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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
degree
Degree may refer to:
As a unit of measurement
* Degree (angle), a unit of angle measurement
** Degree of geographical latitude
** Degree of geographical longitude
* Degree symbol (°), a notation used in science, engineering, and mathematics
...
s 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 th ...
. Analogous to one degree being equal to radians, a square degree is equal to ()
2 steradian
The steradian (symbol: sr) or square radian is the unit of solid angle in the International System of Units (SI). It is used in three-dimensional geometry, and is analogous to the radian, which quantifies planar angles. Whereas an angle in radian ...
s (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 means ...
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
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
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, Norlin Airlann) is a part of the United Kingdom, situated in the north-east of the island of Ireland, that is variously described as a country, province or region. Nort ...
() 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 to the west, and Long Island Sound to the south. Its capita ...
() 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
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 for ...
, covers only .
See also
*
Steradian
The steradian (symbol: sr) or square radian is the unit of solid angle in the International System of Units (SI). It is used in three-dimensional geometry, and is analogous to the radian, which quantifies planar angles. Whereas an angle in radian ...
*
Spat (unit)
The spat (symbol sp), from the Latin ''spatium'' ("space"), is a unit of solid angle. 1 spat is equal to 4 steradians or approximately square degrees of solid angle . Thus it is the solid angle subtended by a complete sphere at its cen ...
*
Minute and second of arc
*
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