An illustration of a solid angle formed by a rectangular pyramid, whose apex angles are both 10 degrees. The resulting solid angle is about 99.747369 square degrees. The size of Moon, as seen from Earth, is shown in yellow for comparison.
Asquare degree (deg2) is a non-SIunit measure ofsolid angle. Other denotations includesq. deg. and (°)2. Just asdegrees are used to measure parts of a circle, square degrees are used to measure parts of asphere.
Analogous to one degree being equal toπ/180 radians, a square degree is equal to (π/180)2steradians (sr), or about1/3283 sr or about3.046×10−4 sr.Thewhole sphere has a solid angle of4πsr which is approximately41253 deg2:
Each square degree is equal to 3600square arcminutes, and each square arcminute is equal to 3600square arcseconds. So, each square degree is equal to 12,960,000 square arcseconds. The whole sphere contains about 148.51 million square arcminutes and about 534.64 billion square arcseconds.
Thefull moon covers only about0.2 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 roughly0.5°), 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 to0.244 deg2 depending on its distance from the Earth.
Viewed from Earth, theSun is roughly half a degree across (the same as the full moon) and covers only0.2 deg2 as well.
It would take210100 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 ofNorthern Ireland (14130 km2) represents a solid angle of1.14 deg2,Connecticut (14357 km2) represents a solid angle of1.16 deg2,Equatorial Guinea (28050 km2) represents a solid angle of2 deg2.
The largest constellation,Hydra, covers a solid angle of1303 deg2, whereas the smallest,Crux, covers only68 deg2.[1]
