The astronomical system of units, formerly called the IAU (1976) System of Astronomical Constants, is a system of measurement developed for use in astronomy. It was adopted by the International Astronomical Union (IAU) in 1976 via Resolution No. 1, and has been significantly updated in 1994 and 2009 (see Astronomical constant).
The system was developed because of the difficulties in measuring and expressing astronomical data in International System of Units (SI units). In particular, there is a huge quantity of very precise data relating to the positions of objects within the Solar System that cannot conveniently be expressed or processed in SI units. Through a number of modifications, the astronomical system of units now explicitly recognizes the consequences of general relativity, which is a necessary addition to the International System of Units in order to accurately treat astronomical data.
The astronomical system of units is a tridimensional system, in that it defines units of length, mass and time. The associated astronomical constants also fix the different frames of reference that are needed to report observations. (See Barycentric and geocentric celestial reference systems.) It is a conventional system, in that neither the unit of length nor the unit of mass are true physical constants, and there are at least three different measures of time.
Astronomical unit of time
The astronomical unit of time is the day, defined as seconds. 365.25 days make up one Julian year. The symbol D is used in astronomy to refer to this unit.
Astronomical unit of mass
The astronomical unit of mass is the solar mass.
The nominal value of the solar GM recommended for use in astrophysics as an exactly defined unit by the IAU is .
Jupiter mass
Jupiter mass ( or ), is the unit of mass approximately equal to the total mass of the planet Jupiter, . Jupiter mass is used to describe masses of the gas giants, such as the outer planets and extrasolar planets. It is also used in describing brown dwarfs and Neptune-mass planets. The nominal value recommended by the IAU for the GM of Jupiter is . It is approximately equal to the mean Earth–Sun distance. It was formerly defined as that length for which the Gaussian gravitational constant (k) takes the value when the units of measurement are the astronomical units of length, mass and time.
The speed of light in IAU is the defined value c<sub>0</sub> = of the SI units. In terms of this speed, the old definition of the astronomical unit of length had the accepted value: