A square degree (deg2) is a non-SIunit 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)2steradians (sr), or about 1/3283 sr or 3.0461741978670859934×10−4 sr.
The whole sphere has a solid angle of 4πsr which is 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)2, or 0.2 square degrees. The moon varies from 0.188 to 0.244 deg2 depending on its distance to the Earth.
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.
Conversely, an average full moon (or the Sun) covers a 2 / 210100 fraction, or less than 1/1000 of a percent (0.00000952381) of the celestial hemisphere, or above-ground "sky".
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 1.14 deg2 and 1.16 deg2, respectively.
The largest constellation, Hydra, covers a solid angle of 1303 deg2, whereas the smallest, Crux, covers only 68 deg2.