The region of “O’Neill’s Bridge.” (Photo by Author).

   Sometimes the most interesting visual challenges are objects that never were. This type of “non-event” can give us pause, to wonder exactly what those skilled observers of yore might have seen… such a challenge comes to us this week in the form of the spurious lunar formation known as O’Neill’s Bridge. This formation lies on the edge of the Mare Crisium along the meeting points of two lunar headlands: Promontorium Lavinium and Olivium, respectively. In the early morning hours of July 29, 1953,observer John O’Neill reported spying a tiny fan of light shining under what appeared to be a natural arc. He was using a 4-inch refractor with a magnification of 125x to 250x. Reports confirming his observation soon spread. The troubling thing was, natural bridges such as those in the American southwest are formed most by water and erosion processes that aren’t present on the Moon. And the dimensions of O’Neill’s bridge would have to be huge; something on the order of 20 miles long and one mile high! Clearly, something odd was at work here. O’Neill’s bridge also garnered a brief controversy in the pre-space travel era when it was suggested that it might be artificial! The bridge also gathered a moment of science fiction fame in Arthur C. Clarke’s novel A Fall of Moondust…

So what did O’Neill and so many others see? Are alien engineers setting up shop on our Moon, building bridges to nowhere?  Part of the mystery and solution lies in the fact that lighting angles on the Moon change dramatically from one apparition to another. Apollo 17 images show a tiny crater almost exactly centered between the two points; the thinking is that at the exact right lighting angle, the straight wall of the crater can look like a bridge between the two points, with perhaps a central peak just grabbing the sunlight to complete the illusion of an arch. A discussion of this came our way via Stephen O’Meara’s outstanding column The Secret Sky in the July 2010 edition of Astronomy magazine, and we felt it was an interesting challenge to share. O’Neill’s bridge seems to be most apparent when the sun angle is at a co-longitude of 127 degrees. This occurs about two days after Full, although the area of O’Neill’s bridge is illuminated from about 3 days after New until just past Full. Optimal viewing dates for the rest of the year are:

19:50UT October 25^th

09:40UT November 24^th

00:05UT December 24^th

Good luck and maybe you’ll see or capture the illusion as it bridges the gap!  

The astro-word for this week is Terminator. This is one of the more ominous sounding science fiction terms mostly thanks to the killer robot movie franchise of the same name. The terminator of an object is simply the dividing line between illumination and darkness. On most populated areas on Earth, you stand directly under the terminator twice a day, once at sunrise and at sunset. On the airless Moon, the terminator can appear abrupt and sharp, and most of the intriguing detail occurs along this line. I especially like to watch crater rims catching the first or last rays of sunlight while the floors are still in darkness, or seeing the tips of the lunar peaks lit while surrounded in darkness. Just what would it be like to camp out on those lunar ridges, watching the sunrise on a two week long “day” as the Earth is perched high overhead?