square degree

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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 :

: $4 \pi \left(\frac\right)^2 \, ^2 = \frac\,\, ^2 = \frac \,\, ^2 \approx 41\,252.96 \,\, ^2$

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 to 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 .

See also

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

References

External links

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{{DEFAULTSORT:Square Degree
Units of solid angle