Popular Astronomy

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Transient Lunar Phenomena (TLP)

TLP are occasional anomalous glows, unusual colourations and odd obscurations of detail sometimes reported by lunar observers. Their exact cause is unknown, so if you see anything on the Moon that you think is unusual and out-of-the ordinary (bearing in mind that the Moon is by no means 'ordinary'!), do report it to the SPA Lunar Section.


The location of around 2000 TLP reports from 554 AD to the present time. The spot area indicates frequency of reports for the feature concerned.

A word of caution, though -- the Moon is full of predictable 'surprises' and it helps to become familiar with the way certain features appear to change as the angle of sunlight illuminating them changes before broadcasting your 'discovery' further afield! The Lunar Section will investigate your report.

The best way of assessing how the tone of the lunar surface appears to change at various illuminations is to use a scale of intensity estimations (see towards bottom of page).

 

A brief history of odd lunar happenings

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. 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.

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. 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.

One of the first serious 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. 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 there might be some unknown threat to their Apollo lunar astronauts which could jeopardize the $25 billion programme.

The mysterious Aristarchus area, imaged from Apollo 15.

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. 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.

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 that shone in late 20th century skies was the least popular of celestial objects under professional astronomical scrutiny, but amateur interest has always been high.

Above (left): A 200x magnification of the region of a transient lunar phenomenon photographed by George Kolovos on 23 May 1985; (right) the same picture superimposed with outlines of surface features: (a) an unnamed crater and (b) Proclus C

Now, in the 21st Century, there has been a profound revival of interest in the Moon. It seems that amateur Moon observation is still capable of revealing significant phenomena.

In hundreds of hours of telescopic lunar observation since 1982 I have been lucky to observe several events that I am fairly certain were anomalous, which goes to show how rare TLP are. Since becoming Lunar Section Director in 1984 I have been privileged to receive many thousands of lunar observations made by hundreds of individuals, and in all that time only a handful of credible TLP incidents have been brought to my attention by section members.

 

Intensity estimations

Lunar craters change markedly in appearance during the course of the lunar cycle (New Moon to Full Moon and back to New Moon). The greatest detail can be seen just after a crater emerges from the terminator (or again two weeks later just before the terminator crosses back over that crater) because of the long shadows cast by all relief features. It is around this time that a drawing of a particular crater is usually made.

On the other hand, the "look" of a crater around a week after its first appearance is utterly different. Now there are no shadows to be seen at all, and no relief detail is directly visible. Instead, what you see are genuine brightness variations of the Moon's surface, indicating how good various parts of the lunar rock and soil are at reflecting the sun's light. A drawing of a crater made under these conditions is a useful complement to one made when the crater is full of shadow near the terminator.

The intensity estimations technique is simply to sketch the boundaries of areas of different brightness, and in addition to estimate those brightnesses on a scale of 0 to 10. In Elger's book The Moon, the author quotes a scale of intensities adopted by the lunar observer Schröter. Although it is a very old system, it cannot be much improved upon, and this is the scale which is used by SPA Lunar Section members. The examples of tones described below are for a general low-power telescopic view (x40) when each particular area is illuminated at its own local lunar midday. Each area will, of course, break down into further gradations of tone under higher powered scrutiny.

0 Black - for the darkest of lunar shadows
1 Very dark greyish black - dark features under extremely shallow illumination
2 Dark grey - the southern half of the floor of Grimaldi
3 Medium grey - the northern half of the floor of Grimaldi
4 Yellow grey (subtle) - general tone of area west of Proclus
5 Pure light grey - general tone of floor of Archimedes
6 Light whitish grey - the ray system of Copernicus
7 Greyish white - the ray system of Kepler
8 Pure white - the southern floor of Copernicus
9 Glittering white - the rim of Tycho
10 Brilliant white - the bright central peak of Aristarchus

These tints are obviously a little hard to picture, but you'll get used to them with practice. The method is a little like that used by planetary observers, and although intensity estimates of this type depend heavily on the individual observer, excellent results can be obtained if the observations of a large number of people are combined.

The following craters should be included in the observing programme of anyone hunting for TLP.

  • Agrippa
  • Copernicus
  • Fra Mauro
  • Plato
  • Alphonsus
  • Endymion
  • Gassendi
  • Ross
  • Aristarchus
  • Eratosthenes
  • Grimaldi
  • Theophilus
  • Atlas
  • Fracastorius
  • Herodotus
  • Tycho