View Full Version : Spherical objects

afterburner

2006-Aug-09, 10:10 PM

Is the Sun a perfect sphere? Or at least the most perfect in our solar system? I've heared of a silicone sphere that was supposed to be the most perfect sphere ever created, but Im not sure how it compares to spheres in nature.

Also, would a star like Antares be a more perfect sphere? Or perhaps some other massive star?

Thanks

01101001

2006-Aug-09, 11:31 PM

Is the Sun a perfect sphere?

Oblate spheroid. It rotates. It roils.

Or at least the most perfect in our solar system? I've heared of a silicone sphere that was supposed to be the most perfect sphere ever created, but Im not sure how it compares to spheres in nature.

Gravity Probe B (http://einstein.stanford.edu/)

The Engineering Story :: Precision Spheres for Gravity Probe B Experiment (GP-B)

From an analysis of the electrical torques on the rotors it was determined that in order to reduce the drift rate due to asphericity to less than 0.0001 arc sec per year, the rotor spheres have to be polished to 1 micro inch (25 nm) or to deviate no more than ± 0.5 micro inch (12 nm) from a perfect sphere.

Not perfect. I'll let you do the math to scale it to sun size.

Tensor

2006-Aug-09, 11:40 PM

Is the Sun a perfect sphere? Or at least the most perfect in our solar system? I've heared of a silicone sphere that was supposed to be the most perfect sphere ever created, but Im not sure how it compares to spheres in nature.

Check back on the those silicon spheres. The oblateness (how much difference from a perfect sphere) is 9×10−6. Which simply means that the difference betweem the equatorial diameter and the polar diameter is 10 km over 1,391,980 km. I can find nothing this precise for Mercury and Venus, whose oblateness is given as zero, a perfect sphere.

Also, would a star like Antares be a more perfect sphere? Or perhaps some other massive star?

Thanks

The oblateness of an object depends on it rotation, gravity, and composition. The faster a less dense, object rotates, the higher the oblateness (check out a picture of Saturn, you can see the difference). While the sun's density is low, it gravity is large and it's rotation is slow. Giving it's oblateness such a small value. Mercury and Venus are comparitively more dense, and rotate much slower, which may explain the zero for their oblatenes.

edit: I see oy vey;) has given you the numbers for the spheres. You should be able to do the math and figure out how it compares to the sun.

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