Excerpt from Practical Astronomy for Engineers

The sun is the central and dominating body of the solar system. It is an intensely heated luminous mass, largely if not wholly gaseous in consti tution. The planets and planetoids, which are relatively cool, revolve about the sun. The satellites revolve about the planets. The paths traced out in the motion of revolution are ellipses. Nearly' circular in form, which vary slowly in size, form, and position. One focus of each elliptical orbit coin cides with the center of the body about which the revolution takes place. Thus, in the case of the planets and planetoids, one of the foci of each. Orbit coincides with the sun, while for the satellites, the coincidence is with the planet to which they belong. In all cases the form of the path is such as would be produced by attractive forces exerted mutually by all members of the solar system and varying in accordance with the Newtonian law Of gravitation. In addition to the motion of revolution, the sun, planets, and some Of the satellites at least, rotate on their axes with respect to the stars.

The planets are eight in number. In order from the sun they are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Their distances from the sun range from thirty - Six million to nearly three thousand million miles. Their diameters vary from about three thousand to nearly ninety thousand miles. Nevertheless, comparatively Speaking, they are small, for their collective mass is but little more than one one-thousandth that of the sun.

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