For the $22^ ext{o}$ halo to form, what specific angle must the exit face be relative to the entry face of the hexagonal ice crystal?
Answer
A $60^ ext{o}$ orientation
The specific geometry required for the $22^ ext{o}$ halo involves precise alignment within the hexagonal ice crystal. The phenomenon requires that the moonlight enter one of the side faces of the crystal and then exit through another side face. The orientation of these two side faces relative to each other must be $60^ ext{o}$. This specific $60^ ext{o}$ relationship between the entry and exit faces is what geometrically constrains the light path, forcing the resulting minimum deviation angle, as observed by us on the ground, to be exactly $22^ ext{o}$.
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