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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.

İHarald 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.