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 / 180 radians, a square degree is equal to (π / 180)2, or about 1 / 3283 or 3.0462×10−4 steradian (0.30462 msr).
The number of square degrees in a whole sphere is approximately 41253 deg2. This is the total area of the 88 modern constellations in the sky.
- A whole sphere has 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 deg/)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.