[@TheUnknowable] Well, it's not the full moon that does it, perse. The full moon gives off Blutz-waves that are a form of Alpha radiation that is absorbed through a Saiyans eyeballs. Due to how weak Blutz-waves are, they can't penetrate the eyelids, which is why a Saiyan has to stare directly into it to transform. Blutz Waves naturally form when sunlight is reflected off of a nearby celestial body and defracted through an atmosphere. Hense why the moon can only transform a Saiyan when it's full. They aren't concentrated enough to reach Earth unless it's a full-moon. Also why, in GT, Goku was able to transform by stating staring at Earth through Tuffles atmosphere.