Aspects of the Low Moon
December 17, 2013
Photographer: David K. Lynch; Dave's Web site
Summary Author: David K. Lynch
The Moon is especially lovely when it's low on the horizon and the terrestrial foreground adds a touch of depth to the scene. Here the full harvest Moon is seen setting over Calabasas Peak in the Santa Monica Mountains during morning twilight on September 19, 2013. Besides its beauty, there are four interesting aspects of the low Moon. Its orange color is due to absorption and scattering of light by the atmosphere and airborne aerosol particles. Much of the shorter wavelength blue and green light is removed from the Moon’s image, leaving red and orange to reach the observer’s eye.
The Moon appears slightly flattened, because light from the lower part of the lunar disk is refracted upward by the Earth's atmosphere more than the upper part. This happens because the amount of atmosphere through which light must pass grows very rapidly toward the horizon and there's enough difference across the 0.5 degree diameter of the lunar disk to apparently squash it.
Note that the upper rim of the Moon has a greenish tinge, while the lower part is reddish. Refraction by air is stronger in the blue-green part of the spectrum than in the red part, so the atmosphere acts like a weak prism, slightly smearing the Moon’s image vertically and separating the colors. Rayleigh scattering removes most of the blue light, leaving the green rim. This green rim is the most common source of the green flash, particularly when viewing the low Sun.
The edges of the Moon are not smooth but show small indentations and irregularities, especially on the left side. These occur when little air pockets (in our atmosphere) with small variations in refractive index pass across the Moon’s face. These “seeing cells” act like little lenses, slightly deflecting light from the Moon. These cells are highly dynamic and are responsible for producing twinkling in starlight. Take a look at tonight's full cold Moon and see how many of these aspects you're able to observe.