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suze
1161709.  Mon Dec 07, 2015 7:21 pm Reply with quote

It's an oblate spheroid, which is "round" under the International Astronomical Union's definition of that term. I don't think anyone could seriously deny that the word "round" is used in a loose sense, presumably for the benefit of the layperson, but it has the advantage of being readily understood.

The Moon is in any case plenty rounder than the Earth. Geodesists define the "flattening" of an oblate spheroid as (semi major axis - semi minor axis) / semi major axis, and the flattening of the Earth is about 1/300. For the Moon, that figure is 1/825. Contrast with Jupiter - the least round of the round planets - which has a flattening of 1/15.

 
Peeeeeteeeee
1161710.  Mon Dec 07, 2015 7:38 pm Reply with quote

Oh, I know, I was just being annoyingly pedantic. Which is completely the right style for that question.

I wonder if there is anything, known to exist, that is really round.

 
gruff5
1161721.  Mon Dec 07, 2015 10:05 pm Reply with quote

I think electrons are thought to be perfectly round? Recent research established that, which I don't really understand, as I always thought electrons weren't like billiard balls at all.

As far as macroscopic objects, the quartz balls for the gyroscopes in the Gravity B space probes were really, really, but not perfectly round.

The old definition of a planet was that it was round (ie excluding the asteroids), but that has now been tightened up to make round objects like the asteroid Ceres and also Eris and Pluto into dwarf planets.

 
crissdee
1161750.  Tue Dec 08, 2015 6:45 am Reply with quote

I would have thought the balls in precision bearings would be as round as it is possible to create something. The tolerances would be in the order of microns at most. PDR could probably come up with specifications for such things.

 
PDR
1161759.  Tue Dec 08, 2015 7:44 am Reply with quote

Well the balls in a typical low-spec ball bearing will be ground to size and eccentricity tolerances of around 0.6nanometres, whilst for the really high-spec ones the tolerance would be around 0.1nm. The surface finish tolerances are (of course) 1-3 orders of magnitude tighter.

But there can be "rounder" things - a droplet of pure water or oil floating inside a spacecraft, for instance.

There have been experiments making ball bearings in orbit by releasing measured quantities of molten metal into a vacuum chamber, but whilst the size and eccentricity are easily achieved the surface finish cannot, so the resulting balls still need at least lapping, and sometimes grinding. TThe processes which are used to make ball bearings are now so widely used that the manufacturing cost of the low precision ones is fractions of a penny each, and only 10-20p each for the really good ones. SO the cost of making them in space is unlikley to be competitive.

PDR

 
gruff5
1161849.  Wed Dec 09, 2015 4:01 am Reply with quote

See my post mentioning the quartz balls (aka bearings) in the Gravity B space probe - the most round things ever manufactured by humans.

As for natural, macroscopic objects - perhaps a neutron star with its incredibly density and gravity would be the most round? If you could find one that was hardly rotating, or even not rotating, that would be an absolute sphere with no features at all.

 
PDR
1161871.  Wed Dec 09, 2015 7:39 am Reply with quote

That would depend on whether you meant "measurably spherical", "discernibly non-spherical" or "absolutely spherical".

Ignoring the detail that you probably can't actually observe the surface of a neutron star, if it is rotating AT ALL (no matter how slowly) then it will not be absolutely spherical because its "gravitoclines" will be distorted by the rotation. The error might be very, very small, to the point of being completely unmeasurable even if you had a large enough vernier micrometer, but as people of science our integrity would never allow us to overlook it.

It would also need to be located either as the sole object in a universe of its own (unlikely, given the high demand for housing in this area) or at a gravitationally neutral point in the universe such that the ambient gravitational field had absolutely no effect on its gravitoclines. This isn't impossible, but it's very unlikely.

And of course the act of measuring its sphericality would inevitably interfere with the local gravitational field and thereby distort the neutron star, but that's as much a philosophical question as a scientific one.

PDR

 
crissdee
1161881.  Wed Dec 09, 2015 8:49 am Reply with quote

PDR wrote:
......... but as people of QI our pedantry would never allow us to overlook it.


Corrected that for you.

 
PDR
1161910.  Wed Dec 09, 2015 11:59 am Reply with quote

You did that like pedantry is somehow a bad thing!

How does that work then?

:o)

PDR

 
Peeeeeteeeee
1161983.  Wed Dec 09, 2015 7:00 pm Reply with quote

I have been told that steel ball bearings could never be perfectly spherical because of the way they are made. Ceramic ones could be, but they are not. They are usually closer.

 
Peeeeeteeeee
1161989.  Wed Dec 09, 2015 7:15 pm Reply with quote

Actually, I seem to remember (not sure where this came from), that the sun is rounder than any planet or moon.

 
PDR
1161991.  Wed Dec 09, 2015 7:22 pm Reply with quote

Peeeeeteeeee wrote:
I have been told that steel ball bearings could never be perfectly spherical because of the way they are made. Ceramic ones could be, but they are not. They are usually closer.


That's a little strange since the finished dimensions of both are done by roller-lapping in spiral disk laps - its the same process.

PDR

 
PDR
1161992.  Wed Dec 09, 2015 7:24 pm Reply with quote

Peeeeeteeeee wrote:
Actually, I seem to remember (not sure where this came from), that the sun is rounder than any planet or moon.


That would be hard to establish, because the "sufrace" of the sun isn't very well defined (what with it being a mass of fusing gas rather than a rock and all).

PDR

 
gruff5
1162013.  Wed Dec 09, 2015 9:25 pm Reply with quote

Yep, I was too casual with my use of the word "absolute" there, PDR. I meant as spherical as it is possible to get in a physical universe. You still have the neutrons themselves providing "lumps & bumps", so it could never be "absolutely" spherical - nothing can be.

I suppose the Sun is rotating at 36 hours per Earth day, which is a lot slower than Jupiter or the other gas giants, so that would make it less of an oblate spheroid than those. Moons will generally have tidal bulges affecting their spherocity.

 
Posital
1162072.  Thu Dec 10, 2015 8:08 am Reply with quote

Even floating drops of water or oil will probably have some residual rotational energy to be accounted for due to turbulence in their production.

 

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