_Wereury, the planet which is nearest the sun, is also, with the exception of the planet oids, the smallest (being only 3 times the size of the moon), and performs its revolution round the sun in the shortest time. Its greatest elongation is never more than 28° 45', and consequently it is never above the horizon more than two hours after sunset, or the same time before sunrise; on this account, and from its small apparent size (5" to 12'), it is seldom distinctly observable by the naked eye. It shines with a peculiarly vivid white or rose-colored light, and exhibits no spots.
Venus, the next in order of distance and period, is to us the most brilliant of all the planets. Its orbit is more nearly a circle than any of the others, and ivhen at its inferior conjunction. it approaches nearer the earth that any other planet, Its apparent angular dimensions thence vary from 10' at the superior, to 70' at the inferior conjunction. Its greatest elongation varies from 45° to 47° 12'. and therefore it can never be above the horizon for much more than three hours after sunset, or the same time before sunrise. While moving from the inferior to the superior conjunction, Venus is a morning star; and during the other half of its synodic period, an evening star. When this planet is at an elongation of 40°, its brilliancy is greatest, far surpassing that of the other planets, and rendering a minute examination through the telescope impossible. At this period it sometimes becomes visible in the day-time, and after sunset is so bright as to throw a distinct shadow. Astronomers have repeatedly attempted to ascertain the nature and characteristics of its surface, but its brightness so dazzles the eyes as to render the cor. reetness of their observations at best doubtful. From the changes in the position of dusky patches on its surface. which have been frequently noticed, it is concluded that it revolves on its axis. and that its equator is inclined to the plane of its orbit at an angle of 75°; but many astronomers (sir John Herschel included) profess to doubt these con clusions. Both Venus and Mercury necessarily exhibit phases like the moon.
The Earth, the next planet iii order, will be found under its own name; it has a single satellite, the MOM NII1'17, the first of the superior planets. Is much inferior in size to the two previous, its volume being about + of the earth's, and, after Mercury, its orbit is much more eccen tric than those of the other planets. When it is nearest to the earth (i.e., in opposition),
its apparent angular diameter is 30'; but When furthest from it (i.e., in conjunction), its diameter is not more than 4". Mars is less known than the rest of the superior planets, owing to its not possessing a satellite, by the motions of which its attractive force (and hence its mass and density) could be estimated. It shines with rt fiery red light. and is a brilliant object in the heavens at midnight when near opposition; when seen through the telescope its surface appears to be covered with irregular blotches, some of them of a red dish, others of a greenish color, while at each pole is it spot of dazzling white. The red spots are surmised to be land; the green, water; while the white spots at the poles are with some reason supposed to be snow, since they decrease when most exposed to the sun, and increase under the contrary circumstances. The phases (q.v.) of Mars range between full, half, full (in conjunction, if visible), and half.
The Plunetoulx.—After Mars in order come the planetoids (q.v.), formerly but improp erly called asteroids.
Jupiter, the next in order, is the largest of all the planets, its bulk being more than 1400 times that of the earth, though, from its small density, its mass is only 338 times more. After Venus it is the brightest of the planets and the largest in apparent size, its angular diameter varying from 30" to 45". When looked at through a telescope, it is seen to be considerably flattened at the poles, owing to its rapid revolution on its own axis; and its surface is crossed in a direction parallel to its equator by three or four distinct and strongly-marked belts, and a few others of a varying nature. Spots also appear and remain for some time on its surface, by means of which its revolution on its axis has been ascertained. This planet is attended by four satellites, which are easily observable through an ordinary telescope, and which Have rendered immense service in the deter mination of longitudes at sea, and of the motion and velocity of light. The satellites, which were discovered by Galileo, were proved by sir William Herschel to revolve on their own axes in the same time that they revolve round their primary. The smallest is about the same size as our moon, the others are considerably larger.