An astronomical constant is any of several

physical constant A physical constant, sometimes fundamental physical constant or universal constant, is a physical quantity that cannot be explained by a theory and therefore must be measured experimentally. It is distinct from a mathematical constant, which has a ...

s used in

astronomy Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest includ ...

. Formal sets of constants, along with recommended values, have been defined 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) 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 2024, © Crown Copyright 2023, ISSN 0737-6421 . is an almanac published by the United Kingdom Hydrographic Office; it also includes data supplied by many scientists from around ...

'' of the

United States Naval Observatory The 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 ...

and HM Nautical Almanac Office. Besides the IAU list of units and constants, also the

International Earth Rotation and Reference Systems Service The International Earth Rotation and Reference Systems Service (IERS), formerly the International Earth Rotation Service, is the body responsible for maintaining global time and reference frame standards, notably through its Earth Orientation P ...

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 exactly equal to ). It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time i ...

and the constant of gravitation which allow transformations between astronomical units and

SI unit The International System of Units, internationally known by the abbreviation SI (from French ), is the modern form of the metric system and the world's most widely used system of units of measurement, system of measurement. It is the only system ...

s. Slightly different values for the constants are obtained depending on the

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 ...

used. Values quoted in barycentric dynamical time (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 centers, federally funded research and development center (FFRDC) in La Cañada Flintridge, California, Crescenta Valley, United States. Founded in 1936 by Cali ...

ephemerides 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 exte ...

) 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. The proper time interval between two events on a world line is the change in proper time ...

) 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 o ...

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

ecliptic The ecliptic or ecliptic plane is the orbital plane of Earth's orbit, Earth around the Sun. It was a central concept in a number of ancient sciences, providing the framework for key measurements in astronomy, astrology and calendar-making. Fr ...

. 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 or The Constant may refer to: Mathematics * Constant (mathematics), a non-varying value * Mathematical constant, a special number that arises naturally in mathematics, such as or Other concepts * Control variable or scientific const ...

Constant