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((Short description|System of measurement developed for use in astronomy))
((Multiple issues|
((Update|inaccurate=yes|date=February 2013))
((disputed|date=November 2015))
))

The '''astronomical system of units''', formerly called the '''IAU (1976) System of Astronomical Constants''', is a [[Systems of measurement|system of measurement]] developed for use in [[astronomy]]. It was adopted by the [[International Astronomical Union]] (IAU) in 1976 via Resolution No. 1,<ref name="IAU76">((citation |author=IAU Commission 4 (Ephemerides) |title=Recommendations [to the XVIth General Assembly, Grenoble, France, 1976] |publisher=IAU |url=http://www.iau.org/static/resolutions/IAU1976_French.pdf |quote=It is recommended that the following list of constants shall be adopted as the 'IAU (1976) System of Astronomical Constants'.))</ref> 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 unit]]s). 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 [[Dimensional analysis|tridimensional]] system, in that it defines units of [[length]], [[mass]] and [[time]]. The associated [[astronomical constant]]s also fix the different [[Frame of reference|frames of reference]] that are needed to report observations.<ref name=Systems>

In particular, there is the ''barycentric celestial reference system'' (BCRS) centered at the [[barycenter]] of the Solar System, and the ''geocentric celestial reference system'' (GCRS) centered at the center of mass of the Earth (including its fluid envelopes) ((cite book |chapter-url=https://books.google.com/books?id=7NdrK4e77CIC&pg=PA105 |author=Dennis D. McCarthy, P. Kenneth Seidelmann |title=Time: from Earth rotation to atomic physics|chapter=Resolution B1.3: Definition of the barycentric celestial reference system and geocentric celestial reference system ''XXIVth International Astronomical Union General Assembly (2000)'' |page=105 |isbn=978-3-527-40780-4 |date=2009 |publisher=[[Wiley-VCH]]))

</ref> The system is a conventional system, in that neither the unit of length nor the unit of mass are true [[physical constant]]s, and there are at least three different measures of time.

== Astronomical unit of time ==
((Main|Day))
The astronomical unit of time is the [[day]], defined as ((val|86400)) [[second]]s. 365.25 days make up one [[Julian year (astronomy)|Julian year]].<ref name="IAU76" /> The symbol ''D'' is used in astronomy to refer to this unit.

== Astronomical unit of mass ==
((Main|Solar mass))
The astronomical unit of mass is the [[solar mass]].<ref name="IAU76" /> The symbol ((Solar mass)) is often used to refer to this unit.
The solar mass (((Solar mass))), ((val|1.98892|e=30|ul=kg)), is a standard way to express [[mass]] in [[astronomy]], used to describe the masses of other [[star]]s and [[galaxy|galaxies]].  It is equal to the mass of the [[Sun]], about ((val|333000)) 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 ((nowrap|1=''G''((Solar mass)) = ((val|1.32712442099|(10)|e=20|u=m<sup>3</sup>⋅s<sup>−2</sup>)))).<ref name=IERS>((cite web |title=Table 1.1: IERS numerical standards |work=IERS technical note no. 36: General definitions and numerical standards |editor=Gérard Petit and Brian Luzum |url=http://tai.bipm.org/iers/conv2010/chapter1/tn36_c1.pdf |publisher=[[International Earth Rotation and Reference Systems Service]] |date=2010 )) For complete document see ((cite book |title=IERS Conventions (2010): IERS technical note no. 36 |editor=Gérard Petit and Brian Luzum |isbn=978-3-89888-989-6 |url=http://www.iers.org/nn_11216/IERS/EN/Publications/TechnicalNotes/tn36.html |publisher=International Earth Rotation and Reference Systems Service |date=2010 |access-date=2012-01-17 |archive-date=2019-06-30 |archive-url=https://web.archive.org/web/20190630104818/https://www.iers.org/nn_11216/IERS/EN/Publications/TechnicalNotes/tn36.html |url-status=dead ))</ref>

=== Jupiter mass ===
((Main|Jupiter mass))
Jupiter mass (((Jupiter mass)) or ''M''<sub>JUP</sub>), is the unit of [[mass]] equal to the total mass of the planet [[Jupiter]], ((val|1.898|e=27|ul=kg)). Jupiter mass is used to describe masses of the [[gas giant]]s, such as the [[outer planet]]s and [[extrasolar planet]]s. It is also used in describing [[brown dwarfs]] and Neptune-mass planets.

