So what's the 'terminator' on the moon? Has Arnold Schwarzenegger headed into space?
Not really...at least not that we know of! The terminator is simply the dividing line on the moon between the light and dark regions. That changes with the phase of the moon, and if you have access to a telescope or binoculars, it's a great spot to observe, since craters and mountains cast shadows that reveal details not ordinarily visible.