SOLAR AND LUNAR HALOS

Greymouth Evening Star, 19 March 1934, Page 10

SOLAR AND LUNAR HALOS

ancient superstitions.

When Octavian —who was to become Augustus—entered Rome after the death of Julius Caesar, the sun was surrounded by a brilliant rain-bcw-tinted ring. Whereby, says Dio Cassius, “the gods gave no uhcertaini warning of the troublous. times to coiiie.” , . Impressive even to the scientific observer, phenomena of this character are perenially strange, if no longer alarming, to the man in the street. Halos have been known to mankind in all ages, yet to this day they have never been relegated to “the dull catalogue of common things?’ They are still popularly regarded as freaks of Nature, abnormal,. quaisi-miraculous. The old chroniclers recorded every conspicuous example of their occurrence with awe and wonder —and the modern chroniclers still do. Since the word “halo” has a diversity of meanings, scientific and otherwise, it is necessary to explain that in meteorological optics this term embraces- all the luminous meteors resulting from the refraction and reflection (but not diffraction) of light by ice crystals in the air. The source of light is generally the sun or the moon, but artificial lights can also produce halos. The crystals concerned in their production are essentially tiny snowflakes, hexagonal or sometimes triangular in system, but infinitely varied in their details. Halos occur in ,a great variety of forms. Some encircle the sun or moon; others appear elsewhere, always in certain definite angular locations with respect to the source of light. Some are rings or arcs, others luminous spots. Some are prismatically coloured, others white. Some are qimrnon, others uncommon, and still others excessively rare. Some, theoretically possible, have never been observed. In cold climates the lower air is often laden with floating ice particles, which produce halos close to the observer. Sir Douglas Mawson tells of seeing them in the Antarctic in the frozen moisture of his own breath.

Of all the tnany members of the halo family one is an exception to wliat we have said about the persistent strangeness of these meteors to the average citizen—and even that one is still a subject of superstitious beliefs. The big white or whitish “ring around the moon,” is familiar to everybody. It is as well known and almost as common as the other kind of lunar appendage, much smaller and distinctly coloured, which Tennyson describes as .... the tender amber round Which the moon about her spreadeth, Moving thro’ a fleecy night.

and which science calls a corona. This is formed in water clouds and is not a halo. The two kinds of ring are much confused in popular weather lore. The halo just mentioned is a com-mon-place sight to the layman, but he views it with a certain amount of interest on account of its traditional reputation as a weather portent. It is supposed to foretell a storm; not necessarily a windstorm, but a. spell of bad weather. One of the old sayings is that a break in the halo shews the direction from which the

wind or rain will come; another, that the number of .stars inside the ring indicates the number of days before the storm’s arrival.

If however, you ask a meteorologist what this ring is “a sign of,” you will not get the sort of answer you expect. He will probably reply: It is a sign that the clouds through which the moon shines are composed of ice.” Prognosticating weather from halos —or from any other portent figuring in weather, folklore—is a most uncertain business. Cirrostratus clouds do spread out in front of travelling barometric, depressions, but the proverbial notion that a halo foretells stormy weather is merely one of thoie that science characterises as “not entirely without foundation.” The sayings about the broken ring and the enclosed stars are wholly fanciful. MOCK SUNS AND MOONS.

The big ring around the moon is called the halo of 22 degrees, in reference to its angular radius. It shows little or no colour, because of its feeble luminosity. The same halo is even more common around the sun, but this solar ring usually escapes the attention of the casual observer, whose eyes are dazzled, by the sun s rays. Dark glasses are an aid in detecting this and other halos. An exceptionally bright solar halo of 22 degrees shows the spectral colours plainly—red at the inner margin and violet at the outer —and can be so conspicuous as to arouse general interest and wonder. Mock suns and mock moons are halos, and there are several kinds, classified according to position. The former are often brilliantly coloured, though certain of the rare varieties are white. Mock suns are common enough to be the subject of several weather proverbs and of at least one poem, written by Wilhelm Muller and set to music by Schubert. Nevertheless, it is probable that a majority of human beings have never seen one, and they were a rare enough sight to our forefathers to figure as prodigies in the annals of ancient, medieval and early modern times. Maurice of Saxony is said to have raised the siege of Magdeburg in April, 1551, because of the appearance of three suns in the sky. As far back as the days of Aristotle, however, mock suns were believed to foretoken bad weather, and the dual role of presaging atmospheric changes and' dire events in the lives of men seems to have been attributed to them through the ages. Mock suns are always seen at. the same height above the horizon as the real sun, and along the line on which they lie there is occasionally visible a white circle, or a fragment of one, which passes through the sun and is parallel to the horizon. This is the mock-sun ring, or parhelic circle. A corresponding ring formed by moonlight is caled the paraselenic circle. , Extending vertically through the luminary there is sometimes seen a columnar streak of light, called a sun-pillar, or moon-pillar, as the case may be. This pillar never shows prismatic colours, but may be reddened by the light of sunset. In an ice-laden atmosphere such pillars are sometimes formed over city lights, giving the effect, when seen at a distance, of a display of the aurora borealis. One of the most beautiful of halos is the.' circumzcnithal arc. It is seen high in the heavens, forming part of a circle—or perhaps in rare cases a complete one—centred at the zenith.