celestial coordinate system

In astronomy, coordinate systems are used for specifying positions of celestial objects (satellites, planets, stars, galaxies, etc.) relative to a given reference frame, based on physical reference points available to a situated observer (e.g. the true horizon and north to an observer on Earth’s surface).[1] Coordinate systems in astronomy can specify an object’s relative position in three-dimensional space or plot merely by its direction on a Celestial Sphere, if the object’s distance is unknown or trivial.

Spherical coordinates, projected on the celestial sphere, are analogous to the geographic coordinate system used on the surface of Earth. These differ in their choice of fundamental plane, which divides the celestial sphere into two equal hemispheres along a great circle. Rectangular coordinates, in appropriate units, have the same fundamental (x, y) plane and primary (x-axis) direction, such as an axis of rotation. Each coordinate system is named after its choice of fundamental plane.

0.1 Coordinate systems

The following table[2] lists the common coordinate systems in use by the astronomical community. The fundamental plane divides the celestial sphere into two equal hemispheres and defines the baseline for the latitudinal coordinates, similar to the equator in the geographic coordinate system. The poles are located at ±90^∘ from the fundamental plane. The primary direction is the starting point of the longitudinal coordinates. The origin is the zero distance point, the ”center of the celestial sphere”, although the definition of celestial sphere is ambiguous about the definition of its center point.


Coordinate system	Center point (origin)	Fundamental plane(0^∘ latitude)	Poles	Latitude	longitude	Primary direction (0^∘ longitude)

Horizontal (also called alt-az or el-az) Coordinate System	Observer	Horizon	zenith, nadir	Altitude (a) or elevation	Azimuth (A)	North or south point of horizon

equatorial coordinate system	Center of the Earth (geocentric), or Sun (heliocentric)	Celestial Equator	Celestial Poles	Declination (δ)	Right ascension (α) or hour angle (h)	March equinox

ecliptic coordinate system	Center of the Earth (geocentric), or Sun (heliocentric)	ecliptic	Ecliptic poles	Ecliptic latitude (β)	Ecliptic longitude (λ)	March equinox

Galactic Coordinate System	Center of the Sun	Galactic plane	Galactic poles	Galactic latitude (b)	Galactic longitude (l)	Galactic Center

Supergalactic Coordinate System		Supergalactic plane	Supergalactic poles	Supergalactic latitude (SGB)	Supergalactic longitude (SGL)	Intersection of supergalactic plane and galactic plane

This article is a derivative work of the creative commons share alike with attribution in [3].

References

[1] Kanas, Nick (2021). ”Star and Solar System Maps: A History of Celestial Cartography”. Research Notes of the AAS. 5 (4). American Astronomical Society: 69.
[2] Majewski, Steve. ”Coordinate Systems”. UVa Department of Astronomy. Archived from the original on 12 March 2016. Retrieved 19 March 2011.
[3] Wikipedia contributors, ”Astronomical coordinate systems,” Wikipedia, The Free Encyclopedia.

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