# What's the most spherical astronomical object in our solar system
**URL:** https://boards.straightdope.com/t/whats-the-most-spherical-astronomical-object-in-our-solar-system/671961
**Category:** Factual Questions
**Created:** [October 22, 2013, 7:39pm UTC](https://boards.straightdope.com/t/whats-the-most-spherical-astronomical-object-in-our-solar-system/671961 "2013-10-22T19:39:35Z")
**Posts on this page:** 12
**Page:** 1
### Author:  [@Great\_Antibob](https://boards.straightdope.com/u/Great_Antibob)
#### Post date: [October 22, 2013, 7:39pm UTC](https://boards.straightdope.com/t/whats-the-most-spherical-astronomical-object-in-our-solar-system/671961/1 "2013-10-22T19:39:35Z")
Inspired by [this thread](http://boards.straightdope.com/sdmb/showthread.php?t=705674), what’s the most spherical object in our solar system? Most large bodies are oblate spheroids, so which is the least oblate?
If it makes things easier, let’s say things that have a mean radius of greater than 10km (or 100km, if that makes things easier).
As for how to define “most spherical”, I guess we can use [sphericity](https://en.wikipedia.org/wiki/Sphericity).
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### Author:  [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)
#### Post date: [October 22, 2013, 7:49pm UTC](https://boards.straightdope.com/t/whats-the-most-spherical-astronomical-object-in-our-solar-system/671961/2 "2013-10-22T19:49:52Z")
[The Sun](http://io9.com/5935742/its-official-the-sun-is-the-most-perfect-natural-sphere-ever-measured#)
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### Author:  [@Great\_Antibob](https://boards.straightdope.com/u/Great_Antibob)
#### Post date: [October 22, 2013, 7:54pm UTC](https://boards.straightdope.com/t/whats-the-most-spherical-astronomical-object-in-our-solar-system/671961/3 "2013-10-22T19:54:43Z")
Thanks. That was fast.
Ok, let’s go with “what’s the most spherical astronomical object in our solar system that isn’t the sun?”
And to make it slightly more interesting, let’s divide that into objects that are greater than 100km in radius and smaller than 100km in radius.
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### Author:  [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)
#### Post date: [October 22, 2013, 8:16pm UTC](https://boards.straightdope.com/t/whats-the-most-spherical-astronomical-object-in-our-solar-system/671961/4 "2013-10-22T20:16:52Z")
> [@Great\_Antibob](#):
>
> Thanks. That was fast.
>
> Ok, let’s go with “what’s the most spherical astronomical object in our solar system that isn’t the sun?”
[Probably Venus, though Mercury is close](http://link.springer.com/article/10.1007%2Fs10569-007-9072-y)
> [@](#):
>
> And to make it slightly more interesting, let’s divide that into objects that are greater than 100km in radius and smaller than 100km in radius.
…erm WAG: [Puck](http://en.wikipedia.org/wiki/Puck_(moon))
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### Author:  [@Gray\_Ghost](https://boards.straightdope.com/u/Gray_Ghost)
#### Post date: [October 23, 2013, 3:40am UTC](https://boards.straightdope.com/t/whats-the-most-spherical-astronomical-object-in-our-solar-system/671961/5 "2013-10-23T03:40:41Z")
Where does Europa rank? I’d think it’d be rather high, given it’s covered with a world-spanning ocean. I have seen, but cannot link, due to this friggin tablet, sites that mention it is basically flat except for a few rapidly eroding impact craters.
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### Author:  [@JKellyMap](https://boards.straightdope.com/u/JKellyMap)
#### Post date: [October 23, 2013, 11:23am UTC](https://boards.straightdope.com/t/whats-the-most-spherical-astronomical-object-in-our-solar-system/671961/6 "2013-10-23T11:23:44Z")
> [@Gray\_Ghost](#):
>
> Where does Europa rank? I’d think it’d be rather high, given it’s covered with a world-spanning ocean. I have seen, but cannot link, due to this friggin tablet, sites that mention it is basically flat except for a few rapidly eroding impact craters.
I don’t think the main factor is topography (mountains, oceans, etc.), but rather overall shape. In other words, most “spherical” planets and moons are oblate enough that the topography is relatively unimportant for this question.
