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Comment by madaxe_again | Hacker News Reader

I'm not sure what you're getting at? I'm not explaining climate change, I'm simply explaining that sea level rise isn't uniform.

The total volume of water increases both due to melting of on-land ice and thermal expansion of the water column - however it is the case that the resulting increased volume is not evenly distributed, due to the factors I mention above.

If you're getting at "crustal thickness" - there are areas of thin or less dense crust where the value of g is lower by a tiny amount - but this tiny amount is enough to cause less water to gather in that area over the vast depth of the water column, as it's essentially uphill to there.

https://en.wikipedia.org/wiki/Gravity_of_Earth#Local_topogra...

https://en.wikipedia.org/wiki/Gravity_anomalies_of_Britain_a...

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In other words the ocean is on a scale which makes it much more complicated than your bathtub.

Any contiguous fluid surrounding a solid body will form a sphere around the system's center of gravity. Currents, tides, and temperature may cause some deformities, but the variations on the force of gravity due to the the solid earth's nonuniformity in shape and density won't effect the 'level' of the sphere of water that surrounds the center of gravity.

> Any contiguous fluid surrounding a solid body will form a sphere around the system's center of gravity.

Did you just disprove tides?

Any contiguous fluid in hydrostatic equilibrium will form a shape such that the potential is constant across connected surfaces. If there were a potential difference, this would cause movement on the surface, which means that the fluid won't be in hydrostatic equilibrium anymore. (This is the mechanism that moves the tides: the constantly changing gravity requires a constant movement of the water.)

The equilibrium surface is a sphere only if the whole fluid is connected (otherwise fluids on different planets would be required to form a single sphere), and if the potential is spherically symmetric. That symmetry almost never holds exactly, it's just a convenient assumption, e.g. when distances are so large that bodies might as well be point masses.

When multiple bodies of comparable mass are involved, e.g. in the solar system, you can't completely ignore their influence. The gravity of sun and moon on the surface of Earth is still strong enough to cause the tides, i.e. differences in water level of several meters. If you measure with sufficient precision, even comparably small objects like tectonic plates can have a non-negligible effect.

EDIT: If you disagree, can you explain how my understanding of the physics is wrong?

You're spot on.

I think we're debating with astroturfers - my comments on this thread have been to -8 and back. This is a climate change related article after all, so all science is just like, your opinion, man, and no more valid than Bob the slime mould's thoughts. Over to you, Bob.

Sorry, but you’re incorrect. Have you ever noticed how the ocean rises and falls sometimes? We call these tides, and they are caused by tidal forces (gravity) predominantly from the moon and sun. This is by far the most commonly observed gravitationally powered distortion from a perfect sphere.

I understand that you might never have seen a body of water large enough to witness the phenomenon, as many folks live far inland, but I assure you, tides are real, and are no conspiracy.

Additionally, the earth rotates rather rapidly. I know this may come as a surprise, but it actually causes the earth to bulge out at the equator - hardly a perfect sphere.

The below articles might help you, but only if you choose to read them under the understanding that you might need to learn something new.

If you don’t want to, that’s also fine.

https://en.m.wikipedia.org/wiki/Tide

https://en.m.wikipedia.org/wiki/Geoid

https://en.m.wikipedia.org/wiki/Equatorial_bulge

https://simple.m.wikipedia.org/wiki/Tide

The intuition that prompted my post was wrong. A fluid encompassing a solid with nonuniform density will not form a sphere, but will be 'warped'. Duh. Disregard my post. Cheers.

No worries. I think I know what you fell into, which is that were the distortion always and only due to gravity, the value of g at actual (not mean) sea level would always be the same, as the additional displaced water would perfectly compensate for the variance caused by any underlying variation.

Sorry that I was snarky - I have a hard time helping myself first thing in the morning. Glad to have shed some light.

I was thinking that any solid -- such as a planet -- can be replaced by a point at its center with equal mass, and its neighbors -- such as a moon -- wouldn't notice. But that's not entirely true.... For instance, replace the earth with an equal size ball of styrofoam, and place two super-dense basketball-sized masses, one in Californis and the other in Africa (I.e. on the opposite sides of the globe.) And, assume, the entire system is the same mass as our earth, assume the 'center of mass' is at the core of the styrofoam planet, and assume the styrofoam is magically able to keep our two Neutron-star-dense basketballs in place. Phew....

Now, the moon probably wouldn't notice s thing. From its point of view, it will 'feel' the same attraction as it would from our planet. But, something much closer -- such as a mile or two of water -- would definitely bunch up around the basketballs and would not form a sphere around the center of our imagine planet.

Is that right?

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