Sun

FIGURE 2-8. Why does the Moon turn the color of blood during a total lunar eclipse? Sunlight enters the atmosphere (the thickness of which is shown greatly exaggerated here) and the blue end of the spectrum is preferentially scattered, making the sky blue. This means that more of the red light makes it through on a route to the Moon. In addition, blue light is refracted more (has its course bent) by the air and fails to make it along the necessary direction (not to scale).

little sunlight to the Moon, and that small fraction that makes it through is predominantly at the red end of the spectrum. That is why in a total lunar eclipse the Moon appears a dark reddish-brown. Any dust suspended in the air will add to these effects.

As the curved shadow of the Earth creeps across the Moon, its boundary is blurred, producing a graded fringe rather than a sharp edge. This is because of both the finite solar diameter and the terrestrial atmosphere, but the presence of a substantial atmospheric loading of dust will cause the normal shadow profile to be altered. After major volcanic eruptions, such as those at Mount St. Helens in Washington and Mount Pinatubo in the Philippines, the dust left lofted in the atmosphere may be immense and take

FIGURE 2-8. Why does the Moon turn the color of blood during a total lunar eclipse? Sunlight enters the atmosphere (the thickness of which is shown greatly exaggerated here) and the blue end of the spectrum is preferentially scattered, making the sky blue. This means that more of the red light makes it through on a route to the Moon. In addition, blue light is refracted more (has its course bent) by the air and fails to make it along the necessary direction (not to scale).

months to years to settle out. It tends to drift around in the upper air, but within a restricted latitude range, resulting in a distinct blob of darkness on the Moon's face during an eclipse. It is as though the Sun is acting as the lamp in a slide projector and the Moon as the screen, throwing an image of the Earth's outline onto the latter. Similar effects were also observed after the Gulf War, when oil-well fires produced vast, dense smoke plumes.

Other features may also be investigated by dint of a lunar eclipse. The Earth's atmosphere is not perfectly spherical, and its profile can be monitored by timing when the eclipse shadow gets to various marker points on the lunar surface, such as well-known craters.

It is only during a lunar eclipse that the influence of our atmosphere in these regards is obvious. Lunar eclipses are quite unlike solar eclipses, then.

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