(Note: the following is one of various posts that were copied here from the Moonwards.com forum for the sake of preserving the early days of the project. It was originally posted there on Dec 13th, 2015.)
Let's say you have enough infrastructure on the Moon to fabricate really good mirrors, small motors, and pivoting joints in bulk. And you make 10 km2 of mirrors and set them up outside so you can very accurately point them. That's 12 GW of sunlight to throw around, accounting for some inefficiency in the mirrors - if your target is at a small angle to the sun from the mirror's position. At a 90 degree angle it would only be 8.5 GW.
I first thought of this idea of reflecting the sun from the Moon as a playful thought experiment in how a permanent human presence on the Moon changes our capabilities. I suggested a more modest 2 km2 of mirrors aimed all in the same direction would create a small star on the Moon's surface visible with the naked eye on Earth, if the mirrors were given modest tracking capabilities. A Moonstar, a little beacon of hope and wonder.
Now let's put on the black hat. If you focused the sunlight of all those mirrors on an area of say 10 m2... what would happen? Given that means you have to make those mirrors super flat, and put motors and pivots on a million units capable of extremely precise positioning and tracking. Or wait, do you? If you manage to focus 10 km2 on an area of Earth that's 1 km2, that's still enough to do royally nasty things. All life caught outside would die in minutes. Those sheltering in houses and such might last a few hours huddled in basements.
That would be 10 000 units with areas of 1000 m2, which would mean circular mirrors with a radius of 18 m. That doesn't sound so very difficult, in 1/6 gravity and no weather. The industrial capacity for something like that isn't really very much. Any development of the Moon for permanent settlement would lead to it after a decade or two.
Why ponder such things? Because the easier it is to do extremely destructive things, the greater the urgency of ensuring nobody does them. And in the case of the Moon, the critical thing to realize is that there is absolutely no way to control an independent colony. They are beyond your reach, they will do what they want. The only thing you can do is ensure they are happy and healthy, and therefore averse to destroying things.
Then the thing to realize is that means we need to start now and go big. You don't leave things that could lead to global chaos to chance, you don't put them off to see what others do. You do them together, properly, and you start now.