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,[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 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.[2] 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.

Astronomical unit of time

The astronomical unit of time is the day, defined as 86400 seconds. 365.25 days make up one Julian year.[1] 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.[1] The symbol S is often used in astronomy to refer to this unit, although M is also common. The solar mass (M), 1.98892×1030 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 333,000 times the mass of the Earth or 1,048 times the mass of Jupiter.

In practice, the masses of celestial bodies appear in the dynamics of the solar system only through the products GM, where G is the constant of gravitation. In the past, GM of the sun could be determined experimentally with only limited accuracy. Its present accepted value is[3] GM=1.327 124 420 99 × 1020±1010 m3s−2

Non IAU units of mass

These are not SI or IAU units, but are used in astronomy.

Jupiter mass

Jupiter mass (MJ or MJUP), is the unit of mass equal to the total mass of the planet Jupiter, 1.8986×1027 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

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.

Equivalent Planetary masses
Solar mass
Solar mass 1
Jupiter masses 1,048
Earth masses 332,950

Astronomical unit of length

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.[1] The dimensions of k2 are those of the constant of gravitation (G), i.e., L3M−1T−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.[4] It is approximately equal to the mean Earth–Sun distance.

The speed of light in IAU is the defined value c0 = 299 792 458 m/s of the SI units. In terms of this speed, the astronomical unit of length has the presently accepted value:[3] 1 ua = c0τA = 1.495 978 707 00 × 1011±3 m., where τA is the transit time of light across the astronomical unit. The astronomical unit of length is determined by the condition that the measured data in the ephemeris match observations, and that in turn decides the transit time τA.

Other units for astronomical distances

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.