Lunar eclipse
Lunar versus solar eclipse

There is often confusion between a solar eclipse and a lunar eclipse. While both involve interactions between the Sun, Earth, and the Moon, they are very different in their interactions.

The Moon does not completely darken as it passes through the umbra because of the refraction of sunlight by Earth's atmosphere into the shadow cone; if Earth had no atmosphere, the Moon would be completely dark during the eclipse. The reddish coloration arises because sunlight reaching the Moon must pass through a long and dense layer of Earth's atmosphere, where it is scattered. Shorter wavelengths are more likely to be scattered by the air molecules and small particles; thus, the longer wavelengths predominate by the time the light rays have penetrated the atmosphere. Human vision perceives this resulting light as red. This is the same effect that causes sunsets and sunrises to turn the sky a reddish color. An alternative way of conceiving this scenario is to realize that, as viewed from the Moon, the Sun would appear to be setting (or rising) behind Earth.

The amount of refracted light depends on the amount of dust or clouds in the atmosphere; this also controls how much light is scattered. In general, the dustier the atmosphere, the more that other wavelengths of light will be removed (compared to red light), leaving the resulting light a deeper red color. This causes the resulting coppery-red hue of the Moon to vary from one eclipse to the next. Volcanoes are notable for expelling large quantities of dust into the atmosphere, and a large eruption shortly before an eclipse can have a large effect on the resulting color.
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