The Moon And Sixpence

The human eye's capability of discerning patterns in detailed scenes or images (where sometimes such patterns do not exist) is well known. A recent personal instance involving the Moon and a southern hemisphere feature which, while real, took the shape of a readily recognisable Earth-bound object is a case in point.

I was testing a newly introduced black and white film to see if it had any qualities that were superior to those of films that I had used for some time. On the night in question the Moon was slightly under two days from full. It is well known that at this time the Moon is an extremely bland object bereft of the surface feature detail which is seen when the Sun is lower in the lunar sky. The desire of the photographer, therefore, is to use a high resolution film with a good tonal range and extended contrast range. With the coming of PCs, I have frequently digitised film images so that they can be manipulated using computer techniques which are easier to apply and often more successful than their purely photographic equivalents.

This was done with several frames from my test run. As I brought one of the scenes up on the monitor, I suddenly noticed what appeared to be a very clear Arabic numeral "6" appearing to the east of the dramatic ray crater Tycho. My first thought was there was a film blemish but a check showed that the object appeared on every frame. This was intriguing so I then launched a search of numerous negatives and prints that I had shot over the past couple of decades, as well as numerous illustrated books.1 This revealed an interesting fact: the feature could be seen clearly at full Moon and for a day or so on either side of full. At other times of the lunar cycle is was not visible. Thus, the feature was something that was revealed when the solar phase angle was nearing, at, and a little after, zero - a time when surface detail is poor but when albedo differences are marked. (Here it is necessary to point out that the "6" will be a "9" for those astronomers, who place lunar south at the top of any reproduction!)

Figure A is a print of the southern hemisphere of a full Moon and is representative of many such images. The location of the feature is arrowed. Figure B is an enlargement showing Tycho and the immediate area of the feature which has been selectively enhanced. The images were taken at the prime focus of an f/9 178mm refractor. The area is dominated by the ejecta blanket and rays from the young (108 million years old) crater Tycho which is some 85 kilometres in diameter. Even images taken with very large aperture, observatory telescopes yield relatively little detail when the Moon is around full but a careful study of the location of the feature, compared with lunar maps and images taken at different solar phase angles, showed the bright curvilinear object to the west (left) to be the eastern rim of the large (114 km diameter) crater Maurolycus, described in one lunar atlas as "a vast walled plain with central peaks".2 Figure C is a section from a frame exposed using eye piece projection (with a 26mm objective) at the refractor when the Moon was a little over 6 days old. Tycho was still in darkness at this time but the craters around the feature, which is ringed, are named for reference. Since the Moon was far from being full in this image the feature is not visible.

Having arrived at this stage there was a necessity to locate higher resolution images. The rich harvest of images obtained by the Clementine spacecraft in the spring of 1994 (2.5m according to deputy science team leader Paul Spudis of Houston's Lunar and Planetary Institute) was readily available since after the mission ended the data were placed on the World Wide Web at URL http://www.nrl.navy.mil/clementine. Lunar Orbiter images were also investigated at http://cass.jsc.nasa.gov/research/lunar_orbiter/.

Figure D is a section from a downloaded 415nm image taken by Clementine's UV-VIS (Ultraviolet/Visible CCD) camera of much of the area shown in (C). The same locating craters are named and the feature ringed. This image confirms a fact revealed by (A) and (B), but with greater clarity - that Maurolycus and the feature (although the latter is dark) are situated on one of the very prominent ejecta rays emanating from Tycho. Further it would appear that it is ejecta from Tycho that has rendered the eastern rim of Maurolycus so bright in most of the images. The dark "6" is readily visible in this frame.

A Lunar Orbiter image (D (LO)) is of much higher resolution and shows the surrounding area extremely well but albedo differences are not so clearly shown.

Finally, presented as Figure E is an extreme enlargement from the Clementine image which has also been contrast stretched. This technique emphasises differences in albedo to the east of Maurolycus. An approximate calculation indicates the feature's north-south length to be somewhat greater than the diameter of Maurolycus - perhaps about 120kms.

The surface elements resulting in the formation of the dark figure "6" are readily seen in this last print. The area generally is a somewhat chaotic mixture of dark maria material and much lighter material presumably ejected when Tycho was formed. While there are numerous dark patches (for example, the crater Barocius and its surroundings) the figure runs in an approximate north-south direction across a wide area of ejecta ray which gives it its visibility. Further, by chance two small craters in close proximity have a higher albedo and create the appearance of the centre of the lower "o" of the figure. Off to the north, light material extending from the east runs westwards just far enough to create the upper part of the figure but not so far as to breach the dark regolith which appears to form the upper left curve of the figure. Its upper right curve is formed partly by the fortuitous distribution of darker and lighter albedo material but also by what may well be a ridge or old crater rim - which is incidentally shown in Map XVII in Edmund Neison's classic book The Moon published over 120 years ago!3

At a time when many seek extra-terrestrial explanations for apparent anomalies on Earth and elsewhere it is necessary to emphasise that at no time did I think that the figure was other than a chance feature observed on the lunar surface at a particular phase angle of the Sun. The intriguing puzzle for me personally is why I didn't notice the "6" on numerous other full Moon images taken over the years. I am even now looking again at the dust jacket of Paul Spudis' book The Once and Future Moon there is the full Moon in colour and, despite the slight loss of quality resulting from reproduction techniques, the "6" east of Tycho is clearly visible. Readers are invited to make the same test of any reasonable quality picture of a Full Moon!

Postscript

The Sun is not to be outshone by the Moon when it comes to phenomena exercising the pattern recognition capabilities of the human brain. Below is an image exposed through a DayStar hydrogen alpha filter. The seeing was not good at the time and the resulting lack of fine detail contributed significantly to a cluster of Sun spots appearing very much like - the face of a dog.

I am grateful to Patrick Moore, Paul Lowman (NASA Goddard Space Flight Centre) and Paul Spudis (Lunar and Planetary Institute) for information and discussion.

This article first appeared in Photon.