Friday, October 30, 2009

A Ring Around The Moon

Jupiter forms the jewel in this ring around the Moon (click to embiggen)

Even on cloudy nights you can see interesting things, I captured this image of a ring around the Moon on a night you might have thought was a waste of time. Technically its a 22 degree halo, not a ring, and fortuitously the halo arc was so close to Jupiter that it looked like a Jewel in the ring. In the original you can see a little colour in the halo.

22 Degree halos are the commonest kind of halo or arc, although they are far less obvious than rainbows, and may easily be missed. They are formed by refraction of light through hexagonal columar ice crystals as the fall to the ground. The halo will form when there is a uniform distribution of randomly oriented crystals in the cirrostratus cloud through which the Moon (or Sun) is shining. Because random orientation is only possible if the crystals are tumbling as they fall, the crystals need to be quite small. This is different from what I've been calling "Moonbows", there are typically 8 degrees away from the Moon, and are caused by bullet shaped crystals.

So not only are you seeing something pretty, you are seeing a dynamic event and some cool physics.


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