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Square degree
From Wikipedia, the free encyclopedia
A square degree is a non-SI unit that can be used to measure solid angles (that is, the area of the projection of a surface onto a unit sphere centered on the point of observation). It is the two-dimensional equivalent of the degree and may be denoted "sq.deg.", "deg²", or a symbol of a square followed by °.
The number of square degrees in the whole sky (a sphere) is 129,600/π (that is, approximately 41,253 sq.deg.), which can be derived from that the fact that the whole sky covers 4π steradians, and one degree is π/180 radians. Thus, one square degree is approximately 1/3283 steradian or 305 microsteradians.
For example, observed from the surface of the Earth, the Moon has a diameter of approximately ½º, so it covers an area of approximately 0.2 sq.deg.
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