Many diagrams of the motion of the Earth-Moon-Sun system show the moon revolving about the center of Earth. Often, overly simplified new-moon-to-full-moon cycle diagrams are used to explain the changing lunar phases. In fact though, Earth and its moon revolve about a common center of mass known as the barycenter (Gr. “heavy” center). The documents here represent the author’s explanation and illustration of the barycenter concept, so often inadequately represented in the educational literature.
Maul, George A., "Motion of the Earth-Moon System about their Barycenter: Planetary Free Spheres Motion" (2015). Ocean Engineering and Marine Sciences Faculty Publications. 5.