Square degree explained

Square degree
Quantity:	Solid angle
Symbol:	deg^2
Units1:	SI units
Inunits1:	≈

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  radians, a square degree is equal to ² steradians (sr), or about  sr or about .

The whole sphere has a solid angle of which is approximately :

4 \pi \left( (180)/(\pi) \right) ^{2} {\deg} ^{2} = (360 ^{2})/(\pi) {\deg} ^{2} = (129600)/(\pi) {\deg} ^{2} ≈ 41252 . 96 {\deg} ^{2}

Examples

• The full moon covers only about 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), so the moon's disk covers a circular area of: ², or 0.2 square degrees. The moon varies from 0.188 to 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 as well.
• It would take times the full moon (or the Sun) to cover the entire celestial sphere.
• Conversely, an average full moon (or the Sun) covers a 2 / fraction, or less than 1/1000 of a percent 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 represents a solid angle of, Connecticut represents a solid angle of, Equatorial Guinea represents a solid angle of .
• The largest constellation, Hydra, covers a solid angle of, whereas the smallest, Crux, covers only .^[1]

See also

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

External links

• Web site: Square Degrees - the Area of something on the sky . The RASC Calgary Centre . 2018-11-05 . 2022-01-21.

Notes and References

1. Web site: RASC Calgary Centre - The Constellations. 2022-02-16. calgary.rasc.ca.