Saturday, May 21, 2011

How to estimate Danjon Scale of Brightness

Lunar eclipses get their colorful red-orange hues from sunlight that is filtered and bent by the Earth's atmosphere around into its shadow. This is the light of all the world's sunrises and sunsets ringing the globe at the time. Bright and colorful eclipses occur when our planet’s upper atmosphere is most transparent. Major volcanic eruptions spew dust and aerosols into the stratosphere, resulting in darker lunar eclipses.
The Moon's brightness varies greatly from eclipse to eclipse, which would tell a lot about the state of the Earth's upper atmosphere if only we understood it better. To help in compiling statistics of this kind, many observers rank each eclipse they see on the five-point Danjon Scale.

Danjon Scale of Brightness

0 Very dark eclipse, Moon almost invisible, especially at midtotality.

1 Dark eclipse, gray or brownish coloration; details distinguishable only with difficulty.

2 Deep red or rust-colored eclipse, with a very dark central part in the umbra and the outer rim of the umbra relatively bright.

3 Brick-red eclipse, usually with a bright or yellow rim to the umbra.

4 Very bright copper-red or orange eclipse,with a bluish, very bright umbral rim.

An even simpler technique was advocated 80 years ago by Harvard astronomer Willard Fisher, who graded mid-totality into three classes depending on the equipment needed to see details on the Moon's surface: (1) the naked eye, (2) a 50-millimeter finderscope or binoculars at 7x, or (3) a 150-mm (6-inch) telescope at about 20x. Fisher reasoned that the Moon's surface brightness is unaltered by the aperture, but the visibility of low-contrast features like lunar maria depends greatly on the image scale. The darker the eclipse, the greater the aperture (and power) needed.

Still another useful gauge is the Moon's stellar magnitude at mideclipse. People who wear thick eyeglasses can simply take them off, turning the Moon and bright stars or planets into blobs of about equal size for easier comparison. Looking through the wrong end of binoculars also works.

courtesy sky and telescope

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