Astronomical constant#Table of astronomical constants

{{distinguish|Cosmological constant}}

{{Short description|Any of several physical constants used in astronomy}}An astronomical constant is any of several physical constants used in astronomy. Formal sets of constants, along with recommended values, have been defined by the International Astronomical Union (IAU) several times: in 1964Resolution No.4 of the [http://www.iau.org/static/resolutions/IAU1964_French.pdf XIIth General Assembly of the International Astronomical Union], Hamburg, 1964. and in 1976Resolution No. 1 on the recommendations of Commission 4 on ephemerides in the [http://www.iau.org/static/resolutions/IAU1976_French.pdf XVIth General Assembly of the International Astronomical Union], Grenoble, 1976. (with an update in 1994{{citation | author = Standish, E. M. | contribution = Report of the IAU WGAS Sub-group on Numerical Standards | url = http://iau-comm4.jpl.nasa.gov/iausgnsrpt.pdf | archive-url = https://web.archive.org/web/20060929065712/http://iau-comm4.jpl.nasa.gov/iausgnsrpt.pdf | url-status = dead | archive-date = 2006-09-29 | title = Highlights of Astronomy | editor = Appenzeller, I. | location = Dordrecht | publisher = Kluwer | date = 1995}}). 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 the [http://www.iau.org/static/resolutions/IAU2009_English.pdf 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) [http://maia.usno.navy.mil/NSFA/NSFA_cbe.html] {{Webarchive|url=https://web.archive.org/web/20160826200953/http://maia.usno.navy.mil/NSFA/NSFA_cbe.html|date=2016-08-26}} The set of constants is widely reproduced in publications such as the Astronomical Almanac of the United States Naval Observatory and HM Nautical Almanac Office.

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.{{cite web |title=Table 1.1: IERS numerical standards |work=IERS technical note no. 36: General definitions and numerical standards |editor=Gérard Petit |editor2=Brian Luzum |url=http://tai.bipm.org/iers/conv2010/chapter1/tn36_c1.pdf |publisher=International Earth Rotation and Reference Systems Service |date=2010 }} For complete document see {{cite book |title=IERS Conventions (2010): IERS technical note no. 36 |editor=Gérard Petit |editor2=Brian Luzum |isbn=978-3-89888-989-6 |url=http://www.iers.org/nn_11216/IERS/EN/Publications/TechnicalNotes/tn36.html |publisher=International Earth Rotation and Reference Systems Service |date=2010 |access-date=2013-02-01 |archive-date=2019-06-30 |archive-url=https://web.archive.org/web/20190630104818/https://www.iers.org/nn_11216/IERS/EN/Publications/TechnicalNotes/tn36.html |url-status=dead }}

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 and the constant of gravitation which allow transformations between astronomical units and SI units. Slightly different values for the constants are obtained depending on the frame of reference used. Values quoted in barycentric dynamical time (TDB) or equivalent time scales such as the T_eph of the Jet Propulsion Laboratory ephemerides represent the mean values that would be measured by an observer on the Earth's surface (strictly, on the surface of the geoid) 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 and proper time) by an observer at the barycentre of the Solar System: these are obtained by the following transformations:

:$\backslash tau\_A(\{\backslash rm\; SI\})\; =\; (1+L\_\{\backslash rm\; B\})^\{\backslash frac\{1\}\{3\}\}\; \backslash tau\_A(\{\backslash rm\; TDB\})\backslash ,$

:$GE(\{\backslash rm\; SI\})\; =\; (1+L\_\{\backslash rm\; B\})\; GE(\{\backslash rm\; TDB\})\backslash ,$

:$GS(\{\backslash rm\; SI\})\; =\; (1+L\_\{\backslash rm\; B\})\; GS(\{\backslash rm\; TDB\})\backslash ,$