Sentence:  The Motions of the ſix Primary Planets round the Sun is So adjuſted, that the Square of the Times of their peroidical ["sic"] Revolutions are as the Cubes of their Diſtances from the Sun : And the ſame thing is found by all Aſtronomers to be true, with Regard to the Motions of the "Secondary Planets" or "Satellites" round their primary ones. [...] But if the Orbit of the <WSD>Satellite Planet</WSD>, inſtead of being a Circle be an "Ellipſis", in whoſe "Focus" the Primary Planet is ſuppoſed to be placed, then will the greater Axis of this Elliptical Orbit, twice advance forward, "viz." in the 2 Quadratures, and twice recede backward, "viz." in the 2 Syzygies, of every Revolution of the Satellite round the Planet. 
Meaning of the word marked with <WSD> tags:  A moon of a planet besides Earth; a secondary planet; a natural planetary satellite.