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Unexplained lights seen on the Moon are a classic example of a Fortean enigma called Transient Lunar Phenomena (TLP). They have been a mystery and a source of wonder to skywatchers since the earliest times. And yet, as astronomer Peter Grego points out, despite a wealth of detailed observations we seem no closer to an understanding of what these anomalous flashes are.

Not long after the telescope was invented at the beginning of the 17th century, astronomers came to realise that the Moon, our only natural satellite, was not as dynamic a world as the Earth. The dark lunar tracts which early astronomers had somewhat optimistically called "maria" (seas) turned out to be nothing more than deceptively smooth plains of solidified lava. Much to astronomers' disappointment it became apparent that there were no appreciable expanses of water, though the new romantic marine nomenclature was retained, regardless - names like Mare Crisium (the Sea of Crises) and Oceanus Procellarum (the Ocean of Storms) were given in a vain attempt to grant the Moon an air of mystery and excitement [1].

In reality, the Moon's surface appeared solid and unchanging. The Moon possessed no appreciable atmosphere and there were no detectable signs of lunar life; the Church breathed a sigh of relief, having been spared the embarrassment of attempting to explain why the book of Genesis forgot to mention that our sister planet was teeming with the products of DNA.

This initial impression of the Moon as being a barren and entirely dead world has been propagated in the astronomical literature ever since Galileo first published his observations in 1610 [2]. It seems, though, the Moon has been receiving an unjustifiably bad astronomical press for nearly three centuries, for reports of its longstanding status rigor mortis have been greatly exaggerated. Lunar observers (mainly amateurs) have noticed that the Moon's surface is occasionally host to anomalous transient lunar phenomena (TLP) which have assumed a variety of forms, including isolated flashes or pulses of light, coloured glows and obscurations of portions of the lunar surface. Just why the science of astronomy has been unwilling to accept that our satellite occasionally displays obvious signs of activity is almost as big a mystery as TLP themselves.

There is no shortage of TLP having been observed by reputable astronomers. William Herschel, one history's greatest astronomers (he discovered the planet Uranus in 1781), observed a red glow in the vicinity of the crater Aristarchus on 4 May 4 1783, at a time when that feature was situated on the unilluminated lunar hemisphere. Through his 225 mm reflecting telescope the glow appeared as bright as a star of magnitude 4. In April 1787 Herschel recorded prominent TLP on several dates, and he became convinced that the lunar surface was experiencing volcanic activity at three separate spots, including Aristarchus. So convinced, in fact, that he invited King George III to view the crater with him using the royal telescope in the grounds of Windsor [3].

One of the first real attempts to catalogue a large number of TLP sightings was made on behalf of NASA and published in a report which gave details of 579 mysterious lunar events dating from 26 November 1540 (pre-telescopic) to 19 October 1967 [4]. The catalogue appeared just a year before Neil Armstrong planted his size 11 boot in the Sea of Tranquillity; strange that such an important and well-funded Moon-landing programme chose to arm itself with some basic historical TLP data only at the very last minute.

NASA's belated enquiries represented a grudging acknowledgement that the Moon might not actually be the dead world it so convincingly advertises itself to be for most of the time. It was in NASA's interest, however, to down-play the idea of an active Moon. Known factors in lunar exploration were hazardous enough to plan for and contend with, without having to admit that lurking somewhere beneath the Moon's surface there might be some unknown, unpredictable and uncontrollable threat to their prospective Apollo lunar astronauts which could jeopardize the $25 billion programme.

More telescopes were turned Moonwards during the nine Apollo Moon missions (six of which made it down to the lunar surface) from 1968 to 1972 than in the entire 270 year history of telescopic observation preceding them. With such intensive monitoring it is hardly surprising that more anomalous lunar events were reported in this period than at any other time before or since [5]. Though many of these observations were a little dubious, made by inexperienced amateur astronomers keen to note anything which appeared out-of-the-ordinary, some were highly plausible because they were seen by independent observers at different sites. From their vantage point in lunar orbit the astronauts also made (in their quieter moments) numerous observations of apparent lunar surface activity of one kind or another.

One of the most notable TLP sightings occurred at 18:45 UT on 19 July 1969, when the crew of Apollo 11 observed the northwest wall of Aristarchus to be displaying some kind of peculiar luminous activity. At the same time, German astronomers Prusse and Witte of the Institute for Space Research in Bochum, observing with a 150 mm refractor, noted brightenings in Aristarchus lasting five to seven seconds [6].

Interest in the Moon - a body which does not often give its secrets away to the casual observer - evaporated soon after the Pacific splashdown of Apollo 17 in December 1972. The professionals had their scientific stations in-situ at six lunar locations (even though some experiments were sending back important data, they were permanently shut-down in 1976 to save money). The lunar geologists were happy to concentrate on probing the rocks which the astronauts had collected - they're still being analysed today.

Sadly, the Moon which shines in late 20th century skies is the least popular of celestial objects under professional astronomical scrutiny.Amateur interest has reverted to its pre-Space Age level - perhaps less, now that amateur Moon-mapping cannot yield any new knowledge of much significance. The Moon is highly neglected by both professional and amateur astronomers alike. Neglected, in fact, to the point where research into TLP - which continue to take place, whether they are being monitored from the Earth or not - now lies solely in the hands of the enthusiastic amateur.

