Information about Equatorial Coordinates

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.

There are two systems to specify the longitudinal (longitude-like) coordinate:

• 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 move across the sky as the Earth spins and orbits under the fixed stars. Over long periods of time, precession and nutation effects alter the earth's orbit and thus the apparent location of the stars. When considering observations separated by long intervals, it is necessary to specify an epoch (frequently J2000.0, for older data B1950.0) when specifying coordinates of planets, stars, galaxies, etc.

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.

The equatorial coordinate system is commonly used by telescopes equipped with 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 [1]

­

•  • [ e] Orbits
Orbit types	Box orbit • Circular orbit • Ecliptic orbit • Elliptic orbit • Highly Elliptical Orbit • Graveyard orbit • Hyperbolic trajectory • Inclined orbit • Osculating orbit • Parabolic trajectory • Capture orbit • Escape orbit • Semi-synchronous orbit • Subsynchronous orbit • Synchronous orbit • Parking orbit
Earth-centered orbits	Geosynchronous orbit • Geostationary orbit • Sun-synchronous orbit • Low Earth orbit • Medium Earth Orbit • Molniya orbit • Near equatorial orbit • Orbit of the Moon • Polar orbit • Tundra orbit
Other orbits	Areosynchronous orbit • Areostationary orbit • Halo orbit • Lissajous orbit • Lunar orbit • Heliocentric orbit • Heliosynchronous orbit
Orbital elements	Inclination () • Longitude of the ascending node () • Eccentricity () • Argument of periapsis () • Semi-major axis () • Mean anomaly at epoch ()
Other orbital parameters	True anomaly () • Semi-minor axis () • Linear eccentricity () • Eccentric anomaly () • Mean longitude () • True longitude () • Orbital period ()
Orbital maneuvers	Bi-elliptic transfer • Geostationary transfer orbit • Gravitational slingshot • Hohmann transfer orbit • Orbital inclination change • Orbit phasing • Space rendezvous
Other orbital mechanics topics	Apsis • Celestial coordinate system • Delta-v budget • Epoch • Ephemeris • Equatorial coordinate system • Gravity turn • Ground track • Interplanetary Transport Network • Kepler's laws of planetary motion • Lagrangian point • List of orbits • n-body problem • Orbit equation • Orbital state vectors • Perturbation • Retrograde and direct motion • Specific orbital energy • Specific relative angular momentum • The Oberth effect