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A square degree (deg
2) is a non-
SI unit
Unit may refer to:
Arts and entertainment
* UNIT, a fictional military organization in the science fiction television series ''Doctor Who''
* Unit of action, a discrete piece of action (or beat) in a theatrical presentation
Music
* ''Unit'' (a ...
measure of
solid angle
In geometry, a solid angle (symbol: ) is a measure of the amount of the field of view from some particular point that a given object covers. That is, it is a measure of how large the object appears to an observer looking from that point.
The poi ...
. 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
A sphere () is a geometrical object that is a three-dimensional analogue to a two-dimensional circle. A sphere is the set of points that are all at the same distance from a given point in three-dimensional space.. That given point is the c ...
. 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 :
:
Examples
* The
full moon
The full moon is the lunar phase when the Moon appears fully illuminated from Earth's perspective. This occurs when Earth is located between the Sun and the Moon (when the ecliptic longitudes of the Sun and Moon differ by 180°). This mea ...
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 km
2, the area of
Northern Ireland
Northern Ireland ( ga, Tuaisceart Éireann ; sco, label=Ulster Scots dialect, Ulster-Scots, Norlin Airlann) is a part of the United Kingdom, situated in the north-east of the island of Ireland, that is #Descriptions, variously described as ...
() represents a solid angle of ,
Connecticut
Connecticut () is the southernmost state in the New England region of the Northeastern United States. It is bordered by Rhode Island to the east, Massachusetts to the north, New York (state), New York to the west, and Long Island Sound to the ...
() represents a solid angle of ,
Equatorial Guinea
Equatorial Guinea ( es, Guinea Ecuatorial; french: Guinée équatoriale; pt, Guiné Equatorial), officially the Republic of Equatorial Guinea ( es, link=no, República de Guinea Ecuatorial, french: link=no, République de Guinée équatoria ...
() represents a solid angle of .
* The largest constellation,
Hydra, covers a solid angle of , whereas the smallest,
Crux
Crux () is a constellation of the southern sky that is centred on four bright stars in a cross-shaped asterism commonly known as the Southern Cross. It lies on the southern end of the Milky Way's visible band. The name ''Crux'' is Latin f ...
, covers only .
See also
*
Steradian
*
Spat (unit)
*
Minute and second of arc
A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The ...
*
List of constellations by area
The International Astronomical Union (IAU) designates 88 constellations of stars. In the table below, they are ranked by the solid angle that they subtend in the sky, measured in square degrees and millisteradians.
These solid angles depend ...
References
External links
*
{{DEFAULTSORT:Square Degree
Units of solid angle