AMATEUR ASTRONOMY:

THE LUNAR ECLIPSE: HOW DARK WILL IT BE ?

As Molly has blogged here before (see our archives for January 2008) there will be a lunar eclipse on the evening of February 20th (in Europe it will occur in the wee hours of the 21st- see the blog here on January 1st for the times of the various parts of the eclipse as per UT and CST-also how to convert for your own time zone). This will be the last total lunar eclipse visible here in North America until December 20-21 of 2010, though a partial eclipse will occur on December 31, 2009. Last chance for almost 3 years. Get your ticket now !

One of the big questions surrounding any eclipse is "how dark will it be ?". Most of North America will be able to view at least totality, and there is always a great interest in exactly how dark the Moon will be at this time. At mid-totality the darkness of the sky allows you to view many stars that are otherwise obscured by the Moon's light. The darkness of an eclipse has been summed up in what has become known as the Danjon Scale. This was originally formulated by the French astronomer André Louis Danjon, and it gives the darkness of the Moon in terms of a parameter called "L" (for luminosité). The stages of the Danjon Scale are as follows, from darkest to brightest.

L= 0 Very dark eclipse. Moon almost invisible at mid-totality.

L= 1 Dark eclipse. Grey or bown colour. Lunar details only visible with difficulty.

L= 2 Deep red or rust-coloured eclipse with a very dark central part in the shadown and the outer edges of the umbra relatively bright.

L= 3 Brick red eclipse with a bright or yellow rim to the shadow.

L= 4 Very bright copper-red or orange eclipsed with a blueish very bright shadow rim.

The Danjon Scale is, of course, subjective, and different viewers may give totally different interpretations of the scale. Different parts of the Moon may also appear to be at different values. How the eclipse will present depends very much on current atmopsheric conditions at the time of the eclipse. The reddish hue that is seen during some eclipses is due to refraction, just as the reddish tint to dawn and dusk is. Generally the larger the amount of particulate dust in the atmosphere the darker the eclipse will seem. The red light that gives the Moon its colour at this time actually comes from the refracted sunlight of dawn and dusk on the Earth via Moonshine. Some folks are interested in hearing about reports of your observations of the Danjon scale in your locality. You can report your observations to Sky and Telescope Magazine or to Richrad Keen at richardkeen@colorado.edu . Wtite it down after the event and send your observations to them. Sky and Telescope also has a great number of useful tools for amateur astronomy, including an almanac of celestial events. Explore the site for more info.