Saturday, August 18, 2007

As previously reported on this blog (see Thursday, August 16th for 'Will You Be Able to See it ?' there will be a second total lunar eclipse this year, coming up in the early morning of Tuesday, August 28th. The entire eclipse will be visible in most of western North America, throughout the Pacific and in parts of eastern Australia. But how dark will it be ?
The Moon does not actually disappear as it enters the shadow of the Earth because the Earth's atmosphere refracts light into the cone of the shadow. The Moon will often appear red during an eclipse because the longer reddish wavelengths of light are less likely to be scattered and diffused by the air. This is the same effect that accounts for the redness of sunrises and sunsets. The condition of the atmosphere has an effect on the colour of the Moon during an eclipse. The dustier the air the more other wavelengths of light are scattered and the darker the reddish hue of the Moon. Thus eclipses that follow a major volcanic eruption produce a redder Moon. The amount of water vapour in the atmosphere at the time of the eclipse also has an effect. Extensive cloud cover along the rim of the Earth during an eclipse also darkens the Moon.
The French astronomer Andre Danjon (1890-1967) devised a five point scale to measure the darkness of an eclipse called, of course, the 'Danjon Scale'. He used a prism that split the image of the Moon into two images. By adjusting a diaphragm to dim one of the images until the sunlit portion of the image was the same as the earthlit portion in the other he could quantify the darkening of the Moon. He used this apparatus to measure the darknesses of eclipses from 1925 to the 1950s. The scale that he devised goes as follows:
L=0 Very dark eclipse. Moon almost invisible, especially at mid-totality
L=1 Dark eclipse. Gray or brownish in colour. Details distinguishable only with difficulty.
L=2 Deep red or rust-coloured eclipse. Very dark central shadow, while outer edges of umbra
relatively bright.
L=3 Brick red eclipse. Umbral shadow has a bright or yellow rim
L=4 Very bright copper-red or orange eclipse. Umbral shadow has a blueish, very bright rim.
If you want to estimate the darkness of an eclipse on this scale it is best done near mid-totality with either the naked eye, binoculars or a small telescope. It is also useful to make an estimate just after the beginning and just before the end of totality to be accurate about the brightness of the outer umbra. Note any variations in colour and brightness in different parts of the umbra as well as the apparent sharpness of the Moon's edge and also note the visibility of lunar features. Both Sky and Telescope Magazine and the astronomer Richard Keen ( ) are interested in amateur astronomer observations of the Danjon scale of the upcoming eclipse. Send any observations to them.
To learn more about the Danjon Scale go to the NASA article on it at

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