Square degree

A unit of solid angle equal to the area of one square degree on a sphere. One square degree is approximately 0.0003046 steradians.

Solid angle: A measure of the two-dimensional extent of a region as seen from a point, expressed in steradians. A square degree is one unit for measuring solid angle.
Square-degree to steradian conversion: The conversion between square degrees and steradians, using one steradian ≈ 3282.8 square degrees. It gives the numerical scale of the unit in SI solid-angle terms.
Celestial sphere: An imaginary sphere onto which celestial objects are projected for describing their directions. Astronomical sky areas are conventionally measured as regions on this sphere.
Full moon: The Moon’s phase when its Earth-facing hemisphere appears fully illuminated. The Moon’s apparent disk covers about 0.2 square degrees, a familiar small sky area.
Degree (angle): A unit of plane angle equal to one three-hundred-sixtieth of a full turn. The square degree derives from squaring the degree used for angular coordinates.
Spherical cap: A region of a sphere bounded by a plane cutting through the sphere. Its area illustrates how angular radius determines a solid angle.
Field of view: The angular extent of the scene visible through an optical instrument or imaging system. Instrument fields are often reported in square degrees when describing sky coverage.
Milky Way: The barred spiral galaxy that contains the Solar System. Its broad band across the sky illustrates why survey coverage is counted in thousands of square degrees.
Steradian: The SI unit of solid angle, equal to the area of a unit sphere subtended by the angle. It is the standard SI unit against which square degrees are converted.
Spherical excess: The amount by which a spherical triangle’s angle sum exceeds 180 degrees. On a unit sphere, spherical excess equals the triangle’s area in steradians.

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