The Orbits of Planets

Proposition 14 Theorm 14: The aphelions and nodes of the orbits of the planets are fixed.

The aphelions are immovable by Prop. XI, Book I; and so are the planes of the orbits, by Prop. I of the same Book. And if the planes are fixed, the nodes must be so too. It is true, that some inequalities may arise from the mutual actions of the planets and comets in their revolutions; but these will be so small, that they may be here passed by.

Corollary 1

The fixed stars are immovable, seeing they keep the same position to the aphelions and nodes of the planets.

Corollary 2

These stars are liable to no sensible parallax from the annual motion of the earth.

And so they can have no force because of their immense distance, to produce any sensible effect in our system.

The stars are promiscuously dispersed in the heavens. Their contrary attractions destroy their mutual actions by Proposition 70 Book 1.

Scholium

Mercury, Venus, the Earth, and Mars are so small that they can act with but little force on each other. Therefore their aphelions and nodes must be fixed, except as they are disturbed by the actions of Jupiter and Saturn, and other higher planets.

Thus, their aphelions move a little in consequece, with respect of the fixed stars in the sesquiplicate proportion of their distances from the sun.

This in 100 years, the aphelions will be carried:

• Mars by 33'20"
• Earth by 17'40"
• Venus by 10'53"
• Mercury by 4'16"

But these motions are so inconsiderable, that we have neglected them in this Proposition,