A square degree (deg²) is a non-SI 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 steradians (sr), or about 1/3283 sr or about 3.046×10⁻⁴ sr.

The whole sphere has a solid angle of 4πsr which is approximately 41253 deg²:

${\displaystyle 4\pi \left({\frac {180}{\pi }}\right)^{2}\,{\deg }^{2}={\frac {360^{2}}{\pi }}\,\,{\deg }^{2}={\frac {129\,600}{\pi }}\,\,{\deg }^{2}\approx 41\,252.96\,\,{\deg }^{2}}$

• 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°), 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 from 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-the-horizon sky.
• Assuming the Earth to be a sphere with a surface area of 510 million km², the area of Northern Ireland (14130 km²) represents a solid angle of 1.14 deg², Connecticut (14357 km²) represents a solid angle of 1.16 deg², Equatorial Guinea (28050 km²) represents a solid angle of 2 deg².
• The largest constellation, Hydra, covers a solid angle of 1303 deg², whereas the smallest, Crux, covers only 68 deg².^[1]

• Steradian
• Spat (angular unit)
• Minute and second of arc
• List of constellations by area