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Physics Lesson 22.3.1 - Main Features of the Moon's Movement
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Welcome to our Physics lesson on Main Features of the Moon's Movement, this is the first lesson of our suite of physics lessons covering the topic of The Moon's Movement. Eclipses. Calendars, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.
Main Features of the Moon's Movement
The Moon also makes one complete cycle around the Earth in an elliptic-shape trajectory in a similar fashion to planetary motion around the Sun, where Earth is located in one of the ellipse foci. This cycle lasts for 27.32 days (a tropical month) when the (unmoveable) stars are taken as a reference frame and 29.53 days (a synodic month) when the Sun is taken as a reference frame. The successive new month begins when the Moon shows the same face it had in the previous month when viewed from Earth. This method for counting lunar months (i.e. to observe the same periodic view of the Moon) is used only when dealing with synodic month.
This difference in month duration is due to the Earth displacement around the Sun during the given period, similar to day duration difference in sidereal and tropical discussed in the previous tutorial (Earth day and solar day).
Aphelion (the longest distance) of Moon's orbit around the Earth is 407 000 km while perihelion (the shortest distance) is 356 000 km. This large difference indicates a long (and pressed) elliptic trajectory, more or less like this:
Now, let's explain why there is a difference of about two days between tropical month (27.32 days) and synodic month (29.53 days). The following figure is very helpful in explaining this point.
The Moon revolves around the Sun in the same direction as Earth does. Let's consider the initial position of Moon at M1 (Earth meanwhile is at E1). When the Moon completes one cycle in respect to remote (unmoveable) stars, it is in the position M2 (Earth is at E2). This process occurs in 27.32 days. During this time, the Earth rotates by 360°/12 = 30° (more accurate measurements give a value of 27°) around the Sun. However, in order to show the same view to the Earth, the Moon must be at M3. It takes about 2 days to the Moon to move from M2 to M3 and complete therefore the cycle of synodic month.
You have reached the end of Physics lesson 22.3.1 Main Features of the Moon's Movement. There are 5 lessons in this physics tutorial covering The Moon's Movement. Eclipses. Calendars, you can access all the lessons from this tutorial below.
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