Page:Astronomy for Everybody.djvu/220

that they would not fill the whole space uniformly, but would be collected into distinct groups. These groups are shown
Fig. 36.—Distribution of the Orbits of the Minor Planets. on the figures of their orbits, given above, and, on a different plan, and more completely, in the second figure, which is arranged on a plan explained as follows: Every planet performs its revolution in a certain number of days, which is greater the farther the planet from the sun. Since the complete circumference of the orbit measures 1,296,000″, it follows that if we divide this number by the time of revolution, the quotient will show through what angle, on the average, the planet moves along its orbit in one day. This angle is called the mean motion of the planet. In the case of the minor planets it ranges from 400″ to more than 1,000″, being greater the shorter the time of revolution and the nearer the planet is to the sun.

Now we draw a vertical line and mark off on it values of the mean motion, from four hundred to one thousand seconds, differing by ten seconds. Between each pair of marks we make as many points as there are planets having mean motions between the limits. For example, between 550″ and 560″ there are three dots. This means