Astronomical constant

An astronomical constant is a physical constant used in astronomy. The astronomical constants include units of astronomical distance (the astronomical unit (A.U.), the light year (ly), and the parsec (pc)), units of time (the sidereal year, the tropical year, and the Gregorian year), and various values for distances, masses, etc. (e.g., the mass and the mean radius of the Earth, and the mass, radius, and luminosity of the Sun). The astronomical constants may also be taken to include such physical constants as the speed of light (c), the gravitational constant (G), and the Planck constant (h).

The values of astronomical constants have been recalculated at various times, perhaps most notably by the Canadian-born astronomer Simon Newcomb (1835-1909).