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Astronomical system of units
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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="IAU1976">{{Citation | editor-last=Müller | editor-first=Edith A. | editor2-last=Jappel | editor2-first=A. | year=1977 | title=Proceedings of the Sixteenth General Assembly Grenoble 1976 | series=Transactions of the International Astronomical Union | section=Resolution No. 1 | volume=16B | page=31 | url=https://archive.org/details/proceedingsofsix0016inte/page/31/ | publisher=D. Reidel | place=Dordrecht, Holland | isbn=90-277-0836-3 }}</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, 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> (See [[Barycentric and geocentric celestial reference systems]].) It 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.
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