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 1964[1] and in 1976[2] (with an update in 1994[3]). In 2009 the IAU adopted a new current set, and recognizing that new observations and techniques continuously provide better values for these constants, they decided[4] to not fix these values, but have the Working Group on Numerical Standards continuously maintain a set of Current Best Estimates.[5] 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.[6]
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 Teph 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:[3]
^Resolution No.4 of the XIIth General Assembly of the International Astronomical Union, Hamburg, 1964.
^Resolution No. 1 on the recommendations of Commission 4 on ephemerides in the XVIth General Assembly of the International Astronomical Union, Grenoble, 1976.
^ abStandish, E. M. (1995), "Report of the IAU WGAS Sub-group on Numerical Standards", in Appenzeller, I. (ed.), Highlights of Astronomy(PDF), Dordrecht: Kluwer, archived from the original (PDF) on 2006-09-29
^Resolution B2 of the XXVIIth General Assembly of the International Astronomical Union, Rio de Janeiro, 2009.
^IAU Division I Working Group on Numerical Standards for Fundamental Astronomy and Astronomical Constants: Current Best Estimates (CBEs) [1] Archived 2016-08-26 at the Wayback Machine
^Gérard Petit; Brian Luzum, eds. (2010). "Table 1.1: IERS numerical standards" (PDF). IERS technical note no. 36: General definitions and numerical standards. International Earth Rotation and Reference Systems Service. For complete document see Gérard Petit; Brian Luzum, eds. (2010). IERS Conventions (2010): IERS technical note no. 36. International Earth Rotation and Reference Systems Service. ISBN 978-3-89888-989-6. Archived from the original on 2019-06-30. Retrieved 2013-02-01.
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