The astronomical system of units, formally 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, and has been slightly updated since then.
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 which 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. The system 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.
The astronomical unit of mass is the solar mass. The symbol S is often used in astronomy to refer to this unit, although M☉ is also common. The solar mass (M⊙), 1.98892×10 30 kg, is a standard way to express mass in astronomy, used to describe the masses of other stars and galaxies. It is equal to the mass of the Sun, about 332,950 times the mass of the Earth or 1,048 times the mass of Jupiter.
These are not SI or IAU units, but are used in astronomy.
Jupiter mass (MJ or MJUP), is the unit of mass equal to the total mass of the planet Jupiter, 1.8986×10 27 kg. 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.
Earth mass (M⊕) is the unit of mass equal to that of the Earth. 1 M⊕ = 5.9742 × 1024 kg. Earth mass is often used to describe masses of rocky terrestrial planets. One Earth mass is 0.00315 times a Jupiter mass.
The astronomical unit of length is that length for which the Gaussian gravitational constant (k) takes the value 0.017 202 098 95 when the units of measurement are the astronomical units of length, mass and time. The dimensions of k2 are those of the constant of gravitation (G), i.e., L3M−1 T−2. The term “unit distance” is also used for the length A while, in general usage, it is usually referred to simply as the “astronomical unit”, symbol AU, au or ua.
An equivalent definition of the astronomical unit is the radius of an unperturbed circular Newtonian orbit about the Sun of a particle having infinitesimal mass, moving with a mean motion of 0.017 202 098 95 radians per day. It is approximately equal to the mean Earth–Sun distance.
|Astronomical Range||Typical Units|
|Distances to satellites||kilometres|
|Distances to near-Earth objects||lunar distance|
|Planetary distances||astronomical units, gigametres|
|Distances to nearby stars||parsecs, light-years|
|Distances at the galactic scale||kiloparsecs|
|Distances to nearby galaxies||megaparsecs|
The distances to distant galaxies are typically not quoted in distance units at all, but rather in terms of redshift. The reasons for this are that converting redshift to distance requires knowledge of the Hubble constant which was not accurately measured until the early 21st century, and that at cosmological distances, the curvature of space-time allows one to come up with multiple definitions for distance. For example, the distance as defined by the amount of time it takes for a light beam to travel to you is different from the distance as defined by the apparent size of an object.