ls
light-second
Lengthexact by definition
A light-second is the distance light travels in vacuum in one second, exactly 299,792,458 metres. It is exact because the metre is defined by fixing the speed of light, so no measurement enters. The Moon is about 1.28 light-seconds away, which is why a radio exchange with a lunar mission has a two-and-a-half second lag.
| 1 ls | 299,792,460 m |
Where the unit came from
The unit is the natural one for spacecraft communication, where the delay is the operational constraint. A Mars rover sits between about 3 and 22 light-minutes from Earth depending on orbital geometry, so it cannot be driven in real time and must be commanded in sequences.
Solvers that use length