Twenty Two Degrees

 

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Drinking coffee this morning, I saw a notification on my phone for a post in the r/scottsdale subreddit. These notifications come at random based on some algorithm.. I followed it and it was, of all things, a post about the Moon!

Someone posted a photograph of the night sky the evening before showing the waxing gibbous moon in an otherwise clear sky with a halo around it asking "what is this?" It must have been taken after I saw the Moon in the previous post, because the sky was clear in the photo. Several of the replies correctly stated that the prominent large halo was due to ice crystals in the upper atmosphere. The angle of the halo is always exactly twenty two degrees, which I assume is correct. Of course, they mean the solid angle.  

There's even a Wikipedia article just about the 22 degree angle!

solid angle. I remember my physics professor at Willamette explaining this concept and I thought it was the coolest thing ever. I had never heard of spherical trigonometry before.