The Danjon Scale of lunar eclipse brightness is a five-point scale useful for measuring the appearance and luminosity of the Moon during a lunar eclipse. It was proposed by André-Louis Danjon when he was measuring the Earthshine on the Moon.
Many factors can affect the appearance of the Moon during a lunar eclipse. The Moon's path through the Earth's umbra is important, but so too are the current conditions of the Earth's atmosphere. While the Earth's shadow blocks any direct light from striking the Moon during a lunar eclipse, some light is refracted through the Earth's atmosphere giving a Moon a red hue. Atmospheric conditions can affect the degree of redness. For example, frequent, large volcanic eruptions that spew significant amounts of volcanic ash into the air are followed by several years of dark, deep red eclipses.
The scale is defined as follows (from NASA):
L = 0 Very dark eclipse.
Moon almost invisible, especially at mid-totality.
L = 1 Dark Eclipse, gray or brownish in coloration.
Details distinguishable only with difficulty.
L = 2 Deep red or rust-colored eclipse.
Very dark central shadow, while outer edge of umbra is relatively bright.
L = 3 Brick-red eclipse.
Umbral shadow usually has a bright or yellow rim.
L = 4 Very bright copper-red or orange eclipse.
Umbral shadow has a bluish, very bright rim.
Determination of the value of L for an eclipse is best done near mid-totality with the naked eye.
