Square degree

A square degree 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, or about sr or about.
The whole sphere has a solid angle of which is approximately :

Subdivisions

Each square degree is equal to 3600 square arcminutes, and each square arcminute is equal to 3600 square 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.

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, 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 and covers only as well.
It would take times the full moon to cover the entire celestial sphere.
Conversely, an average full moon 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.