A Marvelous Night for a Moondance

January 10, 2014


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:

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.

January 10, 2014

A Marvelous Night for a Moondance - January 10, 2014 - {"name"=>"Evan Berkowitz", "twitter"=>"evanberkowitz", "email"=>"evan@evanberkowitz.com", "phone"=>"+1 917-692-5685", "inspire"=>"http://inspirehep.net/search?ln=en&ln=en&p=find+a+%22Evan+Berkowitz%22+or+a+%22E.+Berkowitz%22+not+%22E.H.+Berkowitz%22&of=hb&action_search=Search&sf=earliestdate&so=d&rm=&rg=25&sc=0", "arxiv"=>"http://arxiv.org/a/berkowitz_e_1", "github"=>"http://github.com/evanberkowitz", "linkedin"=>"https://www.linkedin.com/in/evanberkowitz", "google_scholar"=>"https://scholar.google.com/citations?user=hEy9k60AAAAJ", "orcid"=>"http://orcid.org/0000-0003-1082-1374"}