Transient Lunar Phenomena

This is a series of rough notes on mysterious lights that have been seen by telescope on the surface of the moon for centuries. The intention is that this will eventually become a more fully polished article on the subject, but meanwhile it is hoped that the notes are useful to someone.

On the Floor of Plato

Transcribed from:

On the Floor of Plato. By W. R. Birt.

During the last forty-eight years, occasional notices of the spots and marking on the floor of the walled plain Plato have appeared. In consequence of having given considerable attention to Plato and its surroundings in the years 1860 to 1863, I col- lected all the observations of the spots that I became acquainted with, numbering in the whole fifty-six, having reference to four0 teen spots, two or three of which had been observed as craters, one being double. The greatest number observed simultaneously was seven, by Gruithuisen, in the year 1825. On the 23d of February of the present year, Mr. Pratt, of Brighton, observed with his 8-inch silvered-glass reflector eleven spots at the same time. They were not, however, eleven of the spots which had been observed previously, but included four unrecorded and a second double spot. Mr. Pratt has steadily continued his obser- varions up to the present time, and determined the relative posi0 tions of twelve spots by alignment. Mr. Edward Crossley, of Halifax, has also kindly requested his assistant, Mr. Joseph Gled- hill, to make continuous observations on them with his 9.3-inch achromatic by Cooke. The observations made by Mr. Gledhill and Mr. Pratt I have regularly received. The following are the names of the astronomers who have observed the floor of Plato. Gruithuisen, Ma:dler, Challis, Knott, the late Lord Rosse, the late Rev. W. R. Dawes, Baxendell, Dr. Dobie, Birt, Pratt, Cross- ley, Gledhill, and Elger, the number of observations being 297, in fifty series, and the number of spots observed twenty-five, in- cluding the companion of Dawes's double spot. In the annexed Table the number of each spot is given in accordance with the accompanying diagram, the name of the discoverer, the number of times each spot has been observed since the commencement of the present year, 1869, the comparative degree of visibility, that of the central spot, No. 1, being reckoned as unity, or 1.00, and remarks on the positions and general characters of the spots.

Spots on Plato.

No.Discovered byObs. Visibility.Remarks.
0Gruithuisen N.W. of 1, apparently nearer than 23.
1Gruihtuisen321.000 Central, easy, generally of the same appearance.
2Gruihtuisen1 Near the W. rim, seen once only by Gledhill.
3Gruihtuisen23.782 Dawes's double crater, frequently seen single.
4Gruihtuisen27.844 Departs often from typical state; seen double by Pratt.
5Challis20.625 Not seen so frequently as 3.
6Gruihtuisen8.250 Seen lately by Pratt and Gledhill.
7Gruihtuisen8.250 Seen lately by Gledhill, Pratt, and Elger.
8Gruihtuisen Probably the S.E. companion of 3.
9Dobie5.156 A minute spot S.E. of 1, seen by Dawes and Gledhill.
10Pratt6.187 Seen by Gledhill and Elger.
111 A minute spot between 9 and 17, seen once by Gledhill.
12Gledhill3.094 N. of No. 14; seen only by Gledhill.
13Pratt17.531 On curved streak c, now frequently seen.
14Pratt21.656 On central arm of "trident," frequently seen.
15Dawes1 A minute spot due E. of 1, seen once by Gledhill.
16Pratt13.406 On curved streak c, frequently seen.
17Ma:dler21.656 On wedge of "sector," frequently visible.
18Gledhill1 Near N.W. arm of "trident," alignment 1:18:22.
19Gledhill17.531 On curved streak c, frequently seen.
20Knott3.094 Near N. border, probably seen by Gledhill.
21Knott E. of 20, near N. border.
22Ma:dler9.281 Seen lately by Gledhill, Pratt, and Elger.
23Pratt1 Alignment 19:23:4.

