<P> E = − G M m 2 a (\ displaystyle E = - G (\ frac (Mm) (2a))), </P> <P> where a is the semi major axis . </P> <P> Since gravity is a central force, the angular momentum is constant: </P> <P> L _̇ = r × F = r × F (r) r ^ = 0 (\ displaystyle (\ dot (\ mathbf (L))) = (\ mathbf (r)) \ times (\ mathbf (F)) = (\ mathbf (r)) \ times F (r) (\ mathbf (\ hat (r))) = 0) </P>

The major axis of an elliptical path in which the earth