Next Friday brings the March full moon, the Moon of Storms as the American Indians were wont to call it. Between now and then the lunar disc will wax, growing closer and closer to a perfect circle night by night, and in the process it will reflect progressively more light in the direction of the Earth.
So watch out this week if there are showers after dark and who knows? You may observe a moonbow.
The familiar arc of any rainbow is a segment of a circle. If you look at the normal daytime rainbow and imagine where the centre of this circle might be, you will find it to lie on the continuation of an imaginary straight line starting at the sun and passing through your head; we say the arc is centred at the anti-solar point.
So as you face towards a rainbow during the daytime, the sun is always directly behind you.
The reason we see a rainbow at all is that rays of sunlight striking a distant curtain of raindrops are first broken up into their constituent colours by passing through the drops, and then reflected back in our direction from the inside surface of the sphere of water.
But the source of light for a rainbow does not have to be the sun. All you need is any source of illumination, to get her with a shower of raindrops strategically placed in the sky so that the light reflected by them can hit you straight between the eyes. A moonbow, more properly a "lunar rainbow", is a rainbow that originates not from the light of the sun, but from that of the moon.
Sufficient light for a moonbow to be visible is available only within a couple of days of full moon, and even then it is much weaker than its day-time equivalent. Lunar rainbows are seldom coloured, appearing instead a greyish shade of white. This is partly because at very low light levels our eyes provide only black and white vision, but also because the light from the moon is not intrinsically as colourful as that from the sun.
Being reflected from the surface of the moon, it does not contain the full range of spectral colours that make up sunlight, and any rainbow can show only such colours as are present in the light that causes it.
As in the case of a solar rainbow, a lunar rainbow can occur only when the moon is relatively low in the sky. And as also in the case of its daytime equivalent, a lunar rainbow is always on the opposite side of the sky from the full moon.