Posted by: Rob Viens | July 14, 2012

A Marvelous Night for a Moon Ring

On July 14th, it was a:

“Fine day & a prosperous breeze.” (July 14)

And although Darwin does not have anything else to add about the day, he does describe another nighttime phenomenon in is Zoological Notebook – a lunar corona.

“At 11 oclock PM of the 14th of July (off St Catherines) the moon was surrounded by beautifully coloured rings.— Around the disk there was a highly luminous circle edged with red.— The diameter of this (including the moon) was 1°.45′.— Then came one of greenish blue also edged with red, this as broard as to make the diameter of whole halo to be 2°.90′. The appearance only lasted a short time & disappeared gradually.— The sky was of a pale blue; & was traversed with some scattered Cumili driven swiftly along by a Northerly breeze.” (Zoological Notebook)

There are several optical phenomena that are related to the moon, and almost all of these require  a full moon (suggesting that there was a full moon on July 14, 1832).  The one that Darwin is describing here sounds like wither a “moon ring” or a “lunar corona”.  I describe both, but I believe he saw the later.

A moon ring forms when light from the full moon passes through ice crystals in the upper atmosphere (generally in the form of cirrus clouds). As a ray of light enters (and exits) the ice crystals the path it follows is bent, a process known as refraction. So rather than travelling in a straight line between the moon and your eye, the ray of light is refracted equally in all directions away from its central path.  You might think that this would just create a “cloud” of light around the moon – sort of like car headlights coming at you in the fog.  However, since the exact amount of refraction depends on the substances it moves through (in this case air into water ice and then back into air again), all the light rays get bent the same amount.  This translates into a “ring” around the moon.

Moon ring (from Wikipedia Commons):

Moon Ring

In addition, exactly how much a ray of light is bent also depends on the wavelength of light.  Red light waves, for example are bent less that blue light waves (this is the concept behind a prism).  Moonlight, which is reflected sunlight, is “white light” – made up of all of the colors of the spectrum.  So the red portion of that light is bent less, forming an inner ring, and the blue portion is bent more, forming an outer ring. (However, color is not always obvious in a moon ring, as in the picture above.)

A lunar corona forms when light from the moon is diffracted (which is not the same as refraction) around tiny ice crystals or water droplets, also found in high cirrus clouds.  Diffraction occurs when the light encounters objects that are more similar in size to the wavelength of the light (that would mean very tiny ice crystals).  The light wave is diffracted by the surface of the small object (remember, in refraction it passes through the object).  The size of the objects will determine how much the light is diffracted – smaller objects (closer in size to the wavelength of light) will produce larger “coronal rings” around the moon. Like refraction, the color of the light wave will also effect the angle of diffraction – creating colored rings.

Two lunar corona (from Wikipedia Commons):

Lunar corona

Lunar corona

There are also “moonbows” (like a rainbow) that are formed when moonlight passes through water vapor near the Earth’s surface.  However, these to not form rings around the moon, as Darwin describes.

Two things make me think that what Darwin saw was a lunar corona:

  1. Because of the angle of refraction, a halo is often at least 10 times the size of the moon. The moon is about 0.5 degrees across, so Darwin’s numbers suggest an optical phenomena that is 3-6 times the size of the moon itself.
  2. Lunar corona are often more colorful and have reddish colors on the outside (as Darwin describes).  Color is often more subtile in a halo and, if it is present, the red color should only be visible on the inside of the halo.

There you have it  – Darwin’s description of a lunar corona.

By the way – Darwin’s geographic reference place the Beagle near Isla Santa Catarina and the city of Florianopolis in southern Brazil.


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