Let us now go on to the reſt of the Planets. Becauſe the revolutions of the primary Planets about the Sun, and of the ſecondary about Jupiter and Saturn, are phænomena of the ſame kind with the revolution of the Moon about the Earth; and becauſe it has been moreover demonſtrated that the centripetal forces of the primary Planets are directed towards the centre of the Sun, and thoſe of the ſecondary towards the centres of Jupiter and Saturn, in the ſame manner as the centripetal force of the Moon is directed towards the centre of the Earth; and ſince beſides, all theſe forces are reciprocally as the ſquares of the diſtances from the centres, in the ſame manner as the centripetal force of the Moon is as the ſquare of the diſtance from the Earth; we muſt of courſe conclude, that the nature of all is the ſame. Therefore as the Moon gravitates towards the Earth, and the Earth again towards the Moon; ſo alſo all the ſecondary Planets will gravitate towards their primary, and the primary Planets again towards their ſecondary; and ſo all the primary towards the Sun; and the Sun again towards the primary.
Therefore the Sun gravitates towards all the Planets, and all the Planets towards the Sun. For the ſecondary Planets, while they accompany the primary, revolve the mean while with the primary about the Sun. Therefore by the ſame argument, the Planets of both kinds gravitate towards the Sun, and the Sun towards them. That the ſecondary Planets gravitate towards the Sun is moreover abundantly clear from the inequalities of the Moon; a moſt accurate theory of which laid open with a moſt admirable ſagacity, we find explained in the
third book of this Work.
