The Heavens Above: A Popular Handbook of Astronomy

by J. A. Gillet; W. J. Rolfe

Fig. 63.

53. The Force that draws the Different Planets to the Sun Varies inversely as the Squares of the Distances of the Planets from the Sun.—Newton compared the distances jK and eF, over which two planets are drawn towards the sun in the same time, and found these distances to vary inversely as the squares of the distances of the planets from the sun: hence he concluded that all the planets are held in their orbits by gravity. He also showed that this would be true of any two bodies that were revolving around the sun's centre, according to Kepler's Third Law.

54. The Copernican System.—The theory of the solar system which originated with Copernicus, and which was developed and completed by Kepler and Newton, is commonly known as the Copernican System. This system is shown in Fig. 64.

Fig. 64.

II. THE SUN AND PLANETS.

I. THE EARTH.

Form and Size.

55. Form of the Earth.—In ordinary language the term horizon denotes the line that bounds the portion of the earth's surface that is visible at any point.