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Ward et. al define the evection resonance of the Moon with the Earth and Sun as:

occurring when the lunar perigee precession period equals one year.

Does this simply mean that the angle formed by the Sun-Earth-Moon when the Moon was at perigee was the same every month?

I tried looking at the wikipedia definition of Evection but just found it more confusing.

If it helps, I am curious about the lunar evection resonance because it is often cited as a mechanism to explain Moon/Earth angular momentum shedding necessary if one assumes the Giant Impact Hypothesis of Moon formation. See Ward and Canup:

The resonance can exchange angular momentum between the Earth-Moon system and the Earth's orbit.

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The orbit of the moon is elliptical. But the direction of the major axis of this ellipse isn't fixed. Perturbation by the sun causes the axis to rotate towards the East. The time for the moon to go from perigee to perigee is slightly longer than the time it takes for the moon to orbit the Earth.

The direction of the major axis of the moon slowly rotates around the Earth. Now it takes about 8.85 years to make a complete 360-degree rotation.

But early in the history of the Earth-moon system, it might have made a complete rotation in less time. There could have been a time when it took one year to make a 360-degree rotation. This could lead to a resonance that would affect the orbit of the Earth around the sun. In particular, it could transfer angular momentum from the Earth-Moon system to the Earth-Sun system. This can, perhaps, explain why the Earth-Moon system has less angular momentum than would be predicted by a system that formed from a planetary collision.

The paper investigates how such a resonance could form, how it could transfer angular momentum, and how the resonance could eventually be broken.

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Does this simply mean that the angle formed by the Sun-Earth-Moon when the Moon was at perigee was the same every month?

No. It means that one Earth year after the Moon was at perigee, the Moon would once again be at perigee. This could happen for two reasons:

  • The Moon's apsidal line is fixed, or
  • The Moon's apsidal line rotates at one revolution per year.

The former is not the case. The Moon's orbit is markedly non-Keplerian. One key deviation from a Keplerian orbit is the Moon's apsidal precession, which is currently about one revolution per 8.85 years.

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  • $\begingroup$ As the moon's orbit migrated outward from Earth, wouldn't there be many times the perigee coincided yearly? I am left more confused than ever by this answer. $\endgroup$
    – Connor Garcia ♦
    Feb 26, 2021 at 5:15

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