noon
`` A lot , '' said Russ . `` All commerce is based on the assumption that light is instantaneous , but it is n't . All business , anywhere throughout the Solar System , is based on Greenwich time . When a noon signal sent out from Earth reaches Mars , it 's noon there , but as a matter of fact , it is actually 15 minutes or so past noon . When the same signal reaches Callisto , the correct time for the chronometer used in commerce would be noon when it is really a quarter to one . That system simplifies things . Does away with varying times .