O'Neill's Bridge

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O'Neill's Bridge

(unofficial name)

Lat: 15.5°N, Long: 49.3°E, Diameter: km, Depth: km, Rukl: 26

external image normal_Prom-Olivium-Lavinium.jpg

Two LROC views (WAC No.'s M119530553ME and M117169050ME) of the region under different lighting conditions. Both images assembled with LROC_WAC_Previewer. The small shadowed crater near the center of the left image is Proclus PA (no longer an official name), on the outer rim of the much larger circular crater Proclus P, to its left. The shadow cast to the east by Proclus PA's east rim is presumably the feature mistaken as the shadow of a small arch by Wilkins, with its sunlit interior imagined to be the light streaming under the arch. O'Neill himself seems to have interpreted the triangular fan of illumination (which he saw more as an amorphous blob than as a fan) in the right-hand image as light streaming under a larger arch whose shadow falls beyond it to the east.


LPOD Photo Gallery Lunar Orbiter Images


(LAC zone 61B2) LAC map Geologic map LTO map


Promontorium Olivium is the site of the somewhat infamous O'Neill's Bridge, a lighting effect misinterpreted as sunlight streaming through a hole (or "aperture") in the promontory's ridge.

Additional Information

  • On July 30, 1953, as the Sun was setting over the western shore of Mare Crisium, science writer John J. O'Neill observed a patch of light on the mare floor that appeared to emanate from the low spot between Promontorium Olivium and Promontorium Lavinium and spread to the east. Scans of the original observations by O'Neill as they appeared in ALPO's Journal (then known as the Strolling Astronomer) from October 1953 have been kindly provided by astronomical historian Robert Garfinkle
    • external image (pp. 147-148)
    • external image (pp. 149-150)
  • O'Neill was well aware that a bridge would itself cast a shadow on Mare Crisium and that the light passing under it would be seen entirely between that shadow and the bridge. However why he thought the light he saw on the floor of Mare Crisium was streaming under a bridge rather than over a ridge is baffling unless he was going by the absence in his drawing of any shadow immediately adjacent to the supposed bridge, separating the bridge from the light streaming over it as would normally be expected (and as is in fact observed). In any event, he says he thought he was too late to observe the shadow of the arch itself, which he thought was lost beyond the eastern terminus of the bright patch. Also baffling is his assignment to the span of the aperture in his bridge of a length smaller than the 17-mile diameter of Proclus when his drawing shows an amorphous patch of light (which he says is steaming under the bridge) much larger than Proclus.
  • Incorrectly interpreting the pattern he saw as light passing under the arch of a 12-mile wide natural bridge O'Neill sent letters seeking confirmation of his discovery to a number of prominent lunar observers of the day.
  • News of observations by H. P. Wilkins was possibly first reported in a February 1954 issue of the Sunday tabloid the Illustrated, where the artist's drawing also shows the craters (one of them possibly Proclus PA) that Wilkins claims to have occasionally observed near the tips of the two promontories, and seems to suggest a bridge with a long span. Wilkins' decision to share his conclusions with the popular press and radio commentators before submitting his observations to the scrutiny of peer review seems largely responsible for the animosity with which they were later greeted and rejected.
  • Making light of the previous publicity, Wilkins described the bridge more carefully in a brief illustrated talk starting on page 119 of the BAA Journal for February 1954, in which he says that the larger spans reported in the press were misstatements of his remarks, and he clarifies that the small arch shadow he observed is unrelated to the much larger illusion of a fan of light which he thinks had attracted O'Neill's attention.
