A Marvelous Night for a Moondance
Hexagonal ice crystal in a 22˚ halo. CC BY-SA 2.5 from Wikimedia Commons.
When it is cold enough for hexagonal ice crystals to form in the sky, you can sometimes see the 22˚ halo. Its angular radius, as the name suggests, is about 22˚, which means that it is about half as wide as a rainbow (which has an angular radius of around 42˚).
To make the 22˚ halo, light only gets refracted, which just means that it gets bent when entering and leaving the ice crystal.
Water droplet in a rainbow. Public domain from Wikimedia Commons.
In contrast, to make a rainbow the light also has to reflect off the back surface of the water droplet.
This has a few effects:
- You can only see a rainbow if you look in the direction opposite the light source. Instead of being centered on the position of the sun (or moon) in the sky, like the 22˚ halo is, a rainbow is centered 180˚ away from there.
- The colors are reversed, so that red is on the outside of the primary rainbow, but the inside of the 22˚ halo.
- Rainbows are much larger than the 22˚ halo. The reflection explains why it’s roughly a factor of 2 larger.
On 2014-01-10 there was a magnificent lunar halo in Tahoe.
So that you can see the reversed colors and dramatically larger size of a rainbow, here is a photo I took in 2009 of a brilliant rainbow in DC. You can also tell from the shadows that the sun is behind me in this photo, in contrast to the halo.