As the comparative visibility of a spot depends upon its size and brightness, the larger and brighter spots are more likely to be seen in indifferent states of the Earth's atmosphere than those which are small and of less brilliancy. The number of observa- tions, 238, between Fed. 23 and Sept. 27, 1869, afford a prelimi- nary basis for an approximate measure of the degree of visibility of each according to the number of times it has been observed. The present order of visibility is as under:—

No.Visibility. No.Visibility. No.Visibility. No.Visibility.
11.00017.656 16.40610.187
4.8445.625 22.2819.156
3.78213.531 6.25012.094
14.65619.531 7.25020.094

In most cases the degree of visibility is underrated, as it is probable that in another series by the same observers many of the smaller spots may be looked for and seen under circumstances which, in earlier observations, occasioned them to have been over- looked, unless specially sought for. The spots remaining in the same condition, affected only by variations in the Earth's atmo- sphere, the probability is that successive series of about the same number of observations by the same observers will give similar degrees of visibility. It may be remarked that on no occasion have all the spots been seen simultaneously; the greatest num- ber recorded in one watch is thirteen, and these were not all seen at once.


The dotted lines in the diagram are intended to represent the probably boundaries of the lighter markings on the floor of Plato, which are by no means of a sharp of definite character, but ill- defined and passing gradually into the darker hue. Although there is some reason to conclude that they are really permanent, yet they are seldom seen alike. The observations of the delicate markings are not at present sufficiently numerous to allow of an examination into the circumstances that affect their visibility. In conclusion, I would remark that this communication is intended rather as a contribution towards elucidating a confessedly difficult branch of Selenography, than establishing the order of visibility of the spots above named, further observations being required for this object.

Apollo 11 TLP Observation

From the Apollo 11 Technical Air-to-Ground Voice Transcription:

03 04 57 07 CC Roger. And we've got an observation you can make if you have some time up there. There's been some lunar transient events reported in the vicinity of Aristarc hus. Over.

03 04 57 28 LMP Roger. We just went into spacecraft darkness. Until then, why, we couldn't see a thing down below us. But now, with earthshine, the visibility is pretty fair. Looking back behind me, now, I can see the corona from where the Sun has just set. And we'll get out the map and see what we can find around Aristarchus

03 04 57 54 CDR We're coming upon Aristarchus right now - -

03 04 57 55 CC - - Okay. Aristarchus is at angle Echo 9 on your ATO chart. It's about 394 mile s north of track. However, at your present altitude, which is about 167 nautical miles, it ought to be over - that is within view of your horizon: 23 degrees north, 47 west. Take a look and see if you see anything worth noting up there. Over.

03 04 58 34 CDR Both looking.

03 04 58 36 CC Roger. Out.


03 05 04 50 CC Apollo 11, when we lose the S-band, we'd like to get 0MNI Charlie from you. And update my last, that 77 04 was the time when Aristarchus should become visible over your horizon. 77 12 is point of closest approach south of it. Over.

03 05 05 14 LMP Okay. That sounds better because we just went by Copernicus a little bit ago.


03 05 12 51 CMP Hey, Houston. I'm looking north up toward Aristarchus now, and I can't really tell at that distance whether I am really looking at Aristarchus, but there's an area that is considerably more illuminated than the surrounding area. It just has - seems to have a slight amount of fluorescence to it. A crater can be seen, and the area around the crater is quite bright.

03 05 13 30 CC Roger, 11. We copy.

03 05 14 23 LMP Houston, Apollo 11. Looking up at the same area now and it does seem to be reflecting some of the earthshine. I'm not sure whether it was worked out to be about zero phase to - Well, at least there is one wall of the crater that seems to be more illuminated than the others, and that one - if we are lining up with the Earth correctly, does seem to put it about at zero phase. That area is definitely lighter than anything else that I could see out this window. I am not sure that I am really identifying any phosphorescence, but that definitely is lighter than anything else in the neighborhood.

03 05 15 15 CC 11, this is Houston. Can you discern any difference in color of the illumination, and is that an inner or an outer wall from the crater? Over.

03 05 15 34 CMP Roger. That's an inner wall of the crater.

03 05 15 43 LMP No, there doesn't appear to be any color involved in it, Bruce.


Clementine on Aristarchus:

Langrenus lights:

Sean B. Palmer