An
astronomical constant is a
physical constant used in
astronomy. Formal sets of constants, along with recommended values, have been defined by the
International Astronomical Union (IAU) several times: in 1964 and in 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 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.