A square degree (deg2) is a non-SI-compliant unit measure of solid angle. Other denotations include sq. deg. and (°)2. Just as degrees are used to measure parts of a circle, square degrees are used to measure parts of a sphere. Analogous to one degree being equal to π/ radians, a square degree is equal to (π/)2 squared radians (i.e. steradians or sr), or about 1/ sr = 3.0462×10−4 sr (0.30462 msr).
The number of square degrees in a whole sphere is approximately 41253 deg2.
- A whole sphere has approximately 41253 deg2.
- The full moon covers only about 0.2 deg2 of the sky when viewed from the surface of the Earth. The Moon is only a half degree across (i.e. a circular diameter of roughly 0.5 deg), so the moon's disk covers a circular area of: π × (0.5°/)2, or 0.2 square degrees.
- Viewed from Earth, the Sun is roughly half a degree across (the same as the full moon) and covers only 0.2 deg2 as well.
- It would take 210100 times the full moon (or the Sun) to cover the entire celestial sphere.
- Assuming the Earth to be a sphere with a surface area of 510 million km2, the area of Northern Ireland (14130 km2) and Connecticut (14357 km2) represent a solid angle of 1.1 deg2 and 1.2 deg2, respectively.
- The largest constellation, Hydra, covers a solid angle of 1303 deg2, whereas the smallest, Crux, covers only 68 deg2.