=== Earth mass ===
((Main|Earth mass))
Earth mass (((Earth mass))) is the unit of [[mass]] equal to that of the [[Earth]]. 1 ((Earth mass)) = ((val|5.9742|e=24|ul=kg)). Earth mass is often used to describe masses of rocky [[terrestrial planet]]s. It is also used to describe Neptune-mass planets. One Earth mass is ((val|0.00315)) times a Jupiter mass.

{| class="wikitable" border="1"
|+ Equivalent planetary masses
! !! Solar mass
|-
| Solar mass || 1
|-
| Jupiter masses || ((val|1048))
|-
| Earth masses || ((val|332950))
|}

== Astronomical unit of length ==
((Main|Astronomical unit))
The astronomical unit of length is now defined as exactly 149 597 870 700 meters.<ref name="IAUresB2">((citation |contribution=RESOLUTION B2 on the re-definition of the astronomical unit of length |title=RESOLUTION B2 | editor-last = International Astronomical Union |publisher=[[International Astronomical Union]] |place=Beijing, China |date=31 August 2012 | contribution-url = http://www.iau.org/static/resolutions/IAU2012_English.pdf |quote=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))</ref> 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 ((val|0.01720209895)) when the units of measurement are the astronomical units of length, mass and time.<ref name="IAU76" /> The dimensions of ''k''<sup>2</sup> are those of the [[constant of gravitation]] (''G''), i.e., L<sup>3</sup>M<sup>&minus;1</sup>T<sup>&minus;2</sup>. 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.

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 of ((val|0.01720209895)) radians per day.<ref>((SIbrochure8th|page=126)).</ref> 
The [[speed of light]] in IAU is the defined value ''c''<sub>0</sub>&nbsp;=&nbsp;((val|299792458|u=m/s)) of the SI units. In terms of this speed, the old definition of the astronomical unit of length had the  accepted value:<ref name=IERS/> 1&nbsp;au&nbsp;=&nbsp;''c''<sub>0</sub>''τ''<sub>A</sub>&nbsp;=&nbsp;(((val|149597870700|3)))&nbsp;m, where ''τ''<sub>A</sub> 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 the [[ephemeris]] match observations, and that in turn decides the transit time ''τ''<sub>A</sub>.

=== Other units for astronomical distances ===

{|class="wikitable"
!Astronomical range
!Typical units
|-
|Distances to [[satellites]]
|[[kilometre]]s
|-
|Distances to [[near-Earth object]]s
|[[Lunar distance (astronomy)|lunar distance]]
|-
|[[Planet]]ary distances
|[[astronomical unit]]s, [[gigametre]]s
|-
|Distances to nearby [[star]]s
|[[parsec]]s, [[light-year]]s
|-
|Distances at the galactic scale
|[[kiloparsec]]s 
|-
|Distances to nearby [[galaxy|galaxies]]
|[[megaparsec]]s 
|}

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 [[spacetime]] 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.

== See also ==
* [[Astronomical constant]]
* [[Standard gravitational parameter]]
* [[Planetary mass]]
* [[Natural units]]

== References ==
((reflist))

== External links ==
* [http://www.iau.org/public_press/themes/measuring/ The IAU and astronomical units]
* "[http://asa.usno.navy.mil/static/files/2014/Astronomical_Constants_2014.pdf 2014 Selected Astronomical Constants] ((Webarchive|url=https://web.archive.org/web/20131110215339/http://asa.usno.navy.mil/static/files/2014/Astronomical_Constants_2014.pdf |date=2013-11-10 ))" in ((citation | title = The Astronomical Almanac Online | url = http://asa.usno.navy.mil/ | publisher = [[United States Naval Observatory|USNO]]–[[United Kingdom Hydrographic Office|UKHO]] | access-date = 2009-05-05 | archive-date = 2016-12-24 | archive-url = https://web.archive.org/web/20161224174302/http://asa.usno.navy.mil/static/files/2016/Astronomical_Constants_2016.pdf | url-status = dead )).

((Units of length used in Astronomy))
((Systems of measurement))
((Portal bar|Mathematics|Astronomy|Stars|Spaceflight|Outer space|Solar System|Science))
((Authority control))

[[Category:Units of measurement in astronomy|*]]
[[Category:Systems of units]]

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