Transient lunar phenomena (TLP), also known as transient lunar phenomenon, refers to sudden flashes of light; darkenings; green, blue, or violet colorations; reddish, pink, or orange colorations; and floating mists observed on the surface of the Moon. Reports of transient lunar phenomena go back over 1,000 years, but have only really been taken seriously since the early 1960s, when astronomers themselves began to view the phenomena and record it. At least 300 such events have been recorded by modern astronomers, with at least 2,200 reports in the historic literature. Over a third of all reliable incidents emanated from the Aristarchus plateau region on the Moon, on the northwest part of the near side.
References to transient lunar phenomena are scattered throughout the astronomy literature of the last 1,000 years. For instance, on 18 June 1178, five monks from Canterbury reported "a flaming torch" in the northern region of the Moon shortly after sunset, "spewing out, over a considerable distance, fire, hot coals, and sparks." On 19 April 1787, British astronomer Sir William Herschel, discoverer of Uranus, noticed three red glowing spots in the dark part of the Moon, which he attributed to volcanism. Interesting is that around the same time, the aurora borealis were rippling above Padua, Italy — which practically never happens — less than a thousand miles away, and the sunspot cycle was at its most intense.
Transient lunar phenomena are hard to confirm and verify because they are, by nature, transient, with none recorded on film or video and usually only one witness. Scientists have come up with four possible explanations for transient lunar phenomena: impact events, outgassing, electrostatic phenomena, and unfavorable observation conditions or atmospheric effects. Due to the fact that transient lunar phenomena are so rare and distant, it is difficult to test these theories empirically.
Outgassing is something that occurs on practically every rocky body to some degree. Volatile gases, produced by radioactive decay or tidal heating, get trapped in cavities under the Moon's surface. They are then released slowly or in discrete explosions. This correlates well with one of the main sites where TLPs are observed — around floor-fractured craters, which would provide opportunities for sublunar gases to escape.
Impact events occur on the Moon all the time, mostly through micrometeorites. Impacts of slightly larger meteors might appear as flashes on Earth. Meteors of all sizes hit the Moon frequently.
Another possible source of TLPs are electrostatic discharges, caused by charges building up due to friction, solar wind, or other mechanisms. If the charge is large enough and over a big enough area, the eventual discharge may be large enough to observe from the Earth. This has not been confirmed, however.
The last cause of TLPs would be the most mundane — observational relics caused by the Earth's atmosphere. Atmospheric distortion can cause the Moon to appear hazy, especially with a telescope with high resolution.