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Astronomical system of units
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== 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="IAU1976B"/> The dimensions of ''k''<sup>2</sup> are those of the [[constant of gravitation]] (''G''), i.e., L<sup>3</sup>M<sup>−1</sup>T<sup>−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> = {{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 au = ''c''<sub>0</sub>''τ''<sub>A</sub> = ({{val|149597870700|3}}) 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.
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