Rendezvous with a Lunar Rainbow

John R. Brooks


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In 1726, when Benjamin Franklin was 20, he methodically recorded the events that transpired on one of his trips from England to America.

His ship the Berkshire, left England on August 5. On August 30, young Franklin recorded that “the moon being near full as she rose after eight o'clock, there appeared a rainbow in a western cloud, to windward of us. Franklin also had the experience on this trip of witnessing an eclipse of the sun and an eclipse of the moon just 15 days apart.

Aristotle erroneously ascribed a rainbow to the reflection of the sun's rays by rain – rather than to the fact that it was caused by refraction of light. Since this time the physics of rainbow formations has been well clarified and is completely detailed in W. J. Humphreys Physics of the Air (1) and Hans Neubergerís Introduction to Physical Meterorology (2).

On the night of June 20 1961 the Bermuda Race fleet was approaching the finish line off St. David's Head. We, on George Nichols's Concordia sloop Magic, were some 15 miles north of St. David's light when Franklin King, Jr. and I heard the call for our watch. It was 0100 and the night was warm and calm. with little wind. We had seen an eclipse of the full moon at 2115 the previous evening, as we approached Bermuda. The radio reports from Bermuda had many of the larger boats over the finish line but a good portion of the Bermuda Race Fleet was slowly beating its way up to the finish line in the warm zephyrs of the night.

The other watch had sighted the glow of Bermuda lights earlier that evening, and, as the night wore on lights of other competitors appeared in increasing numbers. The wnd had been discouragingly light following the first 36 hours of wind and rain in the Stream. Magic had stayed to the west of the rhumbline early on but even at that we could have tolerated further westing. and it looked like a long soft heat to the finish line. We gained some solace from the thought and the knowledge that there were many lights behind us.

As we came on deck that night the moon was full and was surrounded by some heaped-up cumulous clouds. It was a discouraged 1000-0100 watch that we relieved for they had. made relatively little to windward. With the usual light banter that the off-going watch has for that relieving it, we said good night, assured them that the 0100-0400 watch would see improvement in ground covered, and settled back to await developments.

The wind was still light from the south. The moon was full occasionally covered by large rain clouds, but on the whole radiating enough sunlight to make work on deck quite easy. Shortly after 0200 we entered a rain squall and for a short five minutes or so with the wind still from the south It rained hard. Moments after the rain ceased and the squall passed to leeward the moon again came out from behind a cloud and we returned to our original “lighted deck.”

The moon was on our starboard beam bearing approximately south southwest. A rainbow suddenly appeared on our port beam bearing northeast. It was a complete bow arising from the ocean's horizon forming a perfect semi-circle and disappearing into the ocean without interruption. Our lunar' bow followed the rule that places the center of such a bow the same distance below the horizon as the source of light (moon) is above the horizon and so that the viewer's eye is in line with the light source and the center of the circle formed bv the rainbow. Our bow lacked color but there were shades of gray that appeared to be broken up by the refraction of what light there was. Presumably the lack of color in our spectrum related to the low order of brightness contributed by the sun's reflection from the moon (2). The rainbow lasted for a few minutes. and. as the rain passed by further to the northeast. it disappeared not to return.

Lunar bows have been recorded before and are not rare in certain parts of the world. The are much rarer than solar rainbows because the shower type of rain which is a prerequisite is more likely to occur in daytime than at night, except perhaps over oceans (2). They occur more frequently in tropical island areas. Lobeck (3) pointed out that they are common in the trade wind belt due to the frequency of thunder squalls in this area. Wentworth pointed out that both lunar and solar rainbows were relatively frequent in the Hawaiian Islands “where most of the geographically variable rainfall is of orographic origin. i.e. due to cooling of trade winds in passing over rugged island topography. Here where local showers and mists occur sporadically on days and nights which are generally clear, are ideal conditions for rainbows.”

W. J. Humphreys states that lunar rainbows probably occur frequentlv but are not seen unless certain conditions present: a full moon is required; a clear, dark night is required; neighboring rain squalls are necessary; and a relatively dry atmosphere may well improve the light radiation. Commonly, a lunar bow is best seen soon after dark before a local or heat thunderstorm has fully rained out. This would be from 8-10 p.m. Therefore, in midlatitudes the moon may well be in the right phase during the proper hours only three days each month, and then the proper clarity of sky may exist. Our bow was not in the west, but the timing (0215) was at a different phase in the moonís travels, which accounts for the appearance of the bow in the northeast.

In any one place, according to Humphreys, the chance of having a lunar bow is one in two years. Probably only one time in five is the sky clear enough in front of a night thunderstorm to permit full illumination of the oncoming sheets of rain. Therefore, probably one time in 30 years is the potential frequency of visualization of a lunar bow by any one man.

The remainder of our crew were somewhat skeptical of our watchís report. During the entire race there had been skepticism of much of our verbiage. Be that as it may, two men of notable honesty did see a lunar bow, as had Benjamin Franklin some 238 years before. Our only regret was that, unlike Franklin, we could not report a solar eclipse along with our lunar eclipse and lunar rainbow.

–John R. Brooks, M.D., was a crew member of The Magic 1961 Bermuda Race)


1. Humphreys, W. J.: Physics of the Air. McGraw Hill Co., Inc. New York, 1940. 2. Neuberger, Hans: Introduction to Physical Meteorology. Pennsylvania State Univeersity Press, 1957. 3. Lobeck, A. K.: Science 88: 187, 1938.

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