LUNAR HALOS

Taranaki Daily News, 29 November 1930, Page 13 (Supplement)

LUNAR HALOS

MOCK SUNS AND MOONS

(Rev.

B. Dudley, F.R.A.S.)

A meteorological phenomenon of rare occurrence in temperate latitudes mis observed at Russell last week. The conditions of the upper atmosphere were somewhat unusual, as the phenomenon known as “mock moons” was then observed in Auckland and the corresponding day-time effect of "mock suns” was reported from Cape PaJliser. The phenomenon seen at Russel! is known as a circumzenlt-ha) are and was seen shortly after 5 p.m., while the ordinary sun halo was also very noticeable. It consisted of an are about GO degrees in diameter, with the zenith as its centre, and displayed the colours of the rainbow, with the violet innermost. . The above is from a report which appeared in the Taranaki Daily News on the 20th. instant, and has inspired this article.

Not infrequently there are witnessed round the moon, on nights when the .sky -is thinly veiled with mist, complete circles of delicately coloured light, these iridescent rings are known as lunar halos. The colour is generally very -subdued, although at times it may be so pronounced as to cause some observers to mistake the halo for a lunar rainbow —a totally different phenomenon. The halo, on the one hand, encircles the moon more or less closely, and is therefore on the same- side of the sky with it; the lunar rainbow, however, is always", and of necessity, on that region .of the sky opposite the moon. In'order to see the halo the observer must stand with his face to the moon; but to see the lunar rainbow he must have his back to it. This halo is due to the moon’s light falling upon myriads of tiny, prismshaped -crystals of ice floating in the upper air. It is quite commonly known ■that if a sunbeam is, allowed to fall upon a three-sided bar dr prism of glass, it is bent out of its original course as it passes through the jirishi, a,nd, at the same time, broken up into a diverging sheaf of coloured rays. The light that passes amongst the suspended ice prisms in the higher levels of the atmosphere behaves in exactly; the same way. i

In order to understand what is taking place, as an observer looks at a halo encircling the moon he must try to conceive that, at a certain distance from the apparent position of the moon, the ice-prisms stand in such a relation to the light which is thrown off them from the shining luminary as to bend down towards the eye a comparatively large ■proportion of- the rays. Great numbers ef bl»i ice-prisms which happen to lie at that precise distance arc so ranged as to conspire to throw the luminous beans towards the eye rather than in any other direction. Since this takes ■place at the same distance all-round the moon, the luminous band appears as a ring. The crystals of ice are scattered in the air in all conceivable positions. The lunar rainbow is produced in precisely the same way as is the solar rainbow, the former being feebler in light and- colour for 7 obvious reasons. The “corona” of -coloured light which •sometimes forms round .the moon is qpite different in character from the halo. For one thing, it is of a much smaller size, seldom more y than four times the. breadth of the moon away from its ,rim., The lunar" cotoria stands distinguished from the. halo, also, in that the order'of the succession of col-, ours is reversed. The red occupies a place, at the outer edge of the circle of the corona, while the blue forms the inner margin. In the case of the halo, on 'the . contrary, the ruddier tinges are along the inner border of the ring. The corona, as a matter of fact, is pot caused by frost (ice-crystals), but by mist or fog. It is produced by the water vapour floating in the upper air between the moon and the observer, An.analagous appearance is sometimes witnessed as one looks through a glass window bedewed with moisture at a bright light. An ‘artificial “corona” can be produced by, observing (the flame -off a lamp through plate-glass upon which the minute spherical spores ;of cliibmoss have been sprinkled, and in other ways. . , .

A similar corona (so called) is some,times observable. round • the sun when a light-fleecy cloud is passing over it. This again is different from the solar corona witnessed only' when the sun is totally eclipsed by the passing moon, and which was the theme of a recent article. The true solar corona is of cosmic origin, and belongs to the province of the astronomer, whereas the lunar and ■ solar coronas, together with the halos just described, are meteorological phenomena. •

There was. once a popular notion that ■the number of bright etars visible within the circle round the moon indicated the number of days before a storm would occur. The belief was wholly without foundation. In whatever part of the heavens a lunar halo is seen at least one bright star is almost sure to ■be noticed inside the luminous ring, and the total numbei’ visible depends entirely upon the position of the moon. Moreover, when the sky within the circle is examined, even with a small telescope, hundreds of stars are seen where only one or two could be detected with the unassisted vision. ’ •

The report at the head of this article makes mention of mock moons as being visible in Auckland, at the time -of the lunar halo, and of mock suns seen from Cape Pal-liser (a “corresponding daytime effect”). These are known respectively as Parhelia and Paraselenae, and are frequently witnessed in association with halos. Occasionally the luminous rings under consideration are accompanied by supernumary circles which cijoss or meet them tangentially. These mock creations are observed at the points of intersection with the primary halo. They are due to the more brilliant illumination at those points, the refracted light being, of course, intensified thereat. The circumzenithal arc mentioned as having been seen at Russell was no doubt the segment of one of these secondary circles, so far above, yet adjoining the primary halo, as to have the zenith for its centre. Occasionally there is observed a vertical shaft of white light cutting.through the ring and the centre of the sun. This, in some places, is known as a “sun pillar.” It can but very rarely be seen except after sunset or before sunrise.

The reason why coronal rings are observable less frequently about the sun than about the moon is because they occur in the midst of the glare of bright sunshine, and are thus more likely to be overlooked. The solar . halo ' may be detected quite frequently, when the conditions are as above described, if the eye is screened from the brilliant sunlight by a darkly tinted or smoked glass..