Back in Ireland for a day or two, I had time to stand and stare a little more than usual, and was rewarded in the early hours of Sunday morning by the sight of a most beautiful corona.
"Bless your exciting little life!" I hear you say.
Be that as it may, readers who may have lacked the opportunity in recent days to observe the sky in real life, will at least remember from last Thursday's Weather Eye that over the weekend the moon was very nearly full. As it happened, while I kept my lonely but invigorating vigil, the sky was also nearly clear. But not completely so: every now and then a patch of thin cloud would drift across the lunar disc, and as it did so, a circle of light appeared around the moon, which expanded and contracted, strengthened and faded, reflecting the individuality of each passing cloud.
The colours of the corona stood out clearly in the classic order: it was bluish near the moon, white further out, and terminated in a reddish-brown ring, about three "moons" in diameter, at its very outer edge.
A corona occurs when water droplets in a cloud, by a process called diffraction, cause the tiny waves which make up a beam of light to be diverted from their original path. When a cloud of water droplets lies between an observer and the moon, some of the rays of light which were originally heading a few degrees away from him - which should, so to speak, have missed him altogether - are now diffracted, or bent, towards his eyes. This, happening over a large area of the cloud, results in a circle of light. The water droplets, in a sense, divert more of the moonlight in the observer's direction than he would normally be entitled to expect.
But what about the colours? Sunlight - and hence the light reflected from the surface of the moon - is white; it is a mixture of all the colours of the familiar spectrum. But the process of diffraction affects some individual colours more than others. The short blue wavelengths are bent very little; the longer red wavelengths are diffracted a good deal more.
At the outer edge of the corona, the blue light is not bent sufficiently to reach our eyes. But the red light is - so light waves reaching us from that part of the cloud are predominantly red. Over most of the corona, however, the different colours, first separated by the process of diffraction, are then superimposed again to blur into a uniform white.