The Moon's appearance during a total lunar eclipse can vary enormously from one eclipse to the next. The geometry of the Moon's path through the umbra plays an important role. Perhaps not as apparent, Earth's atmosphere also has a significant effect on the appearance of eclipses. Although the physical mass of Earth blocks off all direct sunlight from the umbra, the planet's atmosphere refracts some of the Sun's rays into the shadow. Earth's atmosphere contains varying amounts of water (clouds, mist, precipitation) and solid particles (dust, organic debris, volcanic ash). This material filters and attenuates the sunlight before it's refracted into the umbra. For instance, large or frequent volcanic eruptions dumping huge quantities of ash into the atmosphere are often followed by very dark, red eclipses for several years. Extensive cloud cover along Earth's limb also tends to darken the eclipse by blocking sunlight.
In 1921 the French astronomer André-Louis Danjon proposed a five point scale for evaluating the visual appearance and brightness of the Moon during total lunar eclipses. Danjon originally proposed the scale as a tool to determine whether the brightness of a lunar eclipse was related to the solar cycle [L'Astronomie (1921), 35, 261-265]. However, it has proven more useful for evaluating the transparency of Earth's atmosphere.
'L' values for various lunar eclipse luminosities of are defined as follows:
|Danjon Scale of Lunar Eclipse Brightness|
|L = 0|| Very dark eclipse. |
Moon almost invisible, especially at mid-totality.
|L = 1|| Dark Eclipse, gray or brownish in coloration. |
Details distinguishable only with difficulty.
|L = 2|| Deep red or rust-colored eclipse. |
Very dark central shadow, while outer edge of umbra is relatively bright.
|L = 3|| Brick-red eclipse. |
Umbral shadow usually has a bright or yellow rim.
|L = 4|| Very bright copper-red or orange eclipse. |
Umbral shadow has a bluish, very bright rim.
The assignment of an 'L' value to lunar eclipses is best done with the naked eye, binoculars or a small telescope near the time of mid-totality. Observers with myopia (near-sightedness) can simply remove their eyeglasses, turning the Moon and bright stars into blurry disks of about equal size for easier comparison. Another method is to view the Moon and comparison stars through the wrong end of a pair of binoculars. In this case, the Moon's image is reduced to appear star-like.
It is useful to examine the Moon's appearance just after the beginning and before the end of totality. The Moon is then near the edge of the shadow and provides an opportunity to assign an 'L' value to the outer umbra. In making any evaluations, you should record both the instrumentation and the time. Also note any variations in color and brightness in different parts of the umbra, as well as the apparent sharpness of the shadow's edge. Pay attention to the visibility of lunar features within the umbra. Notes and sketches made during the eclipse are invaluable in recalling details, events and impressions.
Observers are encouraged to make Danjon brightness estimates and to report them to Sky and Telescope and to .