Astronomy 104: Lecture 3 (Slide 4)

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The Lunar Month:

It takes about a month for the moon to go around the Earth.

Just as we had sidereal and solar days because of the compound motion of the Earth's rotation and orbit around the Sun, so there are two types of lunar months:

1) The Sidereal Month: This is the time it takes for the moon to make one full orbit of the Earth relative to the stars.

A Sidereal Month = 27.3 days

2) The Synodic Month: This is like the solar day on earth. We define a Synodic Month as the time required to go through all of the Lunar phases.

This is different from a Sidereal Month because the Earth-Moon system is orbiting the Sun.
At the end of a Sidereal Month the Earth has moved around the Sun and the moon has further to go until it reaches the New phase.
A Synodic Month is longer than a Sidereal one.

One Synodic Month = 29.53 days

Because of the motion of the moon around the Earth, it rises about 50 minutes later each day.

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