
Proof of community lies in the character of the paths. Planet and particle alike turn out to travel in ellipses, and ellipticity betrays association. How the orbit labels the occupant we shall see, on finding the paths the planets pursue and why 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.
