Editing Astronomical unit (section)

== Usage and significance ==
With the definitions used before 2012, the astronomical unit was dependent on the [[heliocentric gravitational constant]], that is the product of the [[gravitational constant]], ''G'', and the [[solar mass]], {{Solar mass}}. Neither ''G'' nor {{Solar mass}} can be measured to high accuracy separately, but the value of their product is known very precisely from observing the relative positions of planets ([[Kepler's third law]] expressed in terms of Newtonian gravitation). Only the product is required to calculate planetary positions for an ephemeris, so ephemerides are calculated in astronomical units and not in SI units.

The calculation of ephemerides also requires a consideration of the effects of [[general relativity]]. In particular, time intervals measured on Earth's surface ([[Terrestrial Time]], TT) are not constant when compared with the motions of the planets: the terrestrial second (TT) appears to be longer near January and shorter near July when compared with the "planetary second" (conventionally measured in TDB). This is because the distance between Earth and the Sun is not fixed (it varies between {{val|0.9832898912}} and {{val|1.0167103335|u=au}}) and, when Earth is closer to the Sun ([[perihelion]]), the Sun's gravitational field is stronger and Earth is moving faster along its orbital path. As the metre is defined in terms of the second and the speed of light is constant for all observers, the terrestrial metre appears to change in length compared with the "planetary metre" on a periodic basis.

The metre is defined to be a unit of [[proper length]]. Indeed, the [[International Committee for Weights and Measures]] (CIPM) notes that "its definition applies only within a spatial extent sufficiently small that the effects of the non-uniformity of the gravitational field can be ignored".<ref>{{SIbrochure8th|pages=166–67}}</ref> As such, a distance within the Solar System without specifying the [[frame of reference]] for the measurement is problematic. The 1976 definition of the astronomical unit was incomplete because it did not specify the frame of reference in which to apply the measurement, but proved practical for the calculation of ephemerides: a fuller definition that is consistent with general relativity was proposed,<ref name="Huang">{{cite journal |author=Huang, T.-Y. |author2=Han, C.-H. |author3=Yi, Z.-H. |author4=Xu, B.-X. |date=1995 |title=What is the astronomical unit of length? |bibcode=1995A&A...298..629H |journal=[[Astronomy and Astrophysics]] |volume=298 |pages=629–33 }}</ref> and "vigorous debate" ensued<ref name="Dodd">{{cite book |author=Dodd |first=Richard |title=Using SI Units in Astronomy |date=2011 |publisher=Cambridge University Press |isbn=978-0-521-76917-4 |page=76 |chapter=§&nbsp;6.2.3: Astronomical unit: ''Definition of the astronomical unit, future versions'' |chapter-url=https://books.google.com/books?id=UC_1_804BXgC&pg=PA76}} and also p. 91, ''Summary and recommendations''.</ref> until August 2012 when the IAU adopted the current definition of 1 astronomical unit = {{val|149597870700}} [[metre]]s.

The astronomical unit is typically used for [[stellar system]] scale distances, such as the size of a protostellar disk or the [[heliocentric distance]] of an asteroid, whereas other units are used for [[cosmic distance ladder|other distances in astronomy]]. The astronomical unit is too small to be convenient for interstellar distances, where the [[parsec]] and [[light-year]] are widely used. The parsec (parallax [[Minute and second of arc|arcsecond]]) is defined in terms of the astronomical unit, being the distance of an object with a parallax of {{val|1|u=arcsecond}}. The light-year is often used in popular works, but is not an approved non-SI unit and is rarely used by professional astronomers.<ref name="Dodd1">{{cite book |author=Dodd |first=Richard |title=Using SI Units in Astronomy |date=2011 |publisher=Cambridge University Press |isbn=978-0-521-76917-4 |page=82 |chapter=§&nbsp;6.2.8: Light-year |chapter-url=https://books.google.com/books?id=UC_1_804BXgC&pg=PA82}}</ref>

When simulating a [[numerical model of the Solar System]], the astronomical unit provides an appropriate scale that minimizes ([[arithmetic overflow|overflow]], [[arithmetic underflow|underflow]] and [[truncation]]) errors in [[floating point]] calculations.