Equatorial coordinate system
The equatorial coordinate system is probably the most widely used celestial coordinate system, whose equatorial coordinates are:
*
declination
*
right ascension -also
RA-, or hour angle -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.
Encyclopedia
The
equatorial coordinate system is probably the most widely used celestial coordinate system, whose equatorial coordinates are:
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.
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 . 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 when specifying coordinates of planets, stars, galaxies, etc.
The
latitudinal angle of the equatorial system is called declination . It measures the angle of an object above or below the celestial equator. The longitudinal angle is called the right ascension . 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 = 15 degrees in one hour of right ascension.
The equatorial coordinate system is commonly used by telescopes equipped
equatorial mounts by employing
Setting circles. Setting circles in conjunction with a star chart or ephemeris allow a telescope to be easily pointed at known objects on the celestial sphere.
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
