Is the Sun really spherical?

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Offline thedoc

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Is the Sun really spherical?
« on: 22/08/2012 14:30:02 »
Andrew Papworth  asked the Naked Scientists:
I read recently that scientists have measured the Sun's dimensions very accurately and were astonished to find it is almost a perfect sphere.

I always understood that the Sun was largely gas with no clear boundary edge unlike, say, the Earth.   So there is not firm surface from which, for instance, to bounce signals. So how do they do this accurate measurement?

What do you think?
« Last Edit: 22/08/2012 14:30:02 by _system »


Offline CliffordK

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Re: Is the Sun really spherical?
« Reply #1 on: 22/08/2012 19:34:34 »
It might be difficult to "bounce" many signals off the sun.  Certainly nothing in the visible light spectrum.  I would presume one would use the light emitted from the sun for the measurements.

I believe the top of the photosphere of the sun is quite distinct, and could provide a fairly precise measurement of the size of the sun. 

I'm surprised that the equatorial bulge is not greater (it is supposed to be a difference of about 10 km out of  1,380,000 km diameter).

Different parts of the sun rotate at different speeds, but the rotational period at the equator is about 25 days (34 at the poles) which seems relatively slow, although it is a large object. 

The equatorial surface gravity is about 274.0 m/s2, or 28 g.

So, the intense gravity and relatively slow rotation cause it to be quite spherical in shape.

Both Mercury and Venus have very slow rotational periods, and are also listed as having very little equatorial bulge (flattening).