Square degree

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 = 3.0462×10−4 sr (0.30462 msr).

Square degree
Unit ofSolid angle
Symboldeg2 
Conversions
1 deg2 in ...... is equal to ...
   SI units   3.04617×10−4 sr

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

Examples
  • 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.
  • 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.[1]

See also

References
  1. RASC Calgary Centre - The Constellations
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