Page:Astronomy for Everybody.djvu/179

not use miles or other terrestrial measures to express distances between the heavenly bodies, for two reasons. In the first place, they are too short; to use them would be like stating the distance between two cities in centimetres. In the next place, distances in the heavens cannot be fixed with the necessary exactness in our measures, whereas, if we take the sun's distance from the earth as the unit of measure, we can determine other distances between the planets with great precision in terms of this measure. So, to get the distances of the planets from the sun in astronomical measure, we have to divide the last numbers of the preceding table by ten, or insert a decimal point before the last figure of each.

We have not in this table distracted the attention of the reader by using unnecessary decimals. Actually, the distance of Mercury is 0.387, etc.; we have simply called it 0.4 and multiplied it by 10 to get the proportion for comparing with Bode's Law.

The motions of the planets in their orbits take place in accordance with certain laws laid down by Kepler, and therefore known as Kepler's laws. The first of these has already been mentioned; the orbits of the planets are ellipses, of which the sun is in one focus.

The second law is that the nearer the planet is to the sun the faster it moves. With more mathematical exactness, the areas swept over by the line joining the planet and sun in equal times are all equal.

The third law is that the cubes of the mean distances