You can get even more technical than I did and try to tear down the meaning of "light" all you want. The generally accepted meaning of "a light" which would have been the kind of "light" referred to in saying "God put two lights in the sky" is, of course, something that emits light. The moon emits no light. It is only reflective.
Put it this way - consider the average person who existed during the time that the Genesis account was written. If they had been clever enough to fashion a mirror, and then used that mirror to direct the light that the sun provides into a dark-corner of their home, do you think that they would call the mirror "a light?" Fat chance.
And THAT is the exact relationship of object to source of light we're talking about here. And I argue that, had those writer's understood the subtleties in the relationship between the sun and moon and light they never would have written "two lights." They would have made the text more accurate, more correct in the usage of terms. Even the simple fact that I, and some like me, would take issue with this mention as an imperfection should have provoked enough stir in the "divine inspiration" of the text to make sure it was written to an exacting specification if it was ever to be hailed as perfect. In other words - if God exists, He knows the relationship between the objects, and so God could have made sure it was written this way, but He didn't, because... well... you know what I would say.