Sidereal and Synodic Months

The adjacent animation illustrates the reason for the difference in the length of the sidereal month, which is referenced to the distant stars, and the synodic month, which is referenced to the Moon's phases and thus to the relative position of the Sun.

The angular displacement of the Earth for one month of its orbit around the Sun is exaggerated here for purposes of better viewing. Usage: Use "Play" to start and "Stop" to halt. "Home" returns to the beginning. "Step" moves forward through the animation one frame at a time; "Back" moves backward one frame.