Moon Halo

Moon Halo

This was the sight that greeted people looking skyward last night, captured by Nynke Jansen.

This photograph from Nynke Jansen captures the mesmerising spectacle many people in the north of Scotland witnessed when they looked moon ward on March 9th – a bright halo of white light encircling the full moon.

The explanation for these ‘moon halos’ is simple refraction of light through tiny hexagonal shaped ice crystals suspended in high altitide cirrus clouds.  These halos are usually observed extending a 22 degree circle from the moon, because this corresponds to the minimum angle of refraction produced when light enters and leaves a hexagon shaped piece of glass (or ice).  The full range of refraction is between 22 and 50 degrees.

ice

An ice crystal with its beautiful six fold symmetry.  An uncountable number of these drift within high altitude clouds.

Under certain conditions ice crystals will collectively orientate themselves in the atmosphere such that moonlight refracts through them coherently – hence the intense corona like circle of light beginning at 22 degrees from source (the moon).   And because different wavelengths of light are refracted by slightly different amounts, a subtle rainbow effect is also seen.  The 22 degree minimum angle also results in a queer darkening of the sky between the moon and the halo itself as no refracted light gets scattered into this void region.  This phenomena needs to be witnessed with the naked eye, as photographs tend to over saturate the image with white light (as above).  The overall impression is that of a dark portal suspended in the sky, with the moon at centre!

hexagon

When light enters a medium of higher density it bends towards the surface normal, then away from the normal when exiting.

In general crystals of ice in the atmosphere would be randomly orientated, leading to no such phenomena.  The mechanism leading to the crystals orientating themselves along the same plane isn’t fully understood, but supposedly conditions exist for this to happen dozens of times a year.  Either way, it’s a phenomena I’ll be keenly looking out for in the future.

The Mathematics of Refraction

SinØi / SinØr = n

This simple equation from high school physics determines the angle of refraction Ør, after a ray of light enters a more dense medium at an angle Øi to the surface normal. n is the refractive index of the material, and will vary slightly depending on the frequency of the incident light.  The refractive index of ice is 1.309, slightly lower than that of liquid water at 1.333.

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