Everything about Celestial Coordinate System totally explained
In
astronomy, a
celestial coordinate system is a
coordinate system for mapping positions in the sky.There are different celestial coordinate systems each using a
coordinate grid 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.
Coordinate systems
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.
Elevation angle
Elevation angle, also referred to as
altitude, refers to the vertical
angle measured from the geometric
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 isn't 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
the
hour angle.
Let φ be the observer's
latitude.
Let El be the elevation angle and Az the
azimuth angle.
Let θ be the
zenith (or zenith distance, for example 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, for example 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.
This article is based on Jason Harris' Astroinfo which comes along with KStars, a Desktop Planetarium for Linux/KDE. See http://edu.kde.org/kstars/index.phtml
Further Information
Get more info on 'Celestial Coordinate System'.
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