The barycentric celestial reference system (BCRS) is a coordinate system used in

astrometry Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other Astronomical object, celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, th ...

to specify the location and motions of astronomical objects. Its center of coordinates as the center of mass of the entire Solar System, its

barycenter In astronomy, the barycenter (or barycentre; ) is the center of mass of two or more bodies that orbit one another and is the point about which the bodies orbit. A barycenter is a dynamical point, not a physical object. It is an important con ...

. It was created in 2000 by the

International Astronomical Union The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...

(IAU) to be the global standard reference system for objects located outside the gravitational vicinity of

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

: planets, moons, and other Solar System bodies, stars and other objects in the

Milky Way The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...

galaxy, and extra-galactic objects. The geocentric celestial reference system (GCRS), also created by the IAU in 2000, is a similar standard coordinate system used to specify the location and motions of

near-Earth objects A near-Earth object (NEO) is any small Solar System body orbiting the Sun whose closest approach to the Sun (perihelion) is less than 1.3 times the Earth–Sun distance (astronomical unit, AU). This definition applies to the object's orbit aro ...

, such as satellites. Its center of coordinates is the center of mass of the Earth. These systems make it easier for scientists and engineers to compile, share, compare, and convert accurate measurements worldwide, by establishing standards both of measure and of methodology, and providing a consistent framework of operations. The focus of the BCRS is on astronomy: exploration of the

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

and the universe. The BCRS is the system currently used for expressing positional data in astronomical references, such as the Hipparcos star catalog. The focus of the GCRS is somewhat more on the navigation of Earth satellites and the geophysical applications they support. The proper functioning of the

Global Positioning System The Global Positioning System (GPS) is a satellite-based hyperbolic navigation system owned by the United States Space Force and operated by Mission Delta 31. It is one of the global navigation satellite systems (GNSS) that provide ge ...

(GPS) is directly dependent upon the accuracy of satellite measurements as supported by the GCRS.

Purpose and implementation

The BCRS was designed to support the extremely-high-precision measurements of position and motion required in astrometry. One critical factor in achieving that precision lies in how general relativistic effects are determined and measured. Both systems incorporate standards that enable the consistency and ready comparability of the resulting

spacetime In physics, spacetime, also called the space-time continuum, is a mathematical model that fuses the three dimensions of space and the one dimension of time into a single four-dimensional continuum. Spacetime diagrams are useful in visualiz ...

coordinates among astrometric measurements taken worldwide. They provide a

metric tensor In the mathematical field of differential geometry, a metric tensor (or simply metric) is an additional structure on a manifold (such as a surface) that allows defining distances and angles, just as the inner product on a Euclidean space allows ...

to establish a consistent

frame of reference In physics and astronomy, a frame of reference (or reference frame) is an abstract coordinate system, whose origin (mathematics), origin, orientation (geometry), orientation, and scale (geometry), scale have been specified in physical space. It ...

for observations. The tensor achieves consistency in part through its standardization of the reference point for gravity. The geocentric system is simpler, being smaller and involving few massive objects: that coordinate system defines its center as the

center of mass In physics, the center of mass of a distribution of mass in space (sometimes referred to as the barycenter or balance point) is the unique point at any given time where the weight function, weighted relative position (vector), position of the d ...

of the Earth itself. The barycentric system can be loosely thought of as being centered on the Sun, but the

is more complicated. Even the much smaller planets exert gravitational force upon the Sun, causing it to shift position slightly as they orbit. Those shifts are very large in comparison to the measurement precisions that are required for astrometry. Thus, the BCRS defines its center of coordinates as the center of mass of the entire Solar System, its

. This stable point for gravity helps to minimize relativistic effects from any observational frames of reference within the Solar System.

Relationship to other standards

ICRS

The orientation of the BCRS coordinate system coincides with that of the ''

International Celestial Reference System The International Celestial Reference System (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 "sho ...

