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Why Does the Apparent Position of Moonrise Change by About 57 Degrees During a Month?

-- tony & Molli Rathstone | December 5, 2020

Question:

What is happening that makes the moonrise time according to a compass rise from 62 degrees Nov 5, 2020 all the way up to 118 degrees Nov 18th 2020 and then start back down to 65 degrees Nov 30, 2020 today?

It appears to always stay between these 2 degrees going up and down.

at the high point 118 degrees and low point 65 degrees is that when the moon has arrived on the other side of the earth?

-- tony & Molli Rathstone

Answer:

This change in the position of moonrise throughout a given month is due to three factors. First, the Earth’s rotational axis is tilted by about 23.5 degrees relative to the plane in which it orbits around the Sun. Second, the Moon orbits the Earth in a plane that is tilted by about 5.1 degrees relative to the plane in which the Earth orbits around the Sun. Since the Moon makes one complete orbit around the Earth in 28.5 days, you will see the position of the Moon’s rise on the horizon change by twice 23.5+5.1 degrees, or about 57.2 degrees, during one orbit of the Moon around the Sun. This is pretty close to the 118-65 = 53 degrees that you measured using a compass. An animation of the Moon’s orbit might make this easier to visualize.

-- Jeff Mangum

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