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Theastronomical system of units, formerly called theIAU (1976) System of Astronomical Constants, is asystem of measurement developed for use inastronomy. It was adopted by theInternational Astronomical Union (IAU) in 1976 via Resolution No. 1,[1] and has been significantly updated in 1994 and 2009 (seeAstronomical constant).
The system was developed because of the difficulties in measuring and expressing astronomical data inInternational System of Units (SI units). In particular, there is a huge quantity of very precise data relating to the positions of objects within theSolar 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 ofgeneral relativity, which is a necessary addition to theInternational System of Units in order to accurately treat astronomical data.
The astronomical system of units is atridimensional system, in that it defines units oflength,mass andtime. The associatedastronomical constants also fix the differentframes of reference that are needed to report observations.[2] (SeeBarycentric and geocentric celestial reference systems.) It is a conventional system, in that neither the unit of length nor the unit of mass are truephysical constants, and there are at least three different measures of time.
The astronomical unit of time is theday, defined as86400seconds. 365.25 days make up oneJulian year.[3] The symbolD is used in astronomy to refer to this unit.
The astronomical unit of mass is thesolar mass.[3] The symbolM☉ is often used to refer to this unit.The solar mass (M☉),1.98892×1030 kg, is a standard way to expressmass inastronomy, used to describe the masses of otherstars andgalaxies. It is approximately equal to the mass of theSun, about333000 times the mass of theEarth or 1 048 times the mass ofJupiter.
In practice, the masses of celestial bodies appear in the dynamics of the Solar System only through the productsGM, whereG is the constant of gravitation. In the past,GM of the Sun could be determined experimentally with only limited accuracy. Its present accepted value isGM☉ =1.32712442099(10)×1020 m3⋅s−2.[4]
Jupiter mass (MJ orMJUP), is the unit ofmass approximately equal to the total mass of the planetJupiter,1.898×1027 kg. Jupiter mass is used to describe masses of thegas giants, such as theouter planets andextrasolar planets. It is also used in describingbrown dwarfs and Neptune-mass planets.
Earth mass (M🜨) is the unit ofmass approximately equal to that of theEarth. 1M🜨 =5.9742×1024 kg. Earth mass is often used to describe masses of rockyterrestrial planets. It is also used to describe Neptune-mass planets. One Earth mass is0.00315 times a Jupiter mass.
| Solar mass | |
|---|---|
| Solar mass | 1 |
| Jupiter masses | 1048 |
| Earth masses | 332950 |
The astronomical unit of length is now defined as exactly 149 597 870 700 meters.[5] It is approximately equal to the mean Earth–Sun distance. It was formerly defined as that length for which theGaussian gravitational constant (k) takes the value0.01720209895 when the units of measurement are the astronomical units of length, mass and time.[3] The dimensions ofk2 are those of theconstant of gravitation (G), i.e., L3M−1T−2. The term "unit distance" is also used for the lengthA while, in general usage, it is usually referred to simply as the "astronomical unit", symbol au.
An equivalent formulation of the old 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 of0.01720209895 radians per day.[6] Thespeed of light in IAU is the defined valuec0 = 299792458 m/s of the SI units. In terms of this speed, the old definition of the astronomical unit of length had the accepted value:[4] 1 au = c0τA = (149597870700±3) m, whereτA is the transit time of light across the astronomical unit. The astronomical unit of length was determined by the condition that the measured data in theephemeris match observations, and that in turn decides the transit timeτA.
| Astronomical range | Typical units |
|---|---|
| Distances tosatellites | kilometres |
| Distances tonear-Earth objects | lunar distance |
| Planetary distances | astronomical units |
| Distances to nearbystars | parsecs,light-years |
| Distances at the galactic scale | kiloparsecs |
| Distances to nearbygalaxies | megaparsecs |
The distances to distant galaxies are typically not quoted in distance units at all, but rather in terms ofredshift. The reasons for this are that converting redshift to distance requires knowledge of theHubble constant, which was not accurately measured until the early 21st century, and that at cosmological distances, the curvature ofspacetime 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 an observer is different from the distance as defined by the apparent size of an object.
The XXVIII General Assembly of International Astronomical Union … recommends … 1. that the astronomical unit be re-defined to be a conventional unit of length equal to 149 597 870 700 m exactly
