Thursday, February 01, 2007

a lunar halo visible over dc tonight

12:30am: daisy just texted me from pennsylvania to tell me there's a halo around the moon. it looks a lot better there than it does in d.c....

this is actually a fairly common sight and is caused by high, thin, cirrus (read hazy) clouds containing millions of tiny ice crystals scattering the moonlight. each ice crystal acts like a miniature lens.

we see a halo because most of the crystals have a similar elongated hexagonal shape, and the light entering one crystal face and exiting through the opposing face refracts 22 degrees, which corresponds to the radius of the moon halo. luckily, it looks a lot prettier than it sounds...

so, to produce a lunar halo, you must have:

* moonlight, and
* a thin veil of hexagonal ice crystals between our eyes and the moon.

this is very similar to sunlight and rain falling between our eyes and the sun to produce a rainbow.

we also sometimes see a similar sun halo visible during the day.

so now you know!

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