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 several times: in 1964 and in 1976. 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 to not fix these values, but have the Working Group on Numerical Standards continuously maintain a set of Current Best Estimates. 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. The IAU system of constants defines a system of astronomical units for length, mass and time, 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 or equivalent time scales such as the Teph of the Jet Propulsion Laboratoryephemerides represent the mean values that would be measured by an observer on the Earth's surface over a long period of time. The IAU also recommends values in SI units, which are the values which would be measured by an observer at the barycentre of the Solar System: these are obtained by the following transformations:
;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.
