The two-body problem (n = 2) was completely solved by Johann Bernoulli (1667–1748) by classical theory (and not by Newton) by assuming the main point-mass was fixed; this is outlined here. Consider then the motion of two bodies, say the Sun and the Earth, with the Sun fixed, then: m 1 a 1 = G m 1 m 2 r 12 3 ( r 2 − − r 1 ) Sun–Earth m 2 a 2 = G m 1 m 2 r 21 3 ( r 1 − − r 2 ) Earth–Sun