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What Will Cause the Moon to Stop Moving Away From the Earth?

-- Fred | December 3, 2021

Question:

Jeff, one of your earlier articles mentioned that the lunar retreat would stop in about 15 billion years. What will happen to halt the moon’s movement away from earth? Thank you!

-- Fred

Answer:

As explained in the post What Happens as the Moon Moves Away from the Earth?, the increasing separation of the Moon from the Earth, at a rate of about 3.78 cm per year, will stop after about 15 billion years. This motion of the Moon away from the Earth is due to the transfer of energy from the Earth’s rotation through friction to the Moon. This small amount of energy that is fed into the Moon results in the Moon moving to a more distant orbit around the Earth where it also orbits the Earth more slowly. Over time, the amount of energy transferred to the Moon decreases, which will ultimately result in the Earth-Moon distance stabilizing to a constant value. Note, though, that our Sun is expected to enter its Red Giant phase in about 6 to 7 billion years. So, the Sun will engulf the inner planets as its outer layers expand during its Red Giant phase before the Moon will stop moving away from the Earth.

-- Jeff Mangum

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