# why are planets round?

**URL:** <https://boards.straightdope.com/t/why-are-planets-round/248045>\
**Category:** Factual Questions\
**Created:** [June 1, 2004, 11:02am UTC](https://boards.straightdope.com/t/why-are-planets-round/248045 "2004-06-01T11:02:22Z")\
**Posts on this page:** 4\
**Page:** 2

<div class="post-metadata">

**Author:** ![RM\_Mentock](https://avatars.discourse-cdn.com/v4/letter/r/e274bd/32.png) [@RM\_Mentock](https://boards.straightdope.com/u/RM_Mentock)\
**Post date:** [June 3, 2004, 5:01am UTC](https://boards.straightdope.com/t/why-are-planets-round/248045/21 "2004-06-03T05:01:58Z")

</div>

> [@Mathochist](#):
>
> Oh? And there is no comparably defined “geoid” for, say, Venus?

Of course there is, but that is pretty much the point, that **Jurph** and **nocturnal\_tick** were also trying to make. The Earth’s shape is unique–“geoid” is not a description of it, as “oval” or “sphere” or “oblate spheroid” are.

And the topographic variation of the geoid is only a couple hundred meters, an order of magnitude less than the actual “surface”.

[quote]

> [@OldGuy](#):
>
> > [@Mathochist](#):
> >
> > Tipped 177 degrees _from what_?
> 
> The tipping is either measured relative to a line orthogonal to the ecliptic or to a line orthogonal to the palnet’s own plane of rotation. I forget which for sure, but believe it’s the latter.

[It appears to be](http://planetscapes.com/solar/eng/terms.htm#retrograde) the former. Makes sense when you think about it. It seems geocentric, but the Earth’s orbit just happens to align with the general position of the Sun’s own angular momentum.

I went back to **Ice Wolf** ’s link, which says that “The only way to get all the mass as close to planet’s center of gravity as possible is to form a sphere. The technical name for this process is ‘isostatic adjustment.’”

I would have said just the opposite. Isostasy is what allows us to have topography–deviations from the spherical–the mountains are compensated by lower density roots, kinda like icebergs floating in water.

---

<div class="post-metadata">

**Author:** ![Max\_Carnage](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/max_carnage/32/12775_2.png) [@Max\_Carnage](https://boards.straightdope.com/u/Max_Carnage)\
**Post date:** [June 3, 2004, 2:40pm UTC](https://boards.straightdope.com/t/why-are-planets-round/248045/22 "2004-06-03T14:40:28Z")

</div>

Because otherwise they’d fall into the manhole too easily.

No wait, what was the question again?

---

<div class="post-metadata">

**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\
**Post date:** [June 15, 2004, 11:11am UTC](https://boards.straightdope.com/t/why-are-planets-round/248045/23 "2004-06-15T11:11:48Z")

</div>

> [@RM Mentock](#):
>
> Not “a geoid”, _the_ [geoid](http://www.straightdope.com/mailbag/mgeoid.html). There is only one.

This is quite interesting, because the ~oid suffix means “resembling or having a similar appearance to”, but if there’s only one geoid, then aren’t we just saying that the Earth resembles itself?

---

<div class="post-metadata">

**Author:** ![Mathochist](https://avatars.discourse-cdn.com/v4/letter/m/c89c15/32.png) [@Mathochist](https://boards.straightdope.com/u/Mathochist)\
**Post date:** [June 15, 2004, 2:02pm UTC](https://boards.straightdope.com/t/why-are-planets-round/248045/24 "2004-06-15T14:02:49Z")

</div>

> [@Mangetout](#):
>
> This is quite interesting, because the ~oid suffix means “resembling or having a similar appearance to”, but if there’s only one geoid, then aren’t we just saying that the Earth resembles itself?

Ladies and gentlemen, winner of the delayed-action woosh prize…

[Previous page](https://boards.straightdope.com/t/why-are-planets-round/248045.md?page=1)