Astronomical coordinate systems (also called celestial coordinates) are organized arrangements for specifying positions of satellites, planets, stars, galaxies, and other celestial objects relative to physical reference points available to a situated observer (e.g. the true horizon and north cardinal direction to an observer situated on the Earth’s surface).
Astronomers use astronomical coordinates on the sky to define a position (direction unit vector) of a celestial object as seen from a specific location (which can be on earth or in space) in a way similar to geographic latitude and longitude. The most commonly used system is the equatorial celestial coordinate system which has the plane of earth’s equator projected onto the celestial sphere as fundamental plane. Right ascension (RA) is the angle counted in this plane from 0 to 24 hour, similar to the geographic longitude. Declination (Dec) is the angle orthogonal to RA, i.e. the angular distance from the equatorial plane with +90 degree for the celestial north pole and -90 degree for the celestial south pole. An object on the celestial equator has Dec = 0. A more detailed narrative with figures about celestial coordinate systems is given by G.Kaplan.
The celestial coordinate system is established by large-angle, fundamental observations. These types of observations allow us to define a coordinate system (directions of 3 orthogonal axes) from first principles, without prior knowledge of the coordinates of stars. In the past these fundamental observations were provided by transit circle (meridian circle) telescopes at optical wavelengths for about 1500 bright stars. (H.G.Walter, O.J.Sovers, Astrometry of Fundamental Catalogues, Springer 2000). These observations were tied into the complex motion and rotation of the earth as well as other solar system bodies (sun, major and minor planets) to be able to establish a dynamical reference frame, which is made inertial (i.e. rotation free) based on celestial mechanics and law of gravitation.
In 1997 the International Astronomical Union (IAU) adopted the International Celestial Reference System (ICRS) based on more precise Very Long Baseline Interferometry (VLBI) observations with radio telescopes to define the axes of the celestial coordinate system. The catalog of about 600 compact, extragalactic sources (mainly quasars) form the International Celestial Reference Frame (ICRF), the practical realization of the ICRS. The definition of the ICRS thus is independent of the earth’s motion, rotation, and its equator and is no longer a dynamical system, rather a quasi-inertial reference system with the assumption that those quasars do not move noticeably along the sky due to their enormous distances and thus are used as fixed, fiducial points to define the ICRS. The positions of most sources in the ICRF are known to about 0.1 to 1 mas, and there are no angular motions by definition.
Between 1989 and 1993 the European space mission Hipparcos observed (at visible light wavelengths) about 118,000 stars at a mean epoch of 1991.25 to an accuracy of about 1 mas per position coordinate. Because stars move (see below), the Hipparcos Catalog also had to solve for the effects of proper motion and parallax as well as just position. The Hipparcos Celestial Reference System (HCRS) was adopted by the IAU as the optical realization of the ICRS. The coordinate system of Hipparcos was aligned to the fundamental ICRS with a variety of methods (Kovalevsky et al. 1997) with 12 radio stars (visible at optical and radio wavelengths) providing the strongest link.