Tuesday, June 22, 2010

Yet another Ring Around the Moon

While I was a little disappointed that cloud had stopped me seeing Venus near the Beehive cluster, when returning from bread shopping last night (3 boys can go through a LOT of bread), I looked up to see a prefect halo around the Moon. Absolutely brilliant and like the ISS pass I saw, makes my point about keeping on looking up).

The family, watching Master Chef in the warmth of the Living Room, were not excited, but MiddleOne did come out and express delight.

This is a great example of a 22 Degree halo. 22 Degree halos are the commonest kind of halo or arc seen in the sky, although they are far less obvious than rainbows, and may easily be missed.

22 Degree halos are formed by refraction of light through hexagonal columar ice crystals as they 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.

The image above was made by stitching together two separate shots, as the halo was too bigh to fit into the camera field of view.

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