In astronomy, a celestial coordinate system is a coordinate system for mapping positions on the celestial sphere. There are different celestial coordinate systems each using a system of spherical coordinates projected on the celestial sphere, in analogy to the geographic coordinate system used on the surface of the Earth. The coordinate systems differ only in their choice of the fundamental plane, which divides the sky into two equal hemispheres along a great circle. For example, the fundamental plane of the geographic system is the Earth's equator. Each coordinate system is named for its choice of fundamental plane.
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Coordinate systems
 Orientation of the galactic, ecliptic and celestial (earth based) astronomical coordinate systems, projected on the celestial sphere, showing the galactic equator (black) and north galactic pole (NGP), the ecliptic (orange) and north ecliptic pole (NEP), and the celestial equator (blue) and north celestial pole (NCP). Sun and earth not shown to scale but to indicate sun's orbital direction around the galactic center and earth's orbital direction around the sun. |
| Coordinate system | Fundamental plane | Poles | Coordinates | Epoch |
|---|---|---|---|---|
| Horizontal (also called Alt/Az or Az/El) |
horizon | zenith/nadir | elevation (also called altitude) - azimuth - meridian | |
| Equatorial | celestial equator | celestial poles | declination - right ascension or hour angle | B1950, J2000 |
| Ecliptic | ecliptic | ecliptic poles | ecliptic latitude - ecliptic longitude | |
| Galactic | galactic plane | galactic poles | ||
| Supergalactic | supergalactic plane |
Equatorial coordinate system
Popular choices of pole and equator are the older B1950 and the modern J2000 systems, but a pole and equator "of date" can also be used, meaning one appropriate to the date under consideration, such as that at which a measurement of the position of a planet or spacecraft is made. There are also subdivisions into "mean of date" coordinates, which average out or ignore nutation, and "true of date," which include nutation.
Altitude
Altitude, also referred to as Elevation angle, refers to the vertical angle measured from the astronomical horizon (0°) towards the zenith (+90°). It can also take negative values for objects below the horizon, down to the nadir (-90°). Although some will use the term height instead of elevation, this is not recommended as height is usually understood to be a linear distance unit, to be expressed in meters (or any other length unit), and not an angular distance.
The term zenith distance is more often used in astronomy and is the complement of the elevation. That is: 0° in the zenith, 90° on the horizon, up to 180° at the nadir.
Converting coordinates
Equatorial to horizontal coordinates
Let δ be the declination and H the hour angle.
Let φ be the observer's latitude.
Let a be the altitude and A the azimuth.
Let θ be the zenith (or zenith distance, i.e. the 90° complement of Alt).
Then the equations of the transformation are:
Use the inverse trigonometric functions to get the values of the coordinates.
NOTE: Inverse cosine is dual valued, i.e. 160° and 200° both have the same cosine. The above needs to be corrected. If H < 180 (or Pi radians) then Az = 360 - Az as derived from the above equation.
See also
External links
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Wikimedia Commons has media related to: Celestial coordinate systems |
- This article was originally based on Jason Harris' Astroinfo which comes along with KStars, a Desktop Planetarium for Linux/KDE. See http://edu.kde.org/kstars/index.phtml
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