Square degree

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A square degree is a non-SI unit measure of solid angle — that is, the area of the projection of a surface onto a unit sphere centered on the point of observation. It is the three-dimensional equivalent of the degree and is denoted "deg²."

The number of square degrees in the whole sky (a sphere) is $4π(180 / π) 2 = 129600 / π$ (that is, approximately 41,253 deg²), which can be derived from that the fact that the whole sky covers 4π steradians, and one degree is π/180 radians. Thus, one square degree is approximately 1/3283 steradian.

For example, observed from the surface of the Earth, the Moon has a diameter of approximately 0.5°, so it covers an area of approximately 0.2 deg².