'' (ICRS). Both are centered at the barycenter of the Solar System, and both "point" in the same direction. That is, their axes are aligned with that of the ''

International Celestial Reference Frame The International Celestial Reference System (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 "sho ...

'' (ICRF), which was adopted as a standard by the IAU two years earlier (1998). The motivation of the ICRF is to define what "direction" means in space, by fixing its orientation in relation to the

Celestial sphere In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, ...

, that is, to deep-space background. Speaking casually, it does not move in relation to the stars and galaxies; it does not rotate. Determining perfect immobilization of direction is not possible in practice, but we can get much closer than it is even possible for us to measure. The more distant an object is, the less its direction appears to move in relation to us (the

parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different sightline, lines of sight and is measured by the angle or half-angle of inclination between those two lines. Due to perspective (graphica ...

effect). The ICRF thus uses very distant objects, well outside our galaxy, to establish its directional points of reference. The chosen objects also emit

radio wave Radio waves (formerly called Hertzian waves) are a type of electromagnetic radiation with the lowest frequencies and the longest wavelengths in the electromagnetic spectrum, typically with frequencies below 300 gigahertz (GHz) and wavelengths g ...

lengths, which are less subject than other wavelengths to being obscured by celestial gas in front of them. The ICRF adopts coordinates for 212 defining objects, mostly

quasar A quasar ( ) is an extremely Luminosity, luminous active galactic nucleus (AGN). It is sometimes known as a quasi-stellar object, abbreviated QSO. The emission from an AGN is powered by accretion onto a supermassive black hole with a mass rangi ...

s, fixing its orientation with respect to them.

HCRF

The '' ''Hipparcos'' Celestial Reference Frame'' (HCRF) was similar to ICRF, but earlier, used in association with the ''

Hipparcos ''Hipparcos'' was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions and distances of ...

'' satellite, which functioned between 1989 and 1993. That satellite took copious

stellar parallax Stellar parallax is the apparent shift of position (''parallax'') of any nearby star (or other object) against the background of distant stars. By extension, it is a method for determining the distance to the star through trigonometry, the stel ...

measurements at accuracies exceeding anything otherwise available at the time, thus producing a catalog of stars still in wide use today. No such extensive mapping has yet been completed based upon subsequent improvements in measurement capability. With lower precision then, and at optical wavelengths, the ICRS and BCRS can also be specified using the HCRF. That is the means by which BCRS can be used in relation to the Hipparcos star catalog.

Conversion of coordinates

The BCRS and GCRS were also designed so as to make transformations of their coordinates between themselves and other reference systems possible, though the conversions are not by any means straightforward. There are two software libraries of IAU-sanctioned algorithms for manipulating and transforming among the BCRS and other reference systems: the '' Standards of Fundamental Astronomy'' (SOFA) system and the Naval Observatory Vector Astrometry Subroutines (NOVAS). The orientation of the BCRS/ICRS axes also align within 0.02

arcsecond A minute of arc, arcminute (abbreviated as arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of a degree. Since one degree is of a turn, or complete rotation, one arcminute is of a tu ...

of the Earth's mean equator and equinox for the

Fifth Fundamental Catalog The Catalogue of Fundamental Stars is a series of six astrometric catalogues of high precision positional data for a small selection of stars to define a celestial reference frame, which is a standard coordinate system for measuring positions of ...

(FK5)

J2000.0 In astronomy, an epoch or reference epoch is a moment in time used as a reference point for some time-varying astronomical quantity. It is useful for the celestial coordinates or orbital elements of a celestial body, as they are subject to pe ...

epoch.

References

External links

Standards of Fundamental Astronomy (SOFA)

Naval Observatory Vector Astrometry Software (NOVAS)

SuperNOVAS
is a maintained fork of the NOVAS library (above) for C/C++, providing bug fixes, improvements, new features, and online documentation. {{Portal bar, Astronomy, Stars, Spaceflight, Outer space, Solar System Astronomical coordinate systems Astrometry