Caption: An animation of a gravitationally-bound gavitational two-body system with two spherically symmetric, equal-mass astro-bodies orbiting in elliptical orbits around their common center of mass (called the barycenter for orbits) which marked by a red cross.
Features:
a=(1/2)*( r_periapsis +r_apoapsis ) ,where r_periapsis is the periapsis separation and r_apoapsis is the apoapsis separation.