Nowadays , the `` light time for unit distance '' - the inverse of c , expressed in seconds per astronomical unit-is 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 .
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 .