  • More official responses from Wilkins and others were published in the ALPO journal for "January/February 1954" (published and including news from somewhat later; scans of the follow-up articles were also provided by Robert Garfinkle):
    • external image (p. 1)
    • external image (pp. 2-3)
    • external image (pp. 4-5)
    • external image (p. 6)
      • For comparison here are approximations of what Wilkins must have actually seen on February 20, 1954:
        external image <-- re-mapped image by Wolfgang Sorgenfrey corresponding to lighting at 23:13 UT
        external image <-- LOLA simulation of lighting at 03:30 UT (on the morning of February 21)
    • Wilkins notes that the fan of light seen on the mare floor (resulting from the sunlight streaming over ridges tapering to a low point) was unrelated to the presence or absence of a bridge, and although he says he was unable to confirm O'Neill's claim of a 12-mile long opening, he did (he says) observe the shadow of a much smaller arch (and the light shining through its 1-2 mile long aperture) at nearly the same location. In addition to observing what he believed to be an aperture in the ridge, and sometimes rather large craters on their summits, Wilkins said he could see movement in the dusky arcs that punctuate the large fan of light (unrelated to the bridge) on the mare floor. Finally, Wilkins believed there was an elevated dome to the west of the bridge which cast its own larger shadow over the bridge area, hiding the latter shortly after it appears. What he seems to be depicting is the east rim of Proclus P casting its shadow over Proclus PA.
      • Note: There is considerable ambiguity in the scale of Wilkins' drawings. His initial ones from February 1954 seem to be illustrating an arched shadow that could well be coming from the rim of 7-km Proclus PA (and a spot of light that could be its sunlit interior), but he is consistent in his verbal descriptions that the aperture through which sunlight is streaming is just 1.5-2 miles wide. His later drawings (showing a tiny arch adjacent to the ridge extending south from Proclus PA) look quite different, but are much closer to the consistently stated dimension.- JimMosher
    • Wilkins also mentions a steep-walled ravine seen nearby at local sunrise, but his description of it is difficult to follow, and it does not seem to be shown in his February drawings.
  • Wilkins formal report to the BAA about O'Neill's Bridge (illustrated by the drawings from 1954 Feb 20-21) starts on page 205 of the April 1954 issue of their Journal.
  • Tentative reports of bridges at other locations on the Moon were reported in the April 1954 issue of JALPO (Strolling Astronomer) :
    • external image (pp. 45-46)
  • An article by Patrick Moore in the June 30, 1954 issue of VEGA (a journal edited by British amateur Richard Baum, who supplied scans) further recounts Wilkins' earlier observations. Moore himself was unable to observe anything that could be unambiguously attributed to a bridge:
    • external image (p. 66)
    • external image (p. 67)
  • The July 1954 JALPO includes more reports, and refutations of reports, of bridges elsewhere on the Moon:
    • external image (p. 99)
    • external image (p. 99)
  • Wilkins observed both with a 15-inch reflector at his home in Bexleyheath (a section of London) and later with the Mount Wilson 60-in reflector (the later, at least, with a quite high Sun).
  • Wilkins' (south up) Mount Wilson sketch, originally from the BAA Lunar Section's bulletin The Moon, Vol. 3 (1) 1954, is shown here (on the left) as it appears in the Dobbins and Baum, 1998 article in Bibliography:
Lunar Orbiter

