3towers Observatory Moon Halos

One of the 3 radio towers silhouetted by the Moon and surrounded by a Moon Halo.  This picture was taken on a misty foggy evening. 17 mm f/4.5 lens, 30 second exposure, Ektachrome 200 film.  

Tucson usually has very warm dry days and nights, but there are occasional nights when it is overcast or even misty without actually raining.  Sometimes, on these evenings a ring or halo can be seen around the Moon.  It is a wonderful sight and not that infrequent.  There are many Moon halos during the year.  A similar halo can be seen around the Sun on an occasional cloudy day.   

These halos occur when high thin clouds containing tiny ice crystals cover much of the sky.  Each of these crystals acts like a tiny lens, and many of the crystals have a similar elongated hexagonal shape.  As a result, each crystal bends the Moon’s light in a similar direction giving a 22 degree ring of light around the Moon.   The inner edge of the halo has a red color while most of the halo is whitish.  The most common form of high-level clouds are wispy cirrus clouds which are typically found at heights of 20,000 feet or higher.  Cirrus clouds are composed of ice crystals from freezing of water droplets.  Exactly how the crystals form is not fully understood.  These crystals are about 20 microns in diameter, so small that 50 of them side by side would only measure 1 mm.  

Moon Halo and Venus. 8mm f/3.5 lens, 10-second exposure with Nikon D100 digital camera, ISO 1000.

Moon Halo 23 November 2005. 8 mm f3.5 lens and 16 mm f/4.5 lens, respectively, 5-second exposures with Nikon D100 digital camera, ISO 800

Moon Halo on December 21, 2010, the evening after the Total Eclipse of the Moon (same day).  Four and 8 second exposures with 8 mm f/4 lens on Nikon D100 camera.

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