Page:The Solar System - Six Lectures - Lowell.djvu/21



they pursue them. The orbits of the planets are then the first point to consider.

To begin with the Sun. Observation shows not only that the Sun changes its place in the heavens, but changes its size as well. To measurement through a smoked glass, it seems to contract in summer and expand in winter. Plotting the directions it successively takes in the form of a spider, and taking the legs inversely proportionate to the diameters at the times, we find an ellipse, in one of whose foci lies the Sun. The Earth, then, goes round the Sun in an ellipse.

To find the path of a planet, we first get its synodic period, or period with regard to the Sun. Then, from a sufficient number of observations of synodic periods to give their mean, we obtain the sidereal period, or period with reference to the stars.

By considering the angular motions, the two periods are easily seen to be connected by the following equation:—

$$\textstyle \frac{1}{S}=\frac{1}{P}-\frac{1}{E} \,\!$$;




 * Where
 * E = the Earth's period;
 * S = the Planet's synodic period;
 * P = the Planet's sidereal period.
 * }
 * S = the Planet's synodic period;
 * P = the Planet's sidereal period.
 * }
 * P = the Planet's sidereal period.
 * }

From two bearings separated by a sidereal