Being such a creature myself, I have spent many hours looking at the Moon and getting to know its surface. In more than 200 hours of telescopic lunar observation since 1982 I have been lucky to observe only one event that I am fairly certain was a TLP. On 31 May 1985, an apparent hill was seen on the floor of the crater Herodotus (next door to Aristarchus). This was unusual, since Herodotus has a flat floor! In a period of nearly two hours, from 20:00 UT onwards, the hill appeared to flatten and disappear, its sunlit face becoming dimmer, its shadow narrowing and fading. Alas, circumstances ruled out obtaining independent confirmation, but I have since learned that this temporary hill phenomenon has been observed in the past, taking exactly the same form as my own observation but occurring at an opposite surface illumination, ie., an evening sun rather than, in my case, a scene lit by a morning sun [7].

TLP are rare. Since becoming Lunar Section Director of Britain's Society for Popular Astronomy in 1984 I have been privileged to receive many thousands of lunar observations made by hundreds of individuals, and in all that time only two credible TLP incidents have been brought to my attention by section members. On 1 December 1984 a reliable observer in Southam, Warwickshire, saw an homogeneous grey veil over the 20 km diameter floor of the crater Autolycus, and on 4 April 1996 an observer in Warrington was astonished to detect a pronounced bright spot at the centre of the dark floored crater Plato, immediately following a total lunar eclipse [8].

It is certain that unless a really major TLP incident takes place - one which lasts for an extended period of time and is recorded by many independent observers visually, photographically and spectroscopically - then subject will remain on the fringes of lunar science until the re-commencement of manned exploration some time in the 21st century.

A Canterbury Tale

A transient lunar phenomenon, recorded 819 years ago by Kentish men, might account for one of the strangest observations of astronomical history.

On18 June in the year 1178, a group of men at Canterbury in England witnessed a most remarkable event, according to a manuscript written by Gervase, a 12th century monk whose chronicle is preserved in the library of Trinity College, Cambridge. While the five men were admiring the beautiful four-day-old crescent Moon on that warm summer's evening, Gervase reports them to have been startled by "a flaming torch" which suddenly appeared at the lunar limb, "spewing out, over a considerable distance, fire, hot coals and sparks." The Moon is said to have "writhed like a wounded snake" and assumed a blackish appearance shortly after this unprecedented occurrence.

It is now thought that the 1178 event was caused by a sizeable meteoritic impact upon the lunar surface. As such, it was the first of only two major cosmic impacts to have been observed this millennium - the other one happened in July 1994 with the impacts of comet Shoemaker-Levy's fragmented nucleus in the atmosphere of Jupiter. It is ironic that both the 1178 and 1994 events were situated just past the limb of the impacted object, making it impossible to directly observe the site of the impacts at the moment they occurred.

From the rough position of the "flaming torch" described in the chronicle, Jack Hartung of New York University worked out that the impact site lay at around 45 north and 90 east. Looking at photographs taken by spacecraft in lunar orbit, the site became obvious - a bright, fresh-looking 20 km diameter crater called Giordano Bruno, situated just past the lunar limb (36 N,103 E) and surrounded by a prominent system of rays. Some of these rays actually extend past the mean limb around onto the near-side, so they are theoretically visible through binoculars and telescopes. Bruno and its rays are the newest major topographical features in the solar system which are likely to be permanent.

  1. Giovanni Battista Riccioli: Almagestum Novum ("New Almagest"), (Bologna, 1651). Much of Riccioli's lunar nomenclature was adopted as the standard, to which later named features were added. Attempts by his contemporaries to name lunar features after either terrestrial regions (such as the Mediterranean Sea, Sicily and Mount Etna) or famous people of their day (King Philip II's Ocean) were doomed to failure.
  2. Galileo Galilei: Sidereus Nuncius ("Starry Messenger"), (Bartoluzzi, Padua, 1610). Galileo wrote "....the surface of the Moon is neither smooth nor uniform, nor very accurately spherical, as is assumed by a great many is uneven, rough, replete with cavities and packed with protruding eminences."
  3. J L E Dreyer (ed): The Scientific Papers of Sir William Herschel (London, 1912)
  4. Jaylee Burlee, Barbara Middlehurst Patrick Moore and Barbara Welther: NASA Technical Report TR R-277 (NASA, July 1968)
  5. Patrick Moore: Catalogue of Lunar Events, October 1967 - June 1971, Journal of the British Astronomical Association (August 1971) v81 n5 p365-390. In this report an additional 134 TLP were added to the original lunar events catalogue, making a total of 713 reports of varying credibility.
  6. TLP report numbers 637 and 638 detailed in [4], p369 & p380-381.
  7. The observation appeared in my article An Observer's Grand Tour, Popular Astronomy (July 1993) v40 n3 p9. The astronomer V A Firsoff recorded the temporary hill in Herodotus as a "pseudo-peak" on July 15, 1955, and an account of this was published in his book The Old Moon and the New (Sidgwick & Jackson, London, 1969) p182.
  8. From the files of the Lunar Section of the Society for Popular Astronomy.