I had a letter recently from a man who thought he saw a rainbow. And, to quote Lewis Carroll: . . . that was odd, because it was The middle of the night. "Can this be so," he asks, "or did my eyes deceive me?" Can a rainbow really happen in the dark?
Day or night, whenever it occurs, a rainbow is always formed by the reflection of light inside drops of water falling through the air. Usually when we see one, the sun is the source of light. But it does not have to be. The familiar arc of a rainbow is a segment of a circle. When you look at the bow, if you imagine where the centre of this circle might be, you will always find that it lies on the continuation of an imaginary straight line joining the sun and the observer's eye; we say it is centred at the anti-solar point. So as you face directly towards a rainbow during the daytime, the sun is always right behind you.
But we can be even more precise about where a rainbow can appear. If you care to carry out some elementary measurements, you will find that all points on the arc are at an angle of precisely 42 degrees to this imaginary line that joins the sun, the observer, and the anti-solar point.
This is because the rays of sunlight striking distant raindrops are reflected, as the spherical geometry of water drops demands, at an angle of 42 to their original path. All you need to see a rainbow then - apart, of course, from a suitable source of light - is a shower of raindrops strategically placed in the sky so that their reflected light can hit you straight between the eyes. But the source of light for a rainbow does not have to be the sun. Provided there are water drops in the right place, the arc can be seen at night as well, originating from the light of the moon.
Sufficient light is available only at full moon, and even then, moon-rainbows are much weaker than their daytime equivalent. They are seldom coloured, just as feebly illuminated objects seem to appear colourless at night. And like the daytime rainbow, a moon-rainbow is always seen on the opposite side of sky to the full moon.
As it happens, the December full moon occurs tonight, or to be more precise, shortly after 2.30 tomorrow morning. So if there are showers about after dark over the weekend, who knows, as the song says, "you may see a rainbow, too!"