APOD: A Lunar Corona over Turin (2019 Sep 16)

[APOD Robot]

A Lunar Corona over Turin

Explanation: What are those colorful rings around the Moon? A corona. Rings like this will sometimes appear when the Moon is seen through thin clouds. The effect is created by the quantum mechanical diffraction of light around individual, similarly-sized water droplets in an intervening but mostly-transparent cloud. Since light of different colors has different wavelengths, each color diffracts differently. Lunar Coronae are one of the few quantum mechanical color effects that can be easily seen with the unaided eye. The featured lunar corona was captured around full Moon above Turin, Italy in 2014. Similar coronae that form around the Sun are usually harder to see because of the Sun's great brightness.

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[orin stepanek]

Beautiful Photo! Too bad it is vertical; so I can't use it as a background on my computer screen!

[TheOtherBruce]

Aren't they usually rounder than this? The shapes of the corona and the cloud give the impression Someone just lifted the lid on the outer crystal sphere so They could have a look inside...

[Chris Peterson]

Aren't they usually rounder than this? The shapes of the corona and the cloud give the impression Someone just lifted the lid on the outer crystal sphere so They could have a look inside...

The size of the corona is a function of the size of the diffracting particles. If you have clouds with different sized droplets in different zones, you'll get a distorted circle. If the cloud has different sized droplets all mixed together, you'll get a white, or nearly white corona.

[zeitoon]

What quantum mechanical diffraction effect are they talking about? I've been looking at papers and they all use a classical diffraction picture when talking about the lunar corona

[neufer]

What quantum mechanical diffraction effect are they talking about?

I've been looking at papers and they all use a classical diffraction picture when talking about the lunar corona

Classical optics usually refers to geometrical/ray optics (using Snell's Law)
which can handle simple halos & rainbows.

Classical diffraction/interference is required to handle lunar corona & sophisticated rainbows.

APOD is just being a little coy in calling it quantum mechanical
for a situation where photons are clearly not being individually counted.

<<The simplifying assumptions of geometrical optics include that light rays:

• propagate in straight-line paths as they travel in a homogeneous medium
  
  bend, and in particular circumstances may split in two, at the interface between two dissimilar media
  
  follow curved paths in a medium in which the refractive index changes
  
  may be absorbed or reflected.

Geometrical optics does not account for certain optical effects such as diffraction and interference.>>

[Chris Peterson]

What quantum mechanical diffraction effect are they talking about? I've been looking at papers and they all use a classical diffraction picture when talking about the lunar corona

Quantum mechanics is just a more complete way of dealing with classical diffraction. In this case, you could use either and get the same result.

[Goodwin]

Thank You Chris Peterson! For pointing out that it's a diffraction effect, called Rayleigh scattering, that gives different (& small) diffraction angles for different particle sizes as well as for different wavelengths, and not refraction through water droplets or ice crystals as in rainbows, etc. that gives the same (& larger) angle for a given wavelength, independent of particle size. Rayleigh scattering also accounts for the sky being blue and sunsets red.
A more difficult question is why a guy who used to make a living doing diffraction of X-rays didn't immediately think of that....