Square degree
Encyclopedia
A square degree is a non-SI
unit
measure of solid angle
. It is denoted in various ways, including deg2, sq.deg. and (°)². Just as degree
s 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 π /180 radians, a square degree is equal to (π /180)2 or about 1/3283 steradian
. The number of square degrees in a whole sphere is
or approximately 41,253 deg2. This is the total area of the 88 constellations in the list of constellations by area.
For example, observed from the surface of the Earth
, the Moon
has a diameter of approximately 0.5°, so it covers a solid angle of approximately 0.20 deg2, which is 4.8 × 10−6 of the total sky sphere.
Si
Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...
unit
Units of measurement
A unit of measurement is a definite magnitude of a physical quantity, defined and adopted by convention and/or by law, that is used as a standard for measurement of the same physical quantity. Any other value of the physical quantity can be expressed as a simple multiple of the unit of...
measure of solid angle
Solid angle
The solid angle, Ω, is the two-dimensional angle in three-dimensional space that an object subtends at a point. It is a measure of how large that object appears to an observer looking from that point...
. It is denoted in various ways, including deg2, sq.deg. and (°)². Just as degree
Degree (angle)
A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...
s are used to measure parts of a circle, square degrees are used to measure parts of a sphere
Sphere
A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...
. Analogous to one degree being equal to π /180 radians, a square degree is equal to (π /180)2 or about 1/3283 steradian
Steradian
The steradian is the SI unit of solid angle. It is used to describe two-dimensional angular spans in three-dimensional space, analogous to the way in which the radian describes angles in a plane...
. The number of square degrees in a whole sphere is
or approximately 41,253 deg2. This is the total area of the 88 constellations in the list of constellations by area.
For example, observed from the surface of the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
, the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
has a diameter of approximately 0.5°, so it covers a solid angle of approximately 0.20 deg2, which is 4.8 × 10−6 of the total sky sphere.