The resolution which redefines the astronomical unit was proposed to the IAU by the IAU working group "Numerical Standards for Fundamental Astronomy" ("Valeurs numériques pour les constantes de l’astronomie fondamentale") in which scientists from the Observatoire de Paris played a major role.
The astronomical unit - au for short - is approximately equal to the distance of the Earth from the Sun. It is familiar to professional astronomer, to amateurs and even to the general public as a means of expressing astronomical distances, and in particular those in the solar system. The definition used till now was based on a mathematical expression involving the mass of the Sun (MS), the length of the day (D) and a constant k (called Gauss’ constant) whose numerical value was fixed by convention.
Based on a Newtonian concept, its original purpose had been to express exactly the relative distances in the solar system, at a time when the distances themselves could not be determined with any precision. However, the precision of modern measurements of distances within the solar system has rendered this procedure pointless.
Furthermore, till now, its value in meters was determined experimentally, and so was not only model and observation dependent, but was also a function of the reference system used. This value was then used to deduce, in SI units (the international system of units) the solar mass parameter [2], which was thus determined in an indirect manner. However, contemporary dynamic astronomy has to be done within the framework of general relativity, and using a coherent system of units and constants.
The definition of the astronomical unit therefore had to be revised. This has now been done, with the IAU resolution no. 2012 B2 which was adopted officially on the 30th of August 2012. Henceforth, the astronomical unit is defined as being exactly equal to 149 597 870 700 m, a conventional value chosen so as to be consistent with that of the system of astronomical constants used since 2009.
With this definition, the solar mass parameter becomes an empirically determined quantity, which enables therefore a determination of its variations, related to changes in the mass of our star. Since the astronomical unit is in constant use in astronomy, this decision concerns all the sciences of the Universe. Nonethess, this change concerns main the solar system high precision dynamics. In effect, even thought the astronomical unit leads to the definition of the parsec [3]
, the other unit used by astronomers to express distances in the Universe, the relative difference between the two definitions does not exceed 10-10. In other words, : a ten billionth or 0,000 000 01 %. Specialists chose the value of the astronomical to maintain the continuity of the reference systems.
Although this new definition will have no significant effect, taking into account the relative errors of cosmic distances beyond the solar system, astronomers world-wide now have a perfectly defined unit, consistent with general relativity and linked directly to the International System of Units (SI) via the meter.
[1] he SYRTE is a department of the Observatoire de Paris and an "unité mixte de recherche (UMR 8630)" of the Observatoire de Paris, of the CNRS and of the Pierre and Marie Curie University - UPMC.)
[2] The numerical value of the solar mass parameter (product of the solar mass MS and the gravitational constant G) was deduced from the astronomical unit in meters, A, using the equation GMS = A3k2/D2.
[3] The parsec is defined as the distance at which an astronomical unit subtends an angle of one second of arc (about 206 265 au).