Did you know? Moonbows

We’re all familiar with rainbows and the elusive pot of gold at the end. But our neighbour the Moon is also capable of producing the same phenomenon with the same breathtaking effect.

As the name suggests, a Moonbow (also known as a lunar rainbow), is produced by moonlight rather than sunlight. They are rarer than rainbows because a variety of weather and astronomical conditions have to be just right for them to be created.

Moonbows are most easily viewed when the moon is at or nearest its brightest phase, full Moon. The Moonbow image below is over the town of Kihei, seen from Kula, on Maui, Hawaii, US by Arne Kaiser, Creative Commons.

For moonbows to have the greatest chance of appearing, the Moon must be low in the sky, less than 42 degrees above the horizon. In addition, the night sky must be very dark. Since the sky is not completely dark on a rising/setting full Moon, this means they can only be observed two to three hours before sunrise or two to three hours after sunset. And, of course, there must be water droplets (e.g. from rain or spray) opposite the moon.

Moonbows are much fainter than rainbows, due to the smaller amount of light reflected from the surface of the Moon. Because the light is usually too faint for the human eye to discern colours our eyes see it as white light even though all seven colours that make up white light are present. However, the colours do appear in long exposure photographs.

Moonbows are only visible at a point in the night sky that is exactly opposite the Moon in relation to the position of the observer.

Moonbows are mentioned in Aristotle’s work, Meteorology, which was published around 350BC. Other explanations for how they form can be found in books and pamphlets dating from ancient Greek and Roman times, through the Middle Ages, and into the modern Age of Enlightenment that marked the late Victorian era