Motion of the Moon: Sidereal Month vs. Synodic Month

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  • 0:01 The Length of the…
  • 0:41 Angular Diameter
  • 1:22 Sidereal Month
  • 2:39 The Synodic Month
  • 4:14 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will teach you about the sidereal month and synodic month, how long they are, and how they are measured with respect to the stars and sun.

The Length of the Moon's Motions

Here's a cool fact: four weeks. Well, that's not the cool fact. The cool fact is that one lunar day (the time it takes the moon to rotate once on its axis), and the time it takes for the moon to orbit the earth, and the time it takes for the moon to complete one cycle through its phases are all about four weeks in length.

But remember, it's about four weeks in length, not exactly four weeks. Our lesson will go over the latter two points related to the moon's orbit and cycle of phases, how long they truly are, and how they differ from one another when comparing the position of the moon to the stars and sun in the sky.

Angular Diameter

Our planet moves around the sun counterclockwise when viewed from the north. The moon orbits the earth in the same direction, counterclockwise.

It's very easy to see the motion of the moon across the night sky on an hourly basis! The moon moves just a bit more than its angular diameter every hour.

The angular diameter is the angle that's made by two lines starting at an observer and ending on the opposite sides of an object. The moon's angular diameter is about half a degree; thus, in an hour, it moves approximately half a degree to the east. That means in a day, it will move 13 degrees.

Sidereal Month

As a result, the moon's eastward orbit around the earth takes 27.32 days. This is known as the moon's sidereal month, which is the time it takes for the moon to complete a full orbit around the earth with respect to returning to the same place among the stars.

Confused by what this means? Let's take a look at an important image on the screen then:

Sidereal Month
sun and moon lined up

The image you see shows a new moon (in dark blue), the sun in yellow, and a bunch of white dots representing stars in the sky. This is the part of the sky where the moon will begin and end its journey as it completes its orbit. In this image, the arrows show that both the sun and moon move eastwards, just like I said before.

As the moon orbits eastwards, it will eventually come to the same spot among the stars, 27.32 days later. But alas! The sun moves eastwards as well. It doesn't stand still on our sky and wait for the moon. Thus, while the moon is at the same place among the stars, it hasn't returned to the same position in the sky with respect to the sun, only with respect the stars!

The Synodic Month

This is where the synodic month comes in. The amount of time it takes for the lunar phases to cycle through one time and for the moon to return to the same position in the sky relative to the sun is known as the moon's synodic month, and it is 29.53 days in length. It's sometimes called the lunar month.

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