5. For the satellites, the force acting depends on the mass of their central body, a planet. Hence the mass of any planet possessing a satellite becomes known.
6. The force constraining the moon in her orbit is the same gravity as gives terrestrial bodies their weight and regulates the motion of projectiles. [Because, while a stone drops 16 feet in a second, the moon, which is 60 times as far from the centre of the earth, drops 16 feet in a minute.]
7. The moon is attracted not only by the earth, but by the sun also; hence its orbit is perturbed, and Newton calculated out the chief of these perturbations, viz.:—
(The equation of the centre, discovered by Hipparchus.)
(a) The evection, discovered by Hipparchus and Ptolemy.
(b) The variation, discovered by Tycho Brahé.
(c) The annual equation, discovered by Tycho Brahé.
(d) The retrogression of the nodes, then being observed at Greenwich by Flamsteed.