lebrated astronomer and mathematician of Pelusium, in Egypt, who lived in the beginning of the second century of the Christian æra. Ptolemy, whose name is attached to this system, endeavours to prove that the earth, T, in the annexed figure, is immoveable in the centre of the uni verse, and he placed the Sun and the other planets in orbits that surrounded the orbit of the Earth in the following order, beginning with those which he thought were next the Earth; the Moon >; Mercury; Venus ?; the Sun O; Mars &; Jupiter 2; and Saturn : after which, as the Georgium Sidus, or Herschel planet, was not then known, he comes to the fixed stars. When astronomers observed that the planets Mercury and Venus were sometimes nearer and sometimes farther from the Sun, and that Venus never departs from the Sun more than about 47 degrees, and Mercury about 28 degrees and a half; they soon discovered that the orbits of these planets could not be beyond the orbit of the Earth; because, if that had been the case, they would sometimes have appeared opposite to the Sun, that is, the Earth would sometimes have been between the planets and the Sun, which it never is. Hence the Egyptians thought that these two planets were satellites to the Sun, and turned about him, their orbits being carried with him in his revolution about the Earth. And they supposed the Earth immoveable, as the centre of the system, and that the other celestial bodies revolved about her once in twenty-four hours. Copernicus, with a view of obviating the inconveniences of the imaginary systems that had preceded him, began by assuming the diurnal motion of the Earth, or her motion round her own axis: this being admitted, he showed the necessity of her annual motion round the Sun in the ecliptic in something more than 365 days. With these two motions, he explained, with the utmost facility, the phenomena of the stations and motions of the planets. According to this theory, the Sun' S, is the centre of the planetary system, and the several planets, of which the Earth is one, revolve about him in the following order: Mercury; Venus ; the Earth 8; Mars; Jupiter 2; and Saturn. Such was the true theory, and the full extent of it, as was then known; since which, another planet, the Georgium Sidus, has been discovered beyond the bounds of Saturn: and four other small planetary bodies are now known to be perpetually revolving about the Sun, in orbits which are situated 'See a Philosophical Account of the Sun in No. XLIIL between the orbits of Mars and Jupiter. In our figure, the small circles about the primary planet: are made to represent the orbits of their satellites: thus, our Earth has one, the Moon; Jupiter has four; Saturn has seven; and the Georgium Sidus has six. The Planets, like the Earth, are opaque bodies, and nearly spherical. Being opaque in themselves, they become visible only by reflecting the light which they receive from the Sun. The laws by which they are governed were discovered by Kepler, who demonstrated that they must necessarily revolve in elliptical and not in circular orbits. Astronomers have divided the planets into two classes: in the first are the primary planets, viz. Mercury, Venus, the Earth, Mars, Ceres, Pallas, 1 Juno, Vesta, Jupiter, Saturn, and the Georgium Sidus. The second class includes the satellites which belong to some of the primary planets, such is the Moon, attendant upon the Earth; such also are the four moons or satellites that revolve about Jupiter; the seven that revolve about Saturn; and the six that wait on the Georgium Sidus. The primary planets revolve about the Sun, and are divided into the inferior and superior. The former are those whose distance from the Sun is less than that of the Earth, and the latter are those whose distance is greater than that of the Earth. Mercury and Venus are inferior planets, or those whose orbits are included within that of the Earth. Mars, Ceres, Pallas, Juno, Vesta, Jupiter, Saturn, and the Georgium Sidus, are superior planets, all whose orbits are beyond the orbit of the Earth. The planet Mercury is about 3224 miles in diameter, and revolves around the Sun in 88 days, at the distance of 37 millions of miles. When Mercury is viewed with a telescope of high magnifying power, he exhibits nearly the same phases as the Moon, being sometimes horned, and sometimes full. He is always seen on the same side of the heavens as the Sun; of course, when he is visible in the morning it is in the east, just before Sunrise, and when seen in an evening it is in the west, soon after the setting Sun. It is not known whether he turns on his axis, though Schroeter suspected he had discovered his period to be 24 days, 5 hours, and 28 minutes. The characteristics of this planet are thus described: Mercury the first Near bordering on the day, with speedy wheel With seven-fold splendour. Next, Venus to the westward of the Sun The diameter of Venus is not quite so large as that of the Earth, it being 7648 English miles in length, and she performs her revolution about the Sun in 224 days, at the distance of 68 millions of miles. With almost any telescope this planet exhibits the same phases as those of the Moon. Venus enjoys rather more than twice the quantity of light and heat experienced by the Earth. She is an evening star when she appears east of the Sun, and a morning star when she is west of him. By Milton she is described as Fairest of stars, last in the train of night, If better thou belong not to the dawn, Sure pledge of day, that crownest the smiling morn1. Twice in the course of about 120 years, Venus passes over the disk of the Sun: this phenomenon is called the transit of Venus, and by this, astronomers have been enabled to ascertain, with great accuracy, the distance of the Earth from the Sun. Venus is thought to turn on her axis in 23 hours and 20 minutes, and she is said to have an atmosphere fifty miles in height. More distant still our globe terraqueous turns, CHATTERTON. Fair Venus shines Even in the eye of day; with sweetest beam BARBAVLEL |