an astonisliing pitch of splendour, then sink back more slowly to quasi- extinction. Nova Persei, which blazed 22 February, 1901, and was photographically studied by Father Sidgreaves at Stonyhurst, is the most noteworthy recent instance of the phenomenon. ■ Stars, the vicissitudes of which are comprised in cycles of seven to twenty months, or more, are called "long- period variables". About 400 had been recorded down to 1906. Tliey not uncommonly attain, at maximum, to 1,000 times their minimum brightness. Mira, the "wonderful" star in the Whale. di.scovered by David Fabricius in 1596, is the exemplar of the class. The fluctuations of "short-period variables" take place in a few days or hours, and with far more punctuality. A certain proportion of them are "eclipsing stars" (about 35 have .so far been recog- nized as such), which owe their regularly recurring failures of light to the interposition of large satellites. Algol in Perseus, the variations of which were per- ceived by Montanari in 1669, is the best-known specimen. Hundreds of rapid variables have been recently detected among the components of globular clusters; but their course of change is of a totally different nature from that of eclipsing stars. Ed- mund Halley (1656-1742), the second Astronomer Royal, announced in 1718 that the stars, far from being fixed, move onward, each on its own account, across the sky. He arrived at this conclusion by comparing modern with antique observations; and stellar "proper motions" now constitute a wide and expansive field of research. A preliminarv' attempt to regularize them was made by Herschel's determination, in 178-3, of the sun's line of travel. His success depended upon the fact that the apparent displacements of the stars include a common element, transferred by perspective from the solar advance. Tlieir individual, or "peculiar" movements, however, show no certain trace of method. A good many stars, too, have been ascertained to travel at rates probably uncontrollable by the gravitational power of the entire sidereal system. Arcturus, witli its portentous velocity of 250 miles a second, is one of these "runaway" stars. The sun's pace of about 12 miles a second, seems, by comparison, extremely sedate; and it is probably only half the average stellar speed. The apex of the sun's way, or the point towards which its movement at present tends, is located by the best recent investigations near the bright star Vega.
Distances of the Sun .\Js'd St.\rs. — The dis- tances of the heavenly bodies can only be determined (speaking generally) by measuring their parallaxes, in other words, their apparent changes of position when seen from different points of view. That of the Sim is simply the angle subtended at his distance