Editing Astronomical unit (section)

== Developments ==

[[File:Stellarparallax parsec1.svg|thumb|The ''astronomical unit'' is used as the baseline of the triangle to measure [[stellar parallax]]es (distances in the image are not to scale)]]

The unit distance {{mvar|A}} (the value of the astronomical unit in metres) can be expressed in terms of other astronomical constants:
:<math>A^3 = \frac{G M_\odot D^2}{k^2},</math>
where {{mvar|G}} is the [[Newtonian constant of gravitation]], {{Solar mass}} is the solar mass, {{mvar|k}} is the numerical value of Gaussian gravitational constant and {{mvar|D}} is the time period of one day.<ref name="IAUresB2"/>
The Sun is constantly losing mass by radiating away energy,<ref>{{cite journal |author=Noerdlinger, Peter D. |arxiv=0801.3807 |title=Solar Mass Loss, the Astronomical Unit, and the Scale of the Solar System |journal=[[Celestial Mechanics and Dynamical Astronomy]] |bibcode=2008arXiv0801.3807N |volume=0801 |date=2008 |pages=3807}}</ref> so the orbits of the planets are steadily expanding outward from the Sun. This has led to calls to abandon the astronomical unit as a unit of measurement.<ref>{{cite magazine |url=https://www.newscientist.com/article/dn13286-astronomical-unit-may-need-to-be-redefined.html |title=AU may need to be redefined |magazine=[[New Scientist]] |date=6 February 2008}}</ref>

As the speed of light has an exact defined value in SI units and the Gaussian gravitational constant {{mvar|k}} is fixed in the [[astronomical system of units]], measuring the light time per unit distance is exactly equivalent to measuring the product {{mvar|G}}×{{Solar mass}} in SI units. Hence, it is possible to construct ephemerides entirely in SI units, which is increasingly becoming the norm.

A 2004 analysis of radiometric measurements in the inner Solar System suggested that the [[secular variation|secular increase]] in the unit distance was much larger than can be accounted for by solar radiation, +{{val|15|4}} metres per century.<ref>{{cite journal |author1=Krasinsky, G.A. |author2=Brumberg, V.A. |title=Secular increase of astronomical unit from analysis of the major planet motions, and its interpretation |journal=[[Celestial Mechanics and Dynamical Astronomy]] |volume=90 |issue=3–4 |date=2004 |doi=10.1007/s10569-004-0633-z |pages=267–88|bibcode = 2004CeMDA..90..267K |s2cid=120785056 }}</ref><ref name="Anderson">{{cite journal |last1=Anderson |first1=John D. |last2=Nieto |first2=Michael Martin |name-list-style=amp |date=2009 |title=Astrometric Solar-System Anomalies; §2: Increase in the astronomical unit |journal= Proceedings of the International Astronomical Union |volume=5 |issue=S261 |pages=189–97 |arxiv=0907.2469 |bibcode=2009IAU...261.0702A |doi=10.1017/s1743921309990378 |s2cid=8852372}}</ref>

The measurements of the secular variations of the astronomical unit are not confirmed by other authors and are quite controversial.
Furthermore, since 2010, the astronomical unit has not been estimated by the planetary ephemerides.<ref>{{cite journal |display-authors=1 |first1=A. |last1=Fienga |last2=Kuchynka |first2=P. |last3=Manche |first3=H. |last4=Desvignes |first4=G. |last5=Gastineau |first5=M. |last6=Cognard |first6=I. |last7=Theureau |first7=G. |title=The INPOP10a planetary ephemeris and its applications in fundamental physics |journal=[[Celestial Mechanics and Dynamical Astronomy]] |volume=111 |issue=3 |date=2011 |doi=10.1007/s10569-011-9377-8 |page=363 |bibcode=2011CeMDA.111..363F |arxiv=1108.5546|s2cid=122573801 }}</ref>