One night, after wandering outside, under the light of the full moon, you see an amazing ring around the moon. But what is it?
Well, it’s not really a magically significant event so much as a scientific one. It’s actually known as a lunar halo. It happens sometimes when ice particles in the earth’s atmosphere reflect the moonlight.
The science of the lunar halo.
Some people at the Farmer’s Almanac have a great explanation of it and say, “The refraction, reflection, and dispersion of light through ice particles suspended within thin, wispy, high altitude cirrus or cirrostratus clouds causes a lunar halo. Light bents at a 22° angle as it passes through these hexagon-shaped ice crystals. Thus, creating a halo 22 degrees in radius (or 44 degrees in diameter).”
Related to the lunar halo is the phenomenon called a moonbow. Interestingly, because of the way light refracts, a moonbow – which is just like a rainbow, but appearing at night – we can only see it in the part of the sky opposite of where the moon is visible.
Aristotle refers to this in his book Meteorologia, although he does not use the term moonbow. He says, “These are the facts about each of these phenomena: the cause of them all is the same, for they are all reflections. But they are different varieties. They are also distinguished by the surface from which and the way in which the reflection of the sun or some other bright object takes place. We can see the rainbow by day, and it was formerly thought that it never appeared by night as a moon rainbow. This opinion was due to the rarity of the occurrence. The reason is that the colors are not so easy to see in the dark. And that many other conditions must coincide, and all that in a single day in the month. For if there is to be one it must be at full moon, and then as the moon is either rising or setting. So we have only met with two instances of a moon rainbow in more than fifty years.”
Where can we see the moonbow?
Moonbows are not visible everywhere, and they’re fairly uncommon occurrences, as we see in Aristotle’s work. We know a few places for regular moonbow appearances, though. Where they do occur, they’ve become a major attraction, particularly in places like Victoria Falls. Their website says that “the lunar rainbow is best seen at times of high water (April to July) when there is sufficient spray to create the moonbow effect. This spectacle is best witnessed in the early hours after moonrise. That’s before the moon rises too high to create a moonbow that is visible to the ground-based observer.”
According to the folks at Time and Date, there are four requirements for a moonbow to occur. First, the moon has to be sitting fairly low in the sky. In addition, it must be full, or close to it. The surrounding sky has to be very dark for a moonbow to be visible because even a small bit of light will obscure the view. And there have to be water droplets in the air in the opposite direction of the moon.
Thumbnail Image: A photo in Tromsø, Norway. Credit: antony spencer/E+/Getty Images