<P> An astronomical unit (AU) is approximately the average distance between the Earth and Sun . It was redefined in 2012 as exactly 7011149597870700000 ♠ 149 597 870 700 m . Previously the AU was not based on the International System of Units but in terms of the gravitational force exerted by the Sun in the framework of classical mechanics . The current definition uses the recommended value in metres for the previous definition of the astronomical unit, which was determined by measurement . This redefinition is analogous to that of the metre, and likewise has the effect of fixing the speed of light to an exact value in astronomical units per second (via the exact speed of light in metres per second). </P> <P> Previously, the inverse of c expressed in seconds per astronomical unit was measured by comparing the time for radio signals to reach different spacecraft in the Solar System, with their position calculated from the gravitational effects of the Sun and various planets . By combining many such measurements, a best fit value for the light time per unit distance could be obtained . For example, in 2009, the best estimate, as approved by the International Astronomical Union (IAU), was: </P> <Dl> <Dd> light time for unit distance: t = 7002499004783836000 ♠ 499.004 783 836 (10) s </Dd> <Dd> c = 7008299792457999999 ♠ 0.002 003 988 804 10 (4) AU / s = 7002173144632674000 ♠ 173.144 632 674 (3) AU / day . </Dd> </Dl> <Dd> light time for unit distance: t = 7002499004783836000 ♠ 499.004 783 836 (10) s </Dd>

Speed of light is least in which media