An introduction to coordinate systems used in Astronomy

From Wikipedia:

In geometry, a coordinate system is a system which uses one or more numbers, or coordinates, to uniquely determine the position of the points or other geometric elements on a manifold such as Euclidean space.

The following text briefly explains the coordinate systems being used in astronomy, some of which are listed below:


RA (right ascension) and DEC (declination) are the longitudes and latitudes of the sky. RA corresponds to east / west direction, similar to longitude, while DEC measures north / south directions, like latitude.


World Coordinate System (WCS) is a set of transformations that map pixel locations in an image to their real-world units, such as their position on the sky sphere. These transformations can work both forward (from pixel to sky) and backward (from sky to pixel).


The FITS “World Coordinate System” (WCS) standard defines some conventions and keywords to associate coordinates with each pixel of an image.


Flexible Image Transport System (FITS) is a digital file format useful for storage, transmission and processing of scientific and other images. It is the defacto standard used by many sky tessellation softwares — in this case, HEALPix.

HEALPix header files can contain the following three letters, each depicting the coordinate system being used:

  • C:Celestial = ICRS = RA / DEC (equatorial) = FK5 J2000 (default)
  • G:Galactic
  • E:Ecliptic

International Celestial Reference System (ICRS)

ICRS is the current standard celestial reference system adopted by the International Astronomical Union (IAU). Its origin is at the barycenter of the Solar System, with axes that are intended to be “fixed” with respect to space - this is referred to as International Celestial Reference Frame (ICRF). ICRS coordinates are approximately the same as equatorial coordinates.

Fifth Fundamental Catalogue (FK5)

FK5 is part of the “Catalogue of Fundamental Stars” which provides a series of six astrometric catalogues of high precision positional data for a small selection of stars to define a celestial reference frame. J2000 refers to the instant of 12 PM (midday) on 1st January, 2000. FK5 was published in 1991 and added 3,117 new stars.

Galactic coordinate system

The galactic coordinate system is a celestial coordinate system in spherical coordinates, with its origin at the Sun, the primary direction aligned with the approximate center of the Milky Way galaxy, and the fundamental plane parallel to an approximation of the galactic plane but offset to its north. GCS has its own Galactic longitude and Galactic latitude.

Ecliptic coordinate system

A celestial coordinate system commonly used for representing the positions and orbits of Solar System objects. The system’s origin can either be the center of the Sun or the center of the Earth, its primary direction is towards the vernal (northbound) equinox, and it follows a right-handed convention.