I've found an animation in NASA's website, illustrating tidal acceleration effect between the moon and earth. It's the forth animation from the top in this page: https://moon.nasa.gov/moon-in-motion/tides/

According to NASA, the high tide bulge is a little bit behind the moon, not directly below it.

However, according to all other sources that I know (textbooks + Google search results, Wikipedia etc..) the bulge is actually ahead of the moon. This is the reason why the moon slowly spirals away from earth, and the earth rotation to slowly decrease (day duration increases).

If the bulge was behind the moon (as in NASA's page) this would actually cause the moon to spiral inwards, and earth rotation to speed up (day duration decreases). Which is not what's happening in reality.

A good explanation including force diagrams can be found in Wikipedia: "Tidal acceleration", but you can find this in textbooks and lecture notes as well. All agree that tide bulge is ahead of the moon, not behind it.

Is this NASA's mistake??


1 Answer 1


Yes, the bulge points to a point where the Moon will be in the future, the angular discrepancy is around 3 degrees. This is because Earth's rotation frequency is higher than the Earth - Moon orbital frequency, and the Earth rotation is pulling the bulged parts of the Earth somewhat ahead of the Moon.

See the section Other lunar misconceptions/Friction at



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