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, or about 1/ = ×10−4 steradians (0.30462 msr). 3.0462
The number of square degrees in a whole sphere is approximately 253 deg2. This is the total area of the 4188 modern constellations in the sky.
- A whole sphere has approximately 253 deg2. 41
- 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 (130 km2) and 14Connecticut (357 km2) represent a solid angle of 1.1 deg2 and 1.2 deg2, respectively. 14
- The largest constellation, Hydra, covers a solid angle of , whereas the smallest, 1303 deg2Crux, covers only 68 deg2.