The speed of light in IAU is the defined value "c"0 =  of the SI units. In terms of this speed, the old definition of the astronomical unit of length had the accepted value: 1 au = "c"0"τ"A = () m, where "τ"A 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"A.