Transient lunar phenomena (TLP) are relatively brief flashes of light that appear to come from the surface of the Moon. Such flashes of light have been observed for centuries by both amateur and professional astronomers, and several theories have been put forward to explain them. One theory is that TLP are not genuine lunar phenomena, but only random observational errors caused by defects in the telescopes some observers use. Optical instruments such as telescopes or microscopes are complex pieces of machinery; even small flaws in their design can create artificial optical impressions like flashes or other distortions in the visual field. To an observer, a flash of light caused by an instrument error may sometimes look like a real phenomenon taking place in nature. A second theory is that meteors striking the Moon cause TLP. Because the Moon has no atmosphere, meteors often reach the lunar surface. In fact, meteors strike the Moon every day. According to the meteor theory, when meteors strike the lunar surface, they explode, creating enough heat energy to appear as a flash of light to observers on Earth. When people observe TLP, they are actually observing flashes of light generated by meteor impacts. A third theory is that TLP are the result of lunar rocks emitting their own light by a process called thermoluminescence. It is known that the Moon does in fact contain certain rocks that are capable of generating light in this way when they are heated by the Sun. This is strong empirical support that TLP are caused by thermoluminescence.