 3. What are the relative densities of the planets, with respect to one another and to the Sun? Newton gives theory-dependent answers for Jupiter, Saturn, and the Earth in the corollaries to Proposition 8, but the one for the Earth, even in the third edition, depended on a still questionable value for the horizontal solar parallax (required to determine the distance of the Moon from the Earth in astronomical units), and no corroborating evidence for these answers had emerged, such as from the actions of Jupiter and Saturn on one another.