Astronomical system of units
 | It has been suggested that solar mass, jupiter mass, earth mass and lunar mass be merged into this article. (Discuss) |
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 slightly updated since then.
The system was developed because of the difficulties in measuring and expressing astronomical data in 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.
Astronomical unit of time
The astronomical unit of time is the day, defined as 86400 seconds. 36525 days make up one Julian century.[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.
Other units of mass used in Astronomy
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.[2] It is approximately equal to the mean Earth–Sun distance.
Other units used for various astronomical distances
| Astronomical Range | Typical Units |
|---|---|
| Distances to satellites | kilometres |
| Distances to near-Earth objects | lunar distance |
| Planetary distances | astronomical units |
| 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.
References
- ^ a b c d Resolution No. 10 of the XVIth General Assembly of the International Astronomical Union, Grenoble, 1976.
- ^ International Bureau of Weights and Measures (2006), The International System of Units (SI) (PDF) (8th ed.), p. 126, ISBN 92-822-2213-6, archived (PDF) from the original on 2021-06-04, retrieved 2021-12-16.