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Comment by tamimio | Hacker News Reader

> Caused by different densities of rock far beneath Earth's surface, these variations in gravity are small in absolute terms. But they can have particularly large effects on the oceans. Where gravity is weaker, the ocean surface can sit slightly lower relative to Earth's center because water flows away toward areas of stronger gravity. Due to its gravity hole, the sea-surface height around Antarctica is measurably lower than it would otherwise be.

Isn’t that just a result of earth rotation, and not some magical rocks far beneath earth’s surface!? Plus, that’s not how gravity works anyway, this sounds like some high school way to explain gravity.. in fact, no one knows what gravity is, even quantum physics can’t explain gravity.

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There are a lot of rocks beneath earth's surface, and it shouldn't be surprising or magical that some of them are heavier than others. Which does affect local gravity, especially if we talk about the rocks close enough to the surface.

Take for example a look at https://www.esa.int/ESA_Multimedia/Images/2015/04/Bouguer_gr... which has better resolution than the map in the article and tries to correct for oceans and mountains. There are clear deviations from the norm, and they are not shaped in a way that can be explained by simple rotational effects (even though those also exist)

> Plus, that’s not how gravity works anyway, this sounds like some high school way to explain gravity.. in fact, no one knows what gravity is, even quantum physics can’t explain gravity.

There's an impressive amount of irony in these juxtaposed statements.

Geophysicist here. Look up potential fields methods in geophysics.

Such things have been measured and mapped for quite some time.

The grandfather of all modern gravimeters was invented in 1936 by LaCoste and Romberg.

Now, why did dowsing rods suddenly pop into my mind?

If you weren't accounting for earths rotation, the area with lowest gravity would be near the equator due to the larger distance to earths center.

So that distance implies a higher volume of stuff that you're standing on top of, but the mass of the stuff is what counts for gravitational attraction.

Yes, the mass and you distance to it counts. And one of the biggest influences on that mass in absolute terms is earths very dense inner and outer cores.

Due to the "equatorial bulge" (actual term) you are further away from that heavy center, than at the equator. And due to the inverse square law, you lose intensity of that gravitational force very quickly, the further you are away.

Hence: the smallest gravitational force you get is on a mountain in the Andes near the equator.

If you weren't accounting for earth's rotation, why would points on the equator have a larger difference to earth's center?

Because due to rotation the earth bulges outward at the equator. The actual term is "Equatorial bulge".

Due to that "equatorial bulge" you are further away from earths dense center, than at the equator. And due to the inverse square law, you lose intensity of that gravitational force very quickly, the further you are away.

Hence: the smallest gravitational force you get is on a mountain in the Andes near the equator.

> it is weakest beneath the frozen continent of Antarctica *after accounting for Earth's rotation*.

> in fact, no one knows what gravity is

Albert Einstein has entered the chat

We know how it behaves, not what it is [1].

[1] https://en.wikipedia.org/wiki/Graviton

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