Earth and the Moon both move eastward in their orbits. Suppose the Moon moved westward instead, at the same rate as its actual eastward movement. What consequence would this have?
A)1 synodic month would be the same length it is now.
B)1 sidereal month would be the same length it is now.
C)Both synodic and sidereal months would be shorter than they are now.
D)Both synodic and sidereal months would be longer than they are now.

Question

Quizplus · Accepted Answer

The sidereal month is the time it takes for the Moon to return to the same position relative to the stars. This would not be affected by the Moon moving westward instead of eastward, as the stars do not change their position. Therefore, choice B is correct.

The synodic month is the time it takes for the Moon to return to the same phase (e.g. full moon to full moon). This would be affected by the Moon moving westward instead of eastward, as the relative motion between Earth and Moon would be different. However, it is not possible to determine from the given information whether the synodic month would be longer or shorter than it is now, so choices A, C, and D are not correct.

Earth and the Moon Both Move Eastward in Their Orbits