"Lunar X" (Clair-Obscur phenomenon during First Quarter Moon)
|Lat: 25.3°S, Long: 0.9°E, Diam: 70 km, Rükl: 55|
Ron Bee "Lunar X" photographed on 11/17/2007 at 02:26 UT. The "X" is the pattern to the left of the terminator in the upper left. The deeply shadowed craters below the "X" are Werner (top) and Aliacensis (bottom). To the right of the "X", the large low-walled plain is Playfair G, with 47-km Playfair (two-thirds covered by shadow) to its right. The 63-km diameter crater below Playfair G is Apianus.
Werner X Sunrise video by M Jones and E Kotapish of Ft Worth, TX. Exposures taken from 13 Apr 0150UT thru 0320UT at 15 minute intervals. Takahashi TAO130S/Canon XTI/prime focus/500th/ISO200 (1.8Mb)
Werner X April 12 2008.avi
This is a famous "optical feature" on the Moon, which appears like the letter "X" when the terminator is at a suitable position. It is a fine example of how the combination of lighting and topography can combine to produce a pattern that repeats on each lunation, but only for a short time. The illusion of the "X" is created by sunlight falling on the rims/ridges between the craters La Caille, Blanchinus, and Purbach.
The "X" is observable for about 4 hours around the lunar First Quarter.
- It is unknown when or by whom the "X" feature was first observed. The following are the dates when the "X" was first noticed by various observers who later became interested in it:
- In the article cited below, Dave Chapman analyzes the "X" in terms of the sun angles at Werner. However, the event is dependent on sunlight illuminating the features at 25.2S/0.9E, so it is really the sun angle at that position which matters. Aside from differences in foreshortening, the view will be essentially the same whenever the Sun is at a particular angle above the local horizontal (the "sun angle"). For example, according to Chapman, in an observtion of January 25, 2007 the "first pinprick" of light was observed at 2130 UT. This corresponds to a sun angle of -1.97º. At this particular position, and under the circumstances where the "X" is observed, the local sun angle is increasing by about 0.1 degrees every 13 minutes. According to Chapman, the "X" can be seen for a total of about 4.5 hours, during which time the Sun would rise by about 2º (or to a roughly horizontal position). The illusion of a bright "X" on a dark background is most striking for about an hour near the end of that interval.
- The first two images in the illustration at left
(click to see a larger version) show the relatively subtle changes in the pattern that occur as the sun angle increases by about 0.2º (although taken 41 years apart, this pair of images corresponds to the change that occurs each lunation in an interval of about 30 minutes). The final image in the row shows the underlying features that give rise to the pattern, as seen when the Sun is much higher. The shadowed crater on the left is Purbach, while on its right is Blanchinus (cut in half by the right margin). Above them, cut by the top margin, is La Caille. (note: the freeware LTVT software has been used to re-map all three photos to an identical zero-libration scale).
- In Table I of his paper, Chapman lists the dates and times when various observers thought they were seeing a perfect or "Peak X". The corresponding sun angles (at 25.3ºS/0.9ºE) are:
- Note: 2005-02-16 is misprinted in Chapman's Table I as “2005-02-15”; but even when corrected is difficult to reconcile with the description in the text that on this occasion the X “was fully illuminated from 0515 UT until at least 0700 UT.” The March 2007 observation, since it is at a lower sun angle than the illustration used here, probably also refers to the "start" rather than the "peak" of the X.
- Chapman also illustrates his article with examples of the X photographed on January 25, 2007 at 23:45 UT (his Figure 1) and January 26, 2007 at 01:38 UT (his Figure 2). The corresponding sun angles at 25.3ºS/0.9ºE were -0.94º and -0.08º. Hence, his Figure 1 is expected to be very similar to the Ron Bee photo shown above. As indicated in the table reproduced above, the "peak" of the phenomenon (on this occasion) was said to occur at about 01:22 UT (or a sun angle of -0.20º).
- What is regarded as the "peak" is obviously very subjective, for as Consolidated Lunar Atlas Plate F12 (reproduced above) shows, an "X" is still readily visible (if one knows where to look) even at a sun angle of +7.9º, but is no longer a bright cross on a dark background.
- At about the same moment of the Lunar X's appearance, there's also the Lunar V north-northeast of Ukert M. This clair-obscur effect has the same diameter as the Lunar X, and is also observable through small and common telescopes. An interesting photograph of both the Lunar X and the Lunar V (on the First Quarter Moon) was printed in T.W. Webb's Celestial Objects for Common Telescopes (Volume 1: The Solar System). It's between pages 102 and 103 (the F.Q.-moon photographed at the Lick observatory, Mount Hamilton, California). - DannyCaes Dec 17, 2007
- The exact origin of the term "X" or "Lunar X" is unknown.
- In the article cited below, Dave Chapman adopts the name Werner X on the basis of Werner being the nearest prominent crater fully lit before the event, and therefore serving as the best guide for where to look.
- Many other equally informal names have been used, including the Purbach X and the Blanchinus X.
- David M.F. Chapman (2007) The Lunar X Files: a fleeting vision near the crater Werner. Journal of the Royal Astronomical Society of Canada. 101 (2), 51–56 (April 2007 issue). (PDF 2.5 MB)
- Philip S. Harrington (2011) #79 Moon: Lunar X and Lunar V, Cosmic Challenge, pp. 198–200, Cambridge University Press
- Additional information provided by Dana Thompson.