Perhaps the planets were subject to gravity from the sun?. Newton then calculated the force needed to hold the Moon in its orbit, as compared with the force pulling an object to the ground.
(the force; pulling; an object; to the ground) (?. Newton; calculated; the force needed; T:then) (the force; needed; to hold the Moon in its orbit) (the planets; were subject to; gravity)
the force ?. Newton the force the planets
pulling calculated needed were subject to
an object to the ground
the force needed T:then
to hold the Moon in its orbit
gravity