An astronomical constant is any of several physical constants used in

astronomy Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, g ...

. Formal sets of constants, along with recommended values, have been defined by the

International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...

(IAU) several times: in 1964Resolution No.4 of th
XIIth General Assembly of the International Astronomical Union
Hamburg, 1964. and in 1976Resolution No. 1 on the recommendations of Commission 4 on ephemerides in th
XVIth General Assembly of the International Astronomical Union
Grenoble, 1976. (with an update in 1994). In 2009 the IAU adopted a new current set, and recognizing that new observations and techniques continuously provide better values for these constants, they decidedResolution B2 of th
XXVIIth General Assembly of the International Astronomical Union
Rio de Janeiro, 2009. to not fix these values, but have the Working Group on Numerical Standards continuously maintain a set of Current Best Estimates.IAU Division I Working Group on Numerical Standards for Fundamental Astronomy and Astronomical Constants: Current Best Estimates (CBEs

The set of constants is widely reproduced in publications such as the ''

Astronomical Almanac ''The Astronomical Almanac''The ''Astronomical Almanac'' for the Year 2015, (United States Naval Observatory/Nautical Almanac Office, 2014) . is an almanac published by the United States Naval Observatory (USNO) and His Majesty's Nautical Almanac ...

'' of the

United States Naval Observatory United States Naval Observatory (USNO) is a scientific and military facility that produces geopositioning, navigation and timekeeping data for the United States Navy and the United States Department of Defense. Established in 1830 as the Depo ...

and

HM Nautical Almanac Office His Majesty's Nautical Almanac Office (HMNAO), now part of the United Kingdom Hydrographic Office, was established in 1832 on the site of the Royal Observatory, Greenwich (ROG), where ''The Nautical Almanac'' had been published since 1767. HMNAO ...

. Besides the IAU list of units and constants, also the International Earth Rotation and Reference Systems Service defines constants relevant to the orientation and rotation of the Earth, in its technical notes. For complete document see The IAU system of constants defines a system of astronomical units for length, mass and time (in fact, several such systems), and also includes constants such as the

speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit ...

and the

constant of gravitation The gravitational constant (also known as the universal gravitational constant, the Newtonian constant of gravitation, or the Cavendish gravitational constant), denoted by the capital letter , is an empirical physical constant involved in th ...

which allow transformations between astronomical units and

SI unit The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system and the world's most widely used system of measurement. ...

s. Slightly different values for the constants are obtained depending on the frame of reference used. Values quoted in

barycentric dynamical time Barycentric Dynamical Time (TDB, from the French ) is a relativistic coordinate time scale, intended for astronomical use as a time standard to take account of time dilation when calculating orbits and astronomical ephemerides of planets, asteroid ...

(TDB) or equivalent time scales such as the ''T''_eph of the

Jet Propulsion Laboratory The Jet Propulsion Laboratory (JPL) is a federally funded research and development center and NASA field center in the City of La Cañada Flintridge, California, United States. Founded in the 1930s by Caltech researchers, JPL is owned by NASA an ...

ephemerides In astronomy and celestial navigation, an ephemeris (pl. ephemerides; ) is a book with tables that gives the trajectory of naturally occurring astronomical objects as well as artificial satellites in the sky, i.e., the position (and possibly ...

represent the mean values that would be measured by an observer on the Earth's surface (strictly, on the surface of the

geoid The geoid () is the shape that the ocean surface would take under the influence of the gravity of Earth, including gravitational attraction and Earth's rotation, if other influences such as winds and tides were absent. This surface is extended ...

) over a long period of time. The IAU also recommends values in SI units, which are the values which would be measured (in

proper length Proper length or rest length is the length of an object in the object's rest frame. The measurement of lengths is more complicated in the theory of relativity than in classical mechanics. In classical mechanics, lengths are measured based on ...

and

proper time In relativity, proper time (from Latin, meaning ''own time'') along a timelike world line is defined as the time as measured by a clock following that line. It is thus independent of coordinates, and is a Lorentz scalar. The proper time interval ...

) by an observer at the barycentre of the Solar System: these are obtained by the following transformations: :

\tau_A() = (1+L_)^ \tau_A()\,

GE() = (1+L_) GE()\,

GS() = (1+L_) GS()\,

Astronomical system of units

The astronomical unit of time is a time interval of one day (''D'') of 86400 seconds. The astronomical unit of mass is the mass of the Sun (''S''). The astronomical unit of length is that length (''A'') for which the

Gaussian gravitational constant The Gaussian gravitational constant (symbol ) is a parameter used in the orbital mechanics of the Solar System. It relates the orbital period to the orbit's semi-major axis and the mass of the orbiting body in Solar masses. The value of histor ...

(''k'') takes the value when the units of measurement are the astronomical units of length, mass and time.

Table of astronomical constants

;Notes * The theories of

precession Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In oth ...

and

nutation Nutation () is a rocking, swaying, or nodding motion in the axis of rotation of a largely axially symmetric object, such as a gyroscope, planet, or bullet in flight, or as an intended behaviour of a mechanism. In an appropriate reference frame ...

have advanced since 1976, and these also affect the definition of the

ecliptic The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic agains ...

. The values here are appropriate for the older theories, but additional constants are required for current models. † The definitions of these derived constants have been taken from the references cited, but the values have been recalculated to take account of the more precise values of the primary constants cited in the table.

References

2009 Selected Astronomical Constants
in .
2015 Selected Astronomical Constants

External links

— from Nick Strobel'
Astronomy Notes

— James Q. Jacobs {{DEFAULTSORT:Astronomical Constant Physical constants Constants Constant