  The specialised uses of solid angles mean that they are quite unfamiliar
quantities. This article, apart from making solid angles a little more
familiar, brings out several topics of general interest, such as how units are
interrelated and how equations depend on the choice of units. Although the
steradian is commonly used as the unit for solid angle, another unit, the
square degree, is used in astronomy, and a unit introduced here, the solid
degree (with 360 solid degrees in a hemisphere) could be used with benefits
that are similar to those of the degree when it is used as the unit for plane
angle. The article, which is suitable for students at A-level and introductory
undergraduate level, also shows how solid angles can provide a gentle
introduction to crystal structure, spherical trigonometry and non-Euclidean
geometry.
