Why is the period between two successive full Moons NOT equal to the Moon's orbital period, or sidereal month?
A) The Earth-Moon system is also orbiting the Sun.
B) These two time intervals are not related because full-Moon time depends on the Moon's rotation period about its own axis.
C) The Moon's orbit is inclined at about 5° to Earth's orbital plane.
D) The Moon's orbit is elliptical, and the Moon therefore moves irregularly around Earth.

Question

Quizplus · Accepted Answer

The period between two successive full Moons, known as the synodic month, is longer than the Moon's orbital period (sidereal month) because as the Moon orbits Earth, Earth itself is moving around the Sun. This means the Moon has to travel a bit further to reach the same phase position relative to the Sun and Earth, thus extending the time between similar phases (e.g., full Moon to full Moon).

Why Is the Period Between Two Successive Full Moons NOT