Every once in a while clouds pay us an unwelcome visit at Paranal. This can ruin our observations, but on the other hand we can enjoy amazing Moon halos like this one I saw in November.
These haloes are created by hexagonal ice crystals in the atmosphere. Incident light rays are refracted twice inside the crystal, and come out most of the time at 22º relative to the incident rays. When light from the Sun / Moon go through a myriad of small ice crystals, most beams emerge at 22º, and you’ll thus see a bright ring of this size. The inside of the halo is dark because beams can’t be refracted at less than 22º. The inner part of the halo looks red because ice crystals don’t refract red light as strongly as blue light.
For a more detailed description of what’s going on, check out this Twitter thread: twitter.com/astro_jcm/status/1221835742214868993
Our #Flickr friend Juan-Carlos Munoz-Mateos has captured a moon halo over Paranal. These haloes are created by hexagonal ice crystals in the atmosphere.
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