The Danjon Scale of Lunar Eclipse Brightness is a handy method of estimating the brightness of a particular lunar eclipse, and therefore having an easy of comparing the brightness of different lunar eclipses. It identifies five levels of brightness, ranging from very dark to a very bright red.
- About Lunar Eclipses -
A lunar eclipse is an astronomic event in which the moon passes through the Earth's shadow (that is, the region of space in which the Sun's light is blocked by the presence of the Earth). To an observer on Earth, the brightness can appear to vary as a result of its passage through the Earth's shadow but also due to atmospheric and meteorological conditions. However, during any given eclipse, the moon will normally not be completely dark. Instead, it will reflect a small amount of the light refracted (distorted and bent off course) by the Earth's atmosphere. The result is that the Moon may be very dark during a lunar eclipse, but will usually reflect some amount of reddish or orange light. The next lunar eclipse will be noticeable in December 2010.
Beginning in the 1920s, Caen astronomer Andre-Louis Danjon decided that a method was needed to categorize and compare the brightness of different eclipses. Using the amount of light reflected back to Earth, which he referred to as "Earthshine" because it was a product of atmospheric refraction from Earth, he devised a system of measurements of brightness on a scale, ultimately named after himself. Danjon continued making personal measurements of eclipse brightness into the 1950s, and died in 1967 after a successful career at the Strasbourg and Paris observatories.
- The Danjon Scale of Lunar Eclipse Brightness -
Danjon's scale established a five-point chart for lunar brightness values. According to NASA, these range from L0 to L4, where "L" refers to the level of lunar brightness. At L0, the first point on the Danjon Scale, the eclipse is very dark to the point that the Moon appears nearly invisible to a human observer on Earth. At L1, the Moon is still very dark, but an observer can make out the major features with some effort, and can detect a greyish or brownish hue.
Lunar eclipses start to become visually interesting at about L2. Here, the Moon becomes to take on the reddish hue which is characteristic of the atmospheric refraction effect. However, it is still relatively dark, so that the Moon seems more rust-coloured than red. At L3, the Moon is a solid, detectable red colour, sometimes rimmed by an even brighter yellow. Finally, the most visually striking lunar eclipses are categorized as L4 on the Danjon Scale. These exceptionally bright eclipses feature a red or orange moon, tinged by a rim which is even brighter, possibly even bluish in colour.