<p>Given the recently-announced observations from VLT/SPHERE that 10 Hygiea may be sufficiently round to qualify as the second main-belt dwarf planet, I found myself perusing Wikipedia's  <a href="https://en.wikipedia.org/wiki/Hydrostatic_equilibrium" rel="noreferrer">Hydrostatic Equilibrium</a> article, and came across the following passage, which currently has no linked references:</p>

<blockquote>
  <p>The smallest body confirmed to be in hydrostatic equilibrium is the dwarf planet Ceres, which is icy, at 945 km, whereas <strong>the largest body known to not be in hydrostatic equilibrium is the Moon</strong>, which is rocky, at 3,474 km.</p>
</blockquote>

<p>I'd long assumed that the Moon was in hydrostatic equilibrium, given its spherical-to-the eye shape and ranking among the 20 largest solar system objects. If the quoted statement is true, why isn't it in hydrostatic equilibrium?</p>

CC BY-SA 4.0
2019-11-23T16:55:03.960Z
That might be just an issue with wikipedia authors sometimes using non-standard notions, but this passage should have a [citation needed]. Not being in hydrostatic equilibrium would mean that there's a net acceleration inside the body, and I can't see any rocks flying off from the Moon.
AtmosphericPrisonEscape
CC BY-SA 4.0
2019-11-23T18:55:50.017Z
@AtmosphericPrisonEscape - While the Moon is not in hydrostatic equilibrium, the wikipedia article is nonetheless incorrect. There are several larger bodies in the solar system that are not in hydrostatic equilibrium, one of which is our own Earth.
David Hammen
CC BY-SA 4.0
2019-11-23T19:32:15.710Z
@David Is that related to the lunar mascons?
PM 2Ring
CC BY-SA 4.0
2019-11-23T20:03:56.440Z
@PM2Ring - That's part of it ("it" being that the Moon strictly speaking is not in hydrostatic equilibrium). Another aspect is the Moon's fossil tidal bulge mentioned in antispinwards' answer.
David Hammen
CC BY-SA 4.0
2019-11-23T20:06:49.233Z
When I use "hydrostatic equilibrium" in an answer, I almost always pre-qualify that phrase with "more or less". Hydrostatic equilibrium is a spherical cow.
David Hammen
CC BY-SA 4.0
2021-06-17T11:24:36.137Z
@DavidHammen could you amplify this comment. Is it something to do with defining the word "hydrostatic" because surely both the Moon and the Earth are in equilibrium on any interesting timescale? Or is it just  a question of degree of them being slightly discrepant from the precise shape they should be if they were in equilibrium?
ProfRob
CC BY-SA 4.0
2021-06-17T13:40:08.153Z
@ProfRob With regard to the Moon, the accepted answer is correct. The knowledge that the Moon is not in hydrostatic equilibrium goes back to Laplace. The Moon was more or less in hydrostatic equilibrium from its formation to about 4 billion years ago, at which time the Moon's mantle and crust became too rigid to deform to match changes in tidal and rotational stresses.
David Hammen
CC BY-SA 4.0
2021-06-17T13:40:11.270Z
With regard to the Earth, its observed $J_2$ value is slightly inconsistent with the value implied by hydrostatic equilibrium combined with the observed rotation rate and moments of inertia. Moreover, there are places in the far north where sea level is falling. The land is rising, which is a sign the Earth is slowing trying to restore hydrostatic equilibrium.
David Hammen
<the-moon><planetary-formation><hydrostatic-equilibrium>
Is the Moon not in hydrostatic equilibrium?
notovny
# Is the Moon not in hydrostatic equilibrium?

(Asked by: username_0 on 2019-11-23)

Given the recently-announced observations from VLT/SPHERE that 10 Hygiea may be sufficiently round to qualify as the second main-belt dwarf planet, I found myself perusing Wikipedia's [Hydrostatic Equilibrium](https://en.wikipedia.org/wiki/Hydrostatic_equilibrium) article, and came across the following passage, which currently has no linked references:

> The smallest body confirmed to be in hydrostatic equilibrium is the dwarf planet Ceres, which is icy, at 945 km, whereas **the largest body known to not be in hydrostatic equilibrium is the Moon**, which is rocky, at 3,474 km.

I'd long assumed that the Moon was in hydrostatic equilibrium, given its spherical-to-the eye shape and ranking among the 20 largest solar system objects. If the quoted statement is true, why isn't it in hydrostatic equilibrium?

**Tags:** the-moon, planetary-formation, hydrostatic-equilibrium
> **Comments:**
> **username_1**: That might be just an issue with wikipedia authors sometimes using non-standard notions, but this passage should have a [citation needed]. Not being in hydrostatic equilibrium would mean that there's a net acceleration inside the body, and I can't see any rocks flying off from the Moon.  
> **username_2**: @AtmosphericPrisonEscape - While the Moon is not in hydrostatic equilibrium, the wikipedia article is nonetheless incorrect. There are several larger bodies in the solar system that are not in hydrostatic equilibrium, one of which is our own Earth.  
> **username_3**: @David Is that related to the lunar mascons?  
> **username_2**: @PM2Ring - That's part of it ("it" being that the Moon strictly speaking is not in hydrostatic equilibrium). Another aspect is the Moon's fossil tidal bulge mentioned in antispinwards' answer.  
> **username_2**: When I use "hydrostatic equilibrium" in an answer, I almost always pre-qualify that phrase with "more or less". Hydrostatic equilibrium is a spherical cow.  
> **username_4**: @DavidHammen could you amplify this comment. Is it something to do with defining the word "hydrostatic" because surely both the Moon and the Earth are in equilibrium on any interesting timescale? Or is it just  a question of degree of them being slightly discrepant from the precise shape they should be if they were in equilibrium?  
> **username_2**: @ProfRob With regard to the Moon, the accepted answer is correct. The knowledge that the Moon is not in hydrostatic equilibrium goes back to Laplace. The Moon was more or less in hydrostatic equilibrium from its formation to about 4 billion years ago, at which time the Moon's mantle and crust became too rigid to deform to match changes in tidal and rotational stresses.  
> **username_2**: With regard to the Earth, its observed $J_2$ value is slightly inconsistent with the value implied by hydrostatic equilibrium combined with the observed rotation rate and moments of inertia. Moreover, there are places in the far north where sea level is falling. The land is rising, which is a sign the Earth is slowing trying to restore hydrostatic equilibrium.  

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## 0 answers

