The Moon's sidereal period is 2.2 days shorter than the period during which the Moon's phases change because
A) the Moon always keeps the same side turned toward the Earth.
B) the Earth must rotate so an observer can see the Moon.
C) the Moon's orbit is tilted with respect to the Earth's rotational axis.
D) the Earth moves significantly in its orbit around the Sun during that time.

Question

Quizplus · Accepted Answer

The Moon's sidereal period is shorter because as the Moon orbits Earth, Earth is also moving in its orbit around the Sun. This means the Moon must travel a bit further to reach the same phase, resulting in a longer synodic period.

The Moon's Sidereal Period Is 2