June 6, 2020

Astro-Challenge of the Week: Aristarchus & the TLP.

This week, we turn your attention to the waxing crescent Moon and an enduring mystery that surrounds a unique crater; Aristarchus. About 40 km across, this crater was named after the Greek scientist Aristarchus of Samos by map maker Giovanni Riccioli. This lone crater sits on the Aristarchus plateau amid the Mare Oceanus Procellarum. This crater is near the lunar limb and becomes visible during the early waxing crescent phase, and is markedly brighter than the surrounding lunar plains.

In fact, Aristarchus has an albedo of roughly twice that of ordinary craters, and may be noticeable to a sharply trained eye! The Soviet lunar lander Luna 13 stands as the closest successful landing to the crater, at 275 miles distant. And the mystery you may ask? Well, that’s our astro-term of the week: Transient Lunar Phenomena. Over the centuries of men staring at the Moon, anomalous reports of lights, flashes and curious brightenings have dotted observer’s journals. Aristarchus stands as the most frequent setting for TLPs over the years. These were variously attributed to fires, volcanoes, vegetation, and even intelligence on the Moon; none other than William Herschel believed that our nearest neighbor was inhabited by a race of “Moonmen,” and almost right up to the Apollo era, no one was truly certain that at least vegetation was absent on the  lunar surface. TLP’s also make an appearance in Samuel Coleridge’s Rime of the Ancient Mariner, which also became the subject of what was termed the Coleridge Effect in Richard Baum’s The Haunted Observatory. An outstanding classic read on this and other astronomical oddities is A Catalog of Astronomical Anomalies by William Corliss. Of course, no moon-men or exotic lunar flora greeted Apollo astronauts; most TLPs have been attributed to observer error or optical illusions. A nagging handful may have been the occasional meteorite impact, or in the case of Aristarchus, an optical illusion coupled with the changing and oblique light angle of a crater near the lunar limb…still, it’s interesting to wonder just what those astronomers of yore might have seen. In 1911, an area just north of the crater on the Aristarchus plateau was analyzed by Robert Wood in the ultraviolet and found to contain a high concentration of sulfur. This patch is occasionally referred to as “Wood’s spot”. Even more interestingly, Apollo 15 made a direct pass over the Aristarchus plateau in 1971 and detected a high level of alpha particle emissions, highly suggestive of Radon-222 out-gassing. This was later confirmed by Lunar Prospector. Radon-222 only has a half-life of 3.8 days. Could Aristarchus be geologically active? As the Moon waxes towards first quarter, do give Aristarchus and its surrounding environs a close look!


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