Dr. Aaron Titus | Department of Physics, High Point University
PHY1050      Astronomy of Stars, Galaxies, and the Cosmos
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sidereal period

Sidereal period of Moon is the time required for Moon to make one complete revolution about Earth, relative to the background stars.

Synodic period of Moon is the time required for Moon to rotate once around Earth with respect to Earth and Sun. That is, it's the time between the reoccurrence of a phase, such as a full moon in one month to a full moon the next month, or a new moon in one month to the new moon another month.

To understand the difference between sidereal period and synodic period, consider the following images.

In the above picture, Moon is between Earth and Sun. It is a new moon.
After about 27.3 days, Moon has revolved one time around Earth. However, because Earth has also revolved around Sun in that time period, Moon is not yet between Earth and Sun again. Moon's phase is a waning crescent.
After about 2 more days, Moon is now beween Earth and Sun, and its phase is again a new moon. The total time between new moon and new moon again is about 29.5 days.

Watch this video of Moon orbiting Earth and Earth orbiting Sun. You will directly observe why the synodic period of Moon is greater than its sidereal period.

Why do we always see the same face of Moon

This movie shows the rotation of Moon about its axis. The arrow can be thought of as an observer standing on the surface of Moon. It takes 27.3 days for Moon to rotate once about its axis. But this is exactly equal to the time required for Moon to revolve one time around Earth.

Thus, Moon revolves around Earth at the same rate that it rotates about its own axis. This is why we always see the same side of Moon. We will always see the same craters when we look at Moon through the telescope. If we want to see a different part of Moon, we would have to go to a different location on Earth.






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