Lunar RAINBOW. The moon sometimes also exhibits the phenomenon of an iris or rainbow by the refraction of her rays in drops of rain in the night time. This phenomenon is very rare. In the Philosophical Transactions for 1783, however, we have an account of three seen in one year, and all in the same place, communicated in two letters by Marmaduke Tunstall, Esq. The first was seen 27th February 1782, at Greta Bridge, Yorkshire, between seven and eight at night, and appeared "in tolerably distinct colours, similar to a solar one, but more faint: the orange colour seemed to predominate. It happened at full moon; at which time alone they are said to have been always seen. Though Aristotle is said to have observed two, and some others have been seen by Snellius, &c. I can only find two described with any accuracy; viz. one by Plot, in his History of Oxfordshire, seen by him in 1675, though without colours; the other seen by a Derbyshire gentleman at Glapwell, near Chesterfield, described by Thoresby, and inserted in No 331. of the Philosophical Transactions: this was about Christmas, 1710, and said to have had all the colours of the iris solaris. The night was windy; and though there was then a drizzling rain and dark cloud, in which the rainbow was reflected, it proved afterwards a light frost."
Two others were afterwards seen by Mr Tunstall; one on July the 30th, about 11 o'clock, which lasted about a quarter of an hour, without colours. The other, which appeared on Friday October 18. was "perhaps the most extraordinary one of the kind ever seen. It was first visible about nine o'clock, and continued, though with very different degrees of brilliancy, till past two. At first, though a strongly marked bow, it was without colours; but afterwards they were very conspicuous and vivid in the same form as in the solar, though fainter; the red, green, and purple, were most distinguishable. About twelve it was the most splendid in appearance; its arc was considerably a smaller segment of a circle than a solar; its south-east limb first began to fail, and a considerable time before its final extinction: the wind was very high, nearly due west, most part of the time, accompanied with a drizzling rain. It is a singular circumstance, that three of these phenomena should have been seen in so short a time in one place, as they have been esteemed ever since the time of Aristotle, who is said to have been the first observer of them, and saw only two in 50 years, and since by Plot and Thoresby, almost the only two English authors who have spoke of them, to be exceeding rare. They seem evidently to be occasioned by a refraction in a cloud or turbid atmosphere, and in general are indications of stormy and rainy weather: so bad a season as the late summer having, I believe, seldom occurred in England. Thoresby, indeed, says, the one he observed was succeeded by several days of fine serene weather. One particular, rather singular, in the second, viz. of July the 30th, was its being six days after the full of the moon; and the last, though of so long a duration, was
three days before the full: that of the 27th of February was exactly at the full, which used to be judged the only time they could be seen, though in the Encyclopedia there is an account that Weidler observed one in 1719, in the first quarter of the moon, with faint colours, and in very calm weather. No lunar iris, I ever heard or read of, lasted near so long as that on the 18th instant, either with or without colours."
In the Gentleman's Magazine for August 1788 we have an account of a lunar rainbow by a correspondent who saw it. "On Sunday evening the 17th of August (says he), after two days, on both of which, particularly the former, there had been a great deal of rain, together with lightning and thunder, just as the clocks were striking nine, 23 hours after full moon, looking through my window, I was struck with the appearance of something in the sky, which seemed like a rainbow. Having never seen a rainbow by night, I thought it a very extraordinary phenomenon, and hastened to a place where there were no buildings to obstruct my view of the hemisphere: here I found that the phenomenon was no other than a lunar rainbow; the moon was truly 'walking in brightness,' brilliant as she could be; not a cloud was to be seen near her; and over against her, toward the north-west, or perhaps rather more to the north, was a rainbow, a vast arch, perfect in all its parts, not interrupted or broken as rainbows frequently are, but unremittingly visible from one horizon to the other. In order to give some idea of its extent, it is necessary to say, that as I stood toward the western extremity of the parish of Stoke Newington, it seemed to take its rise from the west of Hampstead, and to end, perhaps, in the river Lea, the eastern boundary of Tottenham; its colour was white, cloudy, or grayish, but a part of its western leg seemed to exhibit tints of a faint sickly green. I continued viewing it for some time, till it began to rain; and at length the rain increasing and the sky growing more hazy, I returned home about a quarter or 20 minutes past nine, and in ten minutes came out again; but by that time all was over, the moon was darkened by clouds, and the rainbow of course vanished."