A radius given as an angle rather than a distance. Used to describe the apparent size of round things (the angle they subtend when we look at them) rather than their actual size; this is useful with respect to views of heavenly bodies and when discussing the resolving power of telescopes.

The sun and the moon have almost exactly the same angular radius as viewed from Earth, about quarter of a degree; this is why lunar eclipses and solar eclipses are both possible on Earth. The moon's angular radius varies between about 15 and 17 arcminutes according to how far from the Earth it is. The angular radii of the planets as seen from Earth are measured in arcseconds, which is to say a few 3,600ths of a degree. This may sound pretty small, but it's big enough that the planets generally don't twinkle like stars, which is often the best way to tell that what you're looking at is a planet. The 22º halo and the rarer 46º halo are forms of aerial spectra named for their angular radii; they can sometimes be seen in the sunlight scattered by a cirrostratus on a cold, hazy day.

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