John Carlos Baez @johncarlosbaez@mathstodon.xyz
Public

Brent Mckean captured this amazing moonlit scene with three combined exposures on an icy winter morning in Manitoba, Canada.

The colorful rings are a corona caused by diffraction by ice crystals near the direction of the Moon.

Outside those rings, a 22-degree halo was created by moonlight refracting through ice crystals shaped like hexagonal prisms.

On the left and right are 'moon dogs', caused by light refracting through thin, flat, 6-sided ice crystals.

At the top and bottom of the 22-degree halo are 'upper and lower tangent arcs', created by moonlight refracting through nearly horizontal hexagonal ice prisms.

A few minutes later the halo and arcs disappeared and the sky returned to normal -- with the exception of a single faint moon dog.

Image source: apod.nasa.gov/apod/ap200224.ht

For more on tangent arcs: en.wikipedia.org/wiki/Tangent_

The image shows a celestial spectacle over Manitoba, Canada, on a cold winter morning. The moonlight, refracted, reflected and diffracted by millions of falling ice crystals, creates extraordinary optical phenomena. At the center, the Moon is surrounded by a colored halo, called the corona, generated by the diffraction of small droplets of water or ice. A 22-degree halo is seen around it, formed by the refraction of moonlight through six-sided cylindrical ice crystals. On either side of the moon are "moon dogs," bright spots caused by the refraction of light through thin, hexagonal ice plates floating toward the ground. Above and below the 22-degree halo are upper and lower tangent arcs, created by refraction through nearly horizontal ice cylinders. The sky, captured in three combined exposures, appears surreal, transforming the moon into an icon of another world. Alt text by Nereide: https://bsky.app/profile/drnereide.bsky.social/post/3llzitehztc2z Photograph by Brent Mckean: https://apod.nasa.gov/apod/ap200224.html
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