A compendium of astronomy: being a concise description of the most interesting phenomena of the heavens (1849)

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11 muck inclined to its own, unless they happen to be near one of the Nodes, or crossing points of their orbits; in which case they will be, so to speak, upon the same level. Thus we very seldom see either Mercury or Venus pass across the Sun’s disk ; because the inclination of their orbits to that of the Earth generally causes them to pass either above or below the Sun. In order that a transit , as it is termed, of Mercury or Venus should take place, it is necessary that the planets should be very near one of the points B or D of its orbit, and that the Earth should be in the same line, which very rarely happens. W hat has been stated of the elliptical form of the orbits of the planets, of their inclination to each other, and of the inequalities in their rate of motion, is equally true of those of the moons or satellites by which several of the planets are attended. The Sun is the centre of light, heat, and attraction to the whole system, and round him the planets and comets revolve. His diameter is 882,000 miles, and his bulk is more than 1,300,000 times greater than that of the Earth. When viewed with a telescope, various dark spots are seen on his surface, by the motion of which it is ascertained that he rotates on his axis in 25g days. The appearance which one of these spots successively presents is shown in DIAGRAM V. The spots are by no means constant ; their size and form undergoing great changes from day to day. The diameter of some which have been observed has been as great as 45,000 miles. There can he little doubt that the solid body of the Sun itself is dark, its brightness arising from the luminous atmosphere with which it is surrounded ; and that the spots are openings occasionally formed in this atmosphere, through which the dark mass below is seen.