Caption: A gravitationally-bound gavitational two-body system with a large difference in mass between the two spherically-symmetric astro-bodies orbiting in circular orbits their common center of mass which is marked by a red cross.

Features:

1. If the mass of smaller body were negligible compared to that of the larger body, the more massive body would NOT be seen by the eye to be moving.

2. Circular orbits of this two-body system are determined by Newtonian physics (what is universal about the physical system) and initial conditions (what is peculiar or individual about the physical system).

3. Gravity is, of course, the force that pulls the astro-bodies in orbits.

4. In most real orbits, perturbations cause noticeable deviations from exact two-body system behavior.

5. This two-body system could be, e.g., a close binary star system a close star and planet system, or a close planet and moon system.