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   9� Circular Halo

A fine 9� halo imaged by Harald Edens in the Netherlands. A faint 18� is also visible inside the bright outer halo.

�Harald Edens, reproduced with permission.

  The innermost of the odd radius halos forms when pyramidal crystals have a central prism section. Sunlight passes between a central face and an opposite pyramidal face. These faces are inclined at only 28� and the corresponding minimum deviation for rays passing through them is 9�.  When the crystals are poorly aligned - as they usually are - then a circular halo forms.