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Lunar Circumhorizon Arc
An impressive lunar halo captured in southern Oregon by Michael Blake.  Image ©Michael Blake

Plane of moon's orbit tilts relative to plane of earth's orbit around the sun.

If you are north of Copenhagen or Edinburgh then summer's most notable halo can never be seen. The circumhorizon arc needs a sun at least ~58° high.



All is not lost because the moon can sometimes rise higher than the sun. The moon's orbit is tilted ~5° from the plane of Earth's orbit around the sun. In November and December the near full moon can ride higher in the northern hemisphere skies than does summer's sun.

But those who are latitudinally challenged need not rush out to search for a lunar circumhorizon arc!

The plane of the moon's orbit precesses with a period of ~18.6 years and high latitude dwellers must wait until around winter 2025 to have a really high full moon.

The moon's orbital plane precesses with an 18.6 year period.

Horizontal plate crystals form the circumhorizon arc. The sun's (or moon's) rays enter a near vertical crystal side face and leave through the lower hexagonal face.

The passage through what is effectively a 90° prism separates the colours widely.