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### Author:  [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)
#### Post date: [October 23, 2013, 11:49am UTC](https://boards.straightdope.com/t/whats-the-most-spherical-astronomical-object-in-our-solar-system/671961/7 "2013-10-23T11:49:09Z")
I recall reading back in the 1980’s that NASA had launched a satellite containing some small glass spheres (maybe a couple inches in diameter) for a study about the age or shape of the universe, or something like that. Anyway, they were said to be the most spherical objects ever created; the claim was that if you expanded one to the size of the earth, the deviation from perfect sphericity would have been no more than a few inches.
Since they’ve been into space, does that make them astronomical objects?
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### Author:  [@Darryl\_Lict](https://boards.straightdope.com/u/Darryl_Lict)
#### Post date: [October 23, 2013, 12:29pm UTC](https://boards.straightdope.com/t/whats-the-most-spherical-astronomical-object-in-our-solar-system/671961/8 "2013-10-23T12:29:28Z")
> [@Machine\_Elf](#):
>
> I recall reading back in the 1980’s that NASA had launched a satellite containing some small glass spheres (maybe a couple inches in diameter) for a study about the age or shape of the universe, or something like that. Anyway, they were said to be the most spherical objects ever created; the claim was that if you expanded one to the size of the earth, the deviation from perfect sphericity would have been no more than a few inches.
>
> Since they’ve been into space, does that make them astronomical objects?
They’ve built really spherical ones in 2004 for [Gravity Probe B](http://en.wikipedia.org/wiki/Gravity_Probe_B). Perfect within 40 atoms! Maybe that’s the one you are thinking of.
This [one](http://www.newscientist.com/article/dn14229-roundest-objects-in-the-world-created.html?feedId=online-news_rss20#.Ume_ofkjKzU) looks like the current record holder.
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### Author:  [@JKellyMap](https://boards.straightdope.com/u/JKellyMap)
#### Post date: [October 23, 2013, 12:41pm UTC](https://boards.straightdope.com/t/whats-the-most-spherical-astronomical-object-in-our-solar-system/671961/9 "2013-10-23T12:41:59Z")
> [@Darryl\_Lict](#):
>
> They’ve built really spherical ones in 2004 for [Gravity Probe B](http://en.wikipedia.org/wiki/Gravity_Probe_B). Perfect within 40 atoms! Maybe that’s the one you are thinking of.
>
> This [one](http://www.newscientist.com/article/dn14229-roundest-objects-in-the-world-created.html?feedId=online-news_rss20#.Ume_ofkjKzU) looks like the current record holder.
Cool! I think they need to clarify that “if the balls were blown up to Earth size, the biggest ‘mountain’ would be four feet high,” they mean IF the Earth were a sphere (but retained its topography) rather than oblate.
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### Author:  [@chrisk](https://boards.straightdope.com/u/chrisk)
#### Post date: [October 23, 2013, 1:39pm UTC](https://boards.straightdope.com/t/whats-the-most-spherical-astronomical-object-in-our-solar-system/671961/10 "2013-10-23T13:39:00Z")
> [@JKellyMap](#):
>
> Cool! I think they need to clarify that “if the balls were blown up to Earth size, the biggest ‘mountain’ would be four feet high,” they mean IF the Earth were a sphere (but retained its topography) rather than oblate.
I’m not sure that clarification adds anything. The Earth-size glass would be much less oblate than the real Earth, as well as having smaller topography. But the Earth doesn’t need to be a perfect sphere to imagine blowing a small sphere up to ‘Earth size.’
Of course, a nitpicker would start asking how you compare their sizes, referencing the polar diameter of the Earth or the equatorial diameter or whatever. 😃
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [October 23, 2013, 3:25pm UTC](https://boards.straightdope.com/t/whats-the-most-spherical-astronomical-object-in-our-solar-system/671961/11 "2013-10-23T15:25:20Z")
Actually, Europa is probably prolate, rather than oblate, since it’s mainly distorted by the tides from Jupiter.
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### Author:  [@BlinkingDuck](https://boards.straightdope.com/u/BlinkingDuck)
#### Post date: [October 23, 2013, 4:55pm UTC](https://boards.straightdope.com/t/whats-the-most-spherical-astronomical-object-in-our-solar-system/671961/12 "2013-10-23T16:55:39Z")
WAG is Venus…large object, doesn’t rotate fast.