What is synchronous rotation?

Synchronous rotation refers to the phenomenon where an object's rotational period is equal to its orbital period around another body. In the context of the Earth-Moon system, this means that the Moon takes the same amount of time to complete one full rotation on its axis as it does to orbit the Earth once. This results in the same side of the Moon always facing the Earth, a condition known as "tidal locking."

This concept is significant as it demonstrates the gravitational interactions between bodies in space, where the gravitational forces can influence rotation and orbit over long periods. Such tidal forces cause the synchronization and shape the way celestial objects interact with one another.

The other options describe different concepts related to celestial motions and properties but do not correctly define synchronous rotation. For example, discussing the time it takes to complete an orbit relates to orbital periods generally but does not specify the equality required for synchronization. A fully illuminated Moon refers to a specific phase, while the boundary between illuminated and unilluminated regions on the Moon is known as the terminator, neither of which pertains to the concept of synchronous rotation.