Speed of light
Nowadays, the "light time for unit distance"—the inverse of c (1/c), expressed in seconds per astronomical unit—is measured by comparing the time for radio signals to reach different spacecraft in the Solar System. The position of spacecraft is 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 is obtained. , the best estimate, as approved by the International Astronomical Union (IAU), is: