What is a moonbow?
A moonbow (sometimes known as a lunar rainbow) is an optical phenomenon caused by the refraction of light from the moon being refracted through water droplets in the air.
The amount of light available even from the brightest full moon is far less than that produced by the sun so moonbows are incredibly faint and very rarely seen. The fact that not enough light is produced to excite the cone colour receptors in the human eye also compounds the difficulty we have in seeing moonbows.
Moonbows were first mentioned by Aristotle back in 350BC, and there are certain parts of the world where you are more likely to see them, such as Hawaii.
How are moonbows formed?
Just like daytime rainbows, moonbows need the light from the moon to be reflected and refracted by water droplets at a certain angle to create a rainbow.
Rather than seeing the full spectrum of colours, moonbows often appear to be white to the human eye. However, the colours are there, and long-exposure photographs are a great way of capturing their beauty.
The moon needs to be near to or at its fullest phase and low in the sky (less than 42°), whilst the night sky itself needs to be very dark. There also needs to be rain falling opposite the moon, or as you can see in the image below, another source of water droplets such as a waterfall.
To see a moonbow, the observer has to have the moon behind them (as in an ordinary rainbow). The best time to see moonbows is a couple of hours after sunset or before sunrise.