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It is stated that the Earth is a 'squashed sphere' due to the very slight bulge at the equator. (Thought in reality it's such a small difference, it's essentially spherical)

Typical values:

Equatorial Diameter: 12,756 km Meridional Diameter: 12,714 km

I'm looking at why the Earth has this tiny bulge.

Some sources I read talk about the tidal effect from the moon. But others talk about the Earth spinning causing this.

The books I have (and videos I've watched) just talk about this things separately and don't really explain the difference

Surely the bulge due to a mixture of both? which one has a greater effect? Does the rotational bulge not effect the oceans or something? Does the tidal bulge only effect the water? I'm really confused


marked as duplicate by John Rennie, user191954, Qmechanic Sep 26 '18 at 11:37

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.


The bulge is due to the rotation of the earth. The moon does cause “earth tides” that slightly affect the shape of the earth, but this phenomenon is about five orders of magnitude smaller than the equatorial bulge.

  • $\begingroup$ The size of the effect is a good indication. Also, the shape for the rotational bulge is an oblate spheroid, but the shape for the tidal bulge is a prolate spheroid. $\endgroup$ – Dale Sep 26 '18 at 3:18

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