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 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 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 the 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 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. 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 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:
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 (k) takes the value when the units of measurement are the astronomical units of length, mass and time.
Table of astronomical constants Quantity Symbol Value Relativeuncertainty Ref. Defining constants Gaussian gravitational constant k A3/2S−1/2D−1 defined Speed of light c m/s defined . Mean ratio of the TCG second to the TT second 1 − LG 1 − defined Resolutions Nos. B1.5 and B1.9 of the XXIVth General Assembly of the International Astronomical Union, Manchester, 2000. Mean ratio of the TCB second to the TDB second 1 − LB 1 − defined Resolution 3 of the XXVIth General Assembly of the International Astronomical Union, Prague, 2006. Primary constants Mean ratio of the TCB second to the TCG second 1 − LC 1 − Light-time for Astronomical unit τA A/c Equatorial radius for Earth ae 1.6 Potential of the geoid W0 m2/s2 8.0 Dynamical form-factor for Earth J2 9.2 Flattening factor for Earth 1/ƒ = 1/ 3.4 Geocentric gravitational constant GE m3s2 2.0 Constant of gravitation G m3·kg−1·s−2 1.5 Ratio of mass of Moon to mass of Earth μ = 1/ 4.0 General precession in longitude, per Julian century, at standard epoch 2000 ρ * Resolution 1 of the XXVIth General Assembly of the International Astronomical Union, Prague, 2006. Obliquity of the ecliptic, at standard epoch 2000 ε 23° 26′ 21.406″ * Derived constants Constant of nutation, at standard epoch 2000 N * Resolution No. B1.6 of the XXIVth General Assembly of the International Astronomical Union, Manchester, 2000. Astronomical unit A defined Solar parallax = arcsin(ae/A) π☉ 1.6 † Constant of aberration, at standard epoch 2000 κ Heliocentric gravitational constant = A3k2/D2 GS m3/s2 3.8 Ratio of mass of Sun to mass of Earth = (GS)/(GE) S/E Ratio of mass of Sun to mass of (Earth + Moon) (S/E)(1 + μ) Mass of Sun = (GS)/G S 1.98855 kg 1.0 † System of planetary masses: Ratios of mass of Sun to mass of planet Mercury Venus Earth + Moon Mars Jupiter 1047.3486 Saturn 3497.898 Uranus Neptune Pluto Other constants (outside the formal IAU System) Parsec = A/tan(1") pc 4.0 † Light-year = 365.25cD ly defined † Hubble constant H0 70.1 km/s·Mpc 0.019 Solar luminosity 3.939 W= 2.107 S/D variable,±0.1%
Notes * The theories of precession and nutation have advanced since 1976, and these also affect the definition of the ecliptic. 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 Tables of constants — from Nick Strobel's Astronomy Notes Astronomical constants index — James Q. Jacobs