celestial reference frame

The term celestial reference frame is used for a defined frame of reference based upon a catalog of astronomical objects, chosen for stability (lack of proper motion), generally stars or quasars. The axes of a celestial coordinate system are defined in relation to such a celestial reference frame, more specifically, in relation to locations of its associated catalog's objects. The Catalogues of Fundamental Stars were developed for this purpose, each defining an associated coordinate system: coordinates may be specified as FK4 (4th catalog), commonly using epoch B1950.0/equinox equatorial coordinates, and similarly FK5 (5th catalog) for epoch J2000.0 and J2000.0 equinox. More recently used is the ICRF (International Celestial Reference Frame) based on catalogs of quasars, the latest being ICRF3 based on a set of 303 quasars. The celestial coordinate system using ICRF, i.e., International Celestial Reference System (ICRS), also commonly uses J2000 epoch and equinox, and both FK5-based and ICRS-based coordinates (which differ slightly in their values) are often cited with a J for "J2000".

Given the Earth's (and Sun's) constant movement as well as the fact that all astronomical objects are in relative motion to each other, no exact and unchanging description of the position of objects in the celestial sphere is possible, even after compensating for the Earth's orbit and its precession of the equinoxes (and Chandler wobble). However, given the timescale of our observations and given the long distances to stars, for practical purposes, approximate reference frames are defined that allow such positions to be recorded and used, enabling identification of objects over the course of for years, decades, or centuries (as opposed to gigayears). Such reference frames have been defined by select sets of stars that are chosen as recognizable and for their lack of proper motion, generally meaning bright but very distant stars. These are stars that show no movement relative to each other from our point of view, so angular distances (distances across the sky) can be measured from them, and from these, coordinates can be calculated. More recently, quasars are used since they are also virtually point-like on the celestial sphere (point sources) and their apparent motion on the celestial sphere is far less than any usable star, being at least a million times further away.