The Danjon Scale
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.
During a lunar eclipse, the Moon doesn't become lost against the night sky, it actually has a reddish or coppery hue. This is because light passing through the thin layers of atmosphere at the edge of the Earth is refracted or bent inwards, making the Earth's shadow lighter. Our atmosphere is good at scattering blue light and the predominant colours that get through are from the red end of the spectrum.
The depth (darkness) of the eclipse is affected by many factors, such as how much cloud cover there is on the Earth and how polluted the atmosphere is. A major volcanic eruption just before a lunar eclipse can have a profound effect on the appearance of the totally eclipsed Moon.
The Danjon Scale is a method for measuring the depth of the eclipse. Determination of the value of depth 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 centre of the Earth's umbra.
Very dark eclipse.
Moon almost invisible, especially at mid-totality.
Dark Eclipse, grey or brownish in colouration.
Details distinguishable only with difficulty.
Deep red or rust-colored eclipse.
Very dark central shadow, while outer edge of umbra is relatively bright.
Umbral shadow usually has a bright or yellow rim.
Very bright copper-red or orange eclipse.
Umbral shadow has a bluish, very bright rim.
Last updated on: Friday 8th September 2017