Why is the Moon's synodic period longer than its sidereal period?
A) because most months have more than 27 days
B) because the Sun moves faster than the Moon
C) because Earth has moved along its orbit during a sidereal period
D) because one complete orbit of the Moon around Earth takes longer than the cycle of lunar phases

Question

Quizplus · Accepted Answer

The Moon's synodic period is longer than its sidereal period because as the Moon completes one orbit around Earth (a sidereal month), Earth has moved in its orbit around the Sun. This means the Moon has to travel a bit further to reach the same phase as seen from Earth, thus extending the synodic period (the time between similar phases, e.g., full moon to full moon) compared to the sidereal period (the time it takes to complete one orbit relative to the stars).

Why Is the Moon's Synodic Period Longer Than Its Sidereal