The equatorial coordinate system is probably the most widely used celestial coordinate system, whose equatorial coordinates are: Image File history File links AstroDeclinationRightascension. ... In astronomy, a celestial coordinate system is a coordinate system for mapping positions in the sky. ...

• declination (δ)
• right ascension (α) -also RA-, or hour angle (H) -also HA-

It is the most closely related to the geographic coordinate system, because they use the same fundamental plane, and the same poles. The projection of the Earth's equator onto the celestial sphere is called the celestial equator. Similarly, projecting the geographic poles onto the celestial sphere defines the north and south celestial poles. In astronomy, declination (dec) is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. ... Right ascension (RA; symbol α: Greek letter alpha) is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. ... In astronomy, an objects hour angle (HA) is defined as the difference between the current local sidereal time (LST) and the right ascension () of the object: HAobject = LST - object Thus, the objects hour angle indicates how much sidereal time has passed since the object was on the local... Map of Earth showing lines of latitude (horizontally) and longitude (vertically); large version (pdf) The geographic (earth-mapping) coordinate system expresses every horizontal position on Earth by two of the three coordinates of a spherical coordinate system which is aligned with the spin axis of the Earth. ... The fundamental plane in a spherical coordinate system is a plane which divides the sphere into two hemispheres. ... The equator is an imaginary circle drawn around a planet (or other astronomical object) at a distance halfway between the poles. ... The celestial sphere is divided by the celestial equator. ... The celestial equator is a great circle on the imaginary celestial sphere, which could be constructed by inflating the Earths equator until it intersects with said sphere. ... For other uses of the word pole, see Pole (disambiguation). ... The two celestial poles are the imaginary points where the Earths spin axis intersects the imaginary rotating sphere of gigantic radius, called the celestial sphere. ...

There are two varieties:

• the hour angle system is fixed to the Earth like the geographic coordinate system
• the right ascension system is fixed to the stars (actually, not quite, see precession and nutation). Thus, during a night or a few nights, it appears to rotate across the sky with the stars, but of course it's really the Earth rotating under the fixed sky. Because of the precession and nutation just referred to, when considering long intervals between observations it is necessary to specify an epoch (frequently J2000.0, for older data B1950.0) when specifying coordinates of planets, stars, galaxies, etc.

Left A star is at culmination on an observer's meridian (HA = 0 h), then RA = LST. Right Now the vernal equinox point is at culmination on the the meridian m (LST = 0 h) (Positive angles: RA, counterclockwise; HA and LST, clockwise)

The latitudinal (latitude-like) angle of the equatorial system is called declination (Dec for short). It measures the angle of an object above or below the celestial equator. The longitudinal angle is called the right ascension (RA for short). It measures the angle of an object east of the vernal equinox point. Unlike longitude, right ascension is usually measured in hours instead of degrees, because the apparent rotation of the equatorial coordinate system is closely related to sidereal time and hour angle. Since a full rotation of the sky takes 24 hours to complete, there are (360 degrees / 24 hours) = 15 degrees in one hour of right ascension. Precession refers to a change in the direction of the axis of a rotating object. ... Rotation (green), Precession (blue) and Nutation (red) of the Earth Nutation is a slight irregular motion (etymologically a nodding) in the axis of rotation of a largely axially symmetric object, such as a gyroscope or a planet. ... The J2000. ... The B1950. ... Image File history File links Right_ascension. ... In astronomy, the culmination, at a given point, of a planet, star, constellation, etc. ... This article is about angles in geometry. ... A clockwise motion is one that proceeds like the clocks hands: from the top to the right, then down and then to the left, and back to the top. ... Clockwise can refer to: Clockwise and counterclockwise Clockwise (movie) This is a disambiguation page — a navigational aid which lists other pages that might otherwise share the same title. ... Latitude, usually denoted symbolically by the Greek letter φ, gives the location of a place on Earth north or south of the Equator. ... The First Point of Aries, also called the vernal equinox point, is one of the two points on the celestial sphere where the celestial equator intersects the ecliptic. ... Longitude, sometimes denoted by the Greek letter λ, describes the location of a place on Earth east or west of a north-south line called the Prime Meridian. ... Sidereal time is time measured by the apparent diurnal motion of the vernal equinox, which is very close to, but not identical with, the motion of stars. ... In astronomy, an objects hour angle (HA) is defined as the difference between the current local sidereal time (LST) and the right ascension () of the object: HAobject = LST - object Thus, the objects hour angle indicates how much sidereal time has passed since the object was on the local...

This article originates from Jason Harris' Astroinfo which comes along with KStars, a Desktop Planetarium for Linux/KDE. See http://edu.kde.org/kstars/index.phtml Screenshot of KStars showing the night sky from Hanover. ... Linux (also known as GNU/Linux) is a computer operating system. ... KDE (K Desktop Environment) is a free desktop environment and development platform built with Trolltechs Qt toolkit. ...