Previously , the inverse of 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 .
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 .