On the night of April 19, 1787, astronomer William Herschel noticed an hour of light as bright as the Orion Nebula emanating from the unlit new moon. He likely witnessed a “lunar surface transient” (TLP), a short-term change in the appearance of parts of the moon’s surface.
TLP includes blanching, reddish or purple spots, and misty spots. In fact, over the past 2,000 years, around 3,000 TLPs have been recorded by telescopes, cameras, or simply by people with good eyesight, says Anthony Cook, a research lecturer in physics at Aberystwyth University in the UK.
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From milliseconds to hours
Masahisa Yanagisawa, professor emeritus at the University of Electro-Communications in Japan, told Live Science in an email that the ultra-fast flicker (lasting less than a minute) is likely caused by a meteorite impact. Meteoroids heavier than 0.44 pounds (0.2 kilograms) (about the same weight as a billiard ball) produce a brief flash of light when they hit the moon’s surface. The flash itself comes from impact energy that heats rocks on the moon’s surface, causing them to glow until they cool.
It was long suspected that such lunar impact flashes (LIFs) were flickers, but scientists could not definitively identify them until the 1990s, when high-speed video cameras became readily available for monitoring the moon, Yanagisawa said. Still, he added, the short duration of the flash makes it impossible to rule out factors such as electrical noise inside the camera.
Therefore, to see the flash, observations had to be made simultaneously from two or more separate locations. Despite these limitations, Yanagisawa said, “Some of the flashes were first seen during the Leonid meteor shower in November 1999,” which he documented in a 2002 study published in the journal Icarus.
Since then, hundreds more LIFs have been formally recorded by projects such as the European Space Agency-funded Near-Earth Object Lunar Impacts and Optical Transients (NELIOTA) program. NELIOTA has recorded 193 LIFs over a nine-year period, and these maps suggest that flashes occur in specific hotspots, such as the Oceanus Procella, a potentially tectonically active lunar region.
But Alexios Liakos, the project’s principal investigator and associate researcher at the National Astronomical Observatory of Athens, said this apparent pattern was an observational bias. In fact, a 2024 study he co-authored showed that the moon is being bombarded “almost uniformly by meteorites,” he told Live Science in an email.
In contrast, the moon’s light, which lasts several minutes, may come from radon gas emitted from the moon’s interior. A pair of studies published in The Astrophysical Journal in 2008 and 2009 suggest that such outgassing occurs when gas that accumulates beneath the moon’s surface is released in bursts by triggers such as moonquakes. Radioactive radon emits light as it decays and can be seen from Earth. Furthermore, the spots where longer-lasting light was observed almost overlapped with areas with higher radon concentrations.
But some moonlight, like the one Herschel witnessed, lasts for hours. According to a 2012 study, such sightings may be indirectly linked to the moon. The study suggests that the solar wind, a stream of charged particles originating from the sun, ionizes the moon’s dust particles and blows them into giant clouds 62 miles (100 kilometers) high. These clouds may refract light from stars and other bright objects visible near the moon in the sky, ostensibly illuminating the moon’s surface.
However, some researchers, like Liakos, dispute the existence of long TLPs. “The only longer (and not longer) events I have observed are satellites crossing the moon’s disk,” Liakos said, adding that since 2017, he has not seen a long-lasting TLP during observations of the moon’s night side.
Still, if you see light on the moon, pay attention. It could be an illusion of light reflecting off the satellite, but it could very well be a TLP.
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