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Mathematicians much more often use radii over diameters when discussing about circles and spheres, because in mathematics the radius is more fundamental than the diameter (the sphere is defined using its radius).

But what about in astronomy? Do astronomers and astrophysicist more often use diameters or radii when characterizing planets, dwarf planets, exoplanets and stars?

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  • $\begingroup$ It depends on what you are going to do with it (and no, in mathematics the radius is not more fundamental than the diameter. It may get used more often because it is often the more convenient, but than does not make it more fundamental (whatever that might mean in this context)). $\endgroup$ – Conrad Turner May 18 '15 at 12:54
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Definitely radii, with one notable exception...

When observers talk about how large an object is on the sky, they usually discuss angular size, which is related to the diameter of an object, not the radius. So when discussing the angular size of, say, Alpha Centauri A (a few milliarcseconds), this is related to the diameter of the star, not the radius.

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    $\begingroup$ I agree with @Warrick that radii are more used, but would like to add an important point: Being a branch of physics, astronomy would probably be considered an exact science, but often uncertainties are so large, that we don't really care whether we talk about radius or diameter, instead just quoting a characteristic size of a system. For instance the size of an atom, a molecular cloud, a star-forming region, an intergalactic gas filament, or the size of the Universe during inflation. $\endgroup$ – pela Mar 18 '15 at 8:08

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