On a night with broken skies, when thin, fleecy clouds glide slowly past the moon, the eye is irresistibly drawn to this illuminated centrepiece of the nocturnal landscape.
And very often, as another little cloud appears, beautifully variegated rings of light can be seen around the softly radiant lunar face, rings with a diameter only a few times that of the lunar disc itself. The display is a corona.
The brightest part is called the aureole, which is a whitish disc. But if you look more carefully, you will see that, nearest the moon, the aureole has a bluish tinge, merging into yellowish-white, and that this in its turn has a brownish outer edge. And sometimes this "bulls-eye", as it were, is surrounded by larger and more beautifully coloured rings, in an outgoing sequence of blue-green-red that may be repeated more than once.
A corona is seen whenever a cloud composed of water droplets partially obscures the moon. The droplets cause diffraction, a process whereby the tiny waves which make up a beam of light are diverted from their original path by small obstacles in their way.
The moon reflects the light of the sun towards us at a myriad angles; the effect of the intervening cloud is to divert in our direction some of the rays of light which were originally headed a few degrees away from us, and which should, so to speak, have missed us altogether.
The water droplets, in a sense, divert more of the moonlight in the observer's direction than he or she should normally be entitled to expect, and this, happening over a large area, results in the bright circle of the corona. The different colours are in evidence because the process of diffraction affects some individual colours more than others.
A corona, in fact, is faintly visible in almost every type of cloud. Nor, indeed, is the moon the only celestial sphere to be affected; it is frequently seen around the sun, and sometimes a small, feebly luminous, aureole can be seen around Venus, Jupiter and many of the brighter stars.
The presence of a strong corona is an indication of great uniformity in the size of the water droplets that make up a cloud. And since the swarms of droplets in a cloud have a continual tendency to become of unequal size, the larger ones growing at the expense of smaller members of the population, a strong corona is also an indication that the cloud concerned has probably formed only very recently; that it is, as it were, a "young cloud".