The Moon's appearance during a total lunar eclipse can vary enormously from one eclipse to the next. Obviously, the geometry of the Moon's path through the umbra plays an important role. Not as apparent is the effect that Earth's atmosphere has on total eclipses. Although the physical mass of Earth blocks 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 (meteoric dust, organic debris, volcanic ash). This material significantly filters and attenuates the sunlight before it is 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.
The French astronomer André-Louis Danjon proposed a useful five-point scale for evaluating the visual appearance and brightness of the Moon during total lunar eclipses. L values for various luminosities are defined as follows:
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. It's also useful to examine
the Moon's appearance just after the beginning and just before the end of
totality. The Moon is then near the edge of the shadow, providing an
opportunity to assign an L value to the outer umbra. In making any evaluations,
the instrumentation used and the time should both be recorded.
Also note any variations in colour 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 often invaluable in recalling
important details, events, and impressions.