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Have you ever seen a ring around the Moon in the sky and wondered what causes it?

This is a "moon halo", and it's caused by ice particles suspended in the high atmosphere, typically around 5-10 km in altitude.

Here's a great picture of a moon halo, taken by Rob Sparks (@halfastro), a Science Outreach Specialist with the National Optical Astronomy Observatory in Tuscon, Arizona.

The size of the halo is usually 22-degrees, which is about the width of your hand stretched out at arm's length. But you can get another halo at 46-degrees in the right conditions.

We see the halo because of the shape of the ice crystals. They refract the light from the Moon at exactly the same angle, concentrating the light that reaches your eyes.

In fact, everyone sees their own private halo, because you're only seeing the crystals that happen to be aligning the light for your specific location. Someone a few meters beside you is seeing their own version of the halo.

In the right conditions, you can even get "moon dogs"; bright spots on either side of the Moon, or "light pillars"; bright spots beneath and above the Moon.

Canon 6D camera, using a 14mm f/2.8 lens from Tuscon, Arizona.

by halfastro on 2016-02-10
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