[brummm]

It would redefine the meter too since the speed of light is defined as a constant and the meter derives from it.

[looperhacks]

The meter is defined by the speed of light in a vacuum, that doesn't change on the moon

[ZiiS]

Meter is defined as how far the light travels in a vacuum in 1/299792458th of a second. The actual speed of the light in a vacuum is fixed, so unless you have identical seconds the distance of the meter changes. If you have identical seconds in lower gravity, then you get less of them.