Danjon scale
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The Danjon scale is a five-point scale useful for measuring the appearance and luminosity of the Moon during a total lunar eclipse. It was proposed by André-Louis Danjon in 1921, when postulating that the brightness of a lunar eclipse was related to the solar cycle.[1] An eclipse's rating on the scale is traditionally denoted by the letter L.
The scale
The Danjon scale is described in the following table:
L value | Description |
---|---|
0 | Very dark eclipse. Moon almost invisible, especially at greatest eclipse. |
1 | Dark eclipse, grey or brownish in coloration. Details distinguishable only with difficulty. |
2 | Deep red or rust-colored eclipse. Very dark central shadow, while outer edge of umbra is relatively bright. |
3 | Brick-red eclipse. Umbral shadow usually has a bright or yellow rim. |
4 | Very bright, copper-red or orange eclipse. Umbral shadow has a bluish, very bright rim. |
Determining the value of L
Determination of the value of L for an eclipse is best done near mid-totality with the naked eye. The scale is subjective, and different observers may determine different values. In addition, different parts of the Moon may have different L values, depending on their distance from the center of the Earth's
Factors affecting the value of L
Many factors can affect the appearance of the Moon during a lunar eclipse. The Moon's path through the Earth's
The amount of light refracted affects the brightness of the moon at mid-eclipse, and this depends on several factors.
It is also thought that the solar cycle has some effect on the darkness of lunar eclipses--indeed Danjon established the scale for this reason.
References
- Bibcode:1921LAstr..35..261D.