external image

external image

  • It is compared to a Lunar Orbiter image that has the same lighting as would have been seen on 1954 Jun 17 10:54UT (3:54 am local time), remapped with LTVT to the geometry that would have been observed from Mt. Wilson. The exact time of Wilkins' sketch is not know, but according to Dr. Richard McKim, Director of the BAA's Mars section, Wilkins mentions in his book Mysteries of Space and Time that he observed the Moon after sketching Mars, and the Mars sketch examined by McKim is dated 1954 Jun 17 7h UT (midnight local time). The lunar observations were likely made a short time after and correspond to a slightly higher Sun than the Lunar Orbiter simulation. They show a tiny loop appended to the ridge that extends to the south from the small crater known in the System of Lunar Craters as Proclus PA. McKim also notes that in his book, Wilkins says the loop arch-like shadow is enlarged 2X for clarity relative to the other features (see also the mention of this on p. 180 of an April 1956 review of Wilkins' book). Wilkins elsewhere estimated the length of this arch to be about 1.5-2 miles, and appears to be trying to show in his drawing that over a 30-minute interval its shadow pattern changes in the manner that would be expected for an arch illuminated by a lowering Sun. With his home telescope he might possibly have been looking at Proclus PA itself, the bowl of which seems to be represented in the Mount Wilson sketch by a dark circle with a bird-like double beak extending to the south. Strangely, the axis of the bird's bill is shown diverging from the axis of Promontorium Olivium at an angle of about 30° when all photos suggest they should be nearly parallel or converging.
    • Nigel Longshaw has supplied a scan of the original article from the Moon, which includes an undated earlier sketch by Wilkins showing the arch-shaped shadow of the bridge observed with opposite illumination through an unknown instrument. The editor, F. H. Thornton, is skeptical about the implication of the shadows depicted in all Wilkins' drawings that the arch itself, rather than being an aperture in the face of a lunar ridge, rises above the ridge like a rose arbor in a garden. Thornton calls the bridge at this location "Wilkins' Bridge" to distinguish it from the "O'Neill's Bridge" at a slightly different location which observers had apparently discussed, and been unable to confirm in the previous issue of the Moon (not available here).
    • The Lunar Orbiter image, above, shows 1 km diameter craters at positions that might possibly correspond to Wilkins' sunrise and sunset shadows. They can be seen with illumination from the opposite side on LTO-61B2 and more clearly on Apollo AS17-M-0293 (and neighboring frames).
      external image <-- south up aerial view from Apollo 17
      • Note the equilateral triangle of small craters in the hollow above Proclus PA. The largest (on the left/east) is about 1 km in diameter. That on the right/west is 0.7-0.8 km. Wilkins might conceivably have glimpsed these craters (also visible in the Lunar Orbiter view) at sunrise and sunset and mistaken their interior shadows for arches. These particular craters, opposite the "beak" of Promontorium Olivium, seem a bit too far south. However, although Wilkins seems to show his arches of shadow closer to the bowl of Proclus PA, the locations he intended are uncertain: the landmarks he depicts correlate only vaguely with photos.
  • O'Neill's Bridge was the subject of pointedly skeptical discussion at the November 24, 1954 meeting of the BAA. A detailed, but unillustrated, report is available in the on-line copy of JBAA Vol. 65 starting here. F. H. Thornton and W. H. Steavenson took particularly dim views of Wilkins' observations (Steavenson had been embroiled with Wilkins in a recent dispute over claims about changes in Linné).
  • According to Charles Wood, Wilkins hinted in the 1954 edition of Wilkins and Moore that the bridge might be artificial, although other articles about the incident claim Wilkins always insisted its was natural. All references to O'Neill and his bridge seem to have been expunged from the final 1961 edition of The Moon, although his name is printed just to the west of the gap between the two promontories on Wilkins' map of Section XII (p. 192), and the name is listed as having been proposed by Wilkins (p. 353). - JimMosher
  • The western shore of Mare Crisium as drawn and labeled by Wilkins in a "Folio" of charts published in 1958, shortly before his death (from ALPO Monograph Number 3 Part 4):
    • Trapezium_Wilkins1958_detail.jpg <-- click to see surrounding areaIt is unclear if the name "O'Neill" refers to the crater with the ridges extending out of it (later to be christened Proclus PA in the System of Lunar Craters), or to the tiny break in the east ridge with the light arc drawn next to it (pointed to by the tip of Promontorium Lavinium). Note the extension of the larger west ridge far to the south, which Wilkins had earlier reported seeing. Note also that in Wilkins' shorthand the Cheerio shapes represent craters. Single circles are hills (or crater cones if they have a dot in the center).


  • The unofficial name O'Neill's Bridge was suggested by Wilkins in the January 1954 JALPO article.

LPOD Articles

Wilkins and the Bridge
A Bridge Too Far, An Observation Quite Good

LROC articles

Natural Bridge on the Moon! (near farside crater King).


  • Ashbrook, Joseph. 1954. "Is there a Bridge on the Moon?”. Sky and Telescope Vol. 13 (April issue), p. 205.
  • Dobbins, Thomas A. and Baum, Richard M. 1998. "O'Neill's Bridge Remembered". Sky & Telescope, Vol. 95, Issue 1 (January), pp. 105-108.
  • Graham, Francis. 1995. "The O'Neill bridge: discovery, analysis and subsequent track in literature to the present". Selenology Vol. 14, 4 (reprinted in Selenology Today Vol. 10, pp. 34-40.
  • Morgan, Phil. 2010. "The ‘mystery’ of O’Neill’s ‘bridge’. BAA Lunar Section Circular, Vol. 47, No. 11 (November 2010), pp. 3-5.
  • Wilkins, H. P. (and others) 1955. "O'Neill's Bridge". JBAA Vol. 65, No. 2, pp. 62-69.
  • Wood, Charles. 2003. O’Neill Unbridged (webpage).