think of how an infrared thermometer works - it is measuring the EM emissions (and reflections) of IR. The surface is not actually colder in moonlight, in fact it is slightly warmer, (by millionths of a degree, maybe) since it is recieving more energy. Your instrumentation for this experiment is inappropriate. Use a mercury thermometer or a thermocouple
-What to consider for future tests: Scientific experiments should be designed to challenge a hypothesis, not confirm it