The Brightness And Color Of The Moon In Total Eclipse

During a total solar eclipse the Sun's disk gets very dark indeed— you can't see it—but the same is not true of a total lunar eclipse. While it is entirely within the umbra the lunar disk brightness drops to about one part in 5,000 that of the near-full moon, and so it can still be seen. One needs no sophisticated equipment to recognize that the normal bluish-white Moon appears a reddish-brown during the eclipse, and many describe the Moon as taking the color of blood. How does any sunlight at all get to the Moon to provide it with some dim yet red illumination?

The answer lies with the atmosphere. Our planet possesses a considerable atmosphere, and that makes its edge somewhat fuzzy. On the other hand, the Moon has no atmosphere of which to speak, and so it casts a shadow whose sharpness is limited only by the Sun's finite size: when a solar eclipse reaches totality it is sudden and abrupt.

Why does red light preferentially get to the Moon during a total lunar eclipse? This occurs because of inequalities in the atmospheric transmission of different wavelengths of light. This effect actually occurs all the time and is obvious once one thinks about it. At sunset the image you see of our star as it sinks below the western horizon is much redder than at midday because the air molecules between your eyes and the Sun scatter light at the blue end of the spectrum more than at the red end, allowing more of the red light to reach the planet's surface directly, but by the same token making the sky look blue.

The images of the Sun and Moon at rising and setting are also distorted somewhat, producing oval rather than circular profiles. This is due to refraction (that is, bending of light) in the atmosphere; it is similar to the way in which your arm seems to develop a sharp kink when thrust through the surface of a swimming pool. The amount of refraction produced in the atmosphere again depends upon the wavelength of the light in question, just as white light passing through a prism is split into the constituent colors of the rainbow. (It is a fallacy that the Sun and Moon are actually larger in size at rising or setting. This is an illusion produced by having reference objects visible along the horizon, as compared with none when the orbs are overhead.)

The atmosphere can thus produce coloration through two means. One is the fact that the blue end of the spectrum is more efficiently scattered by individual air molecules. The second is that the amount of refraction similarly varies across the spectrum.

At sunset the Sun looks red, but think of the light passing ten miles above your head, skimming through the atmosphere. The blue light is largely being scattered, making the sky blue, and also being refracted to such an extent that it is directed more towards the ground, pushing it deeper into the atmosphere and therefore suffering even more scattering. The red light is more likely to escape scattering and may be refracted by just enough to direct it towards the Moon. What is happening is shown schematically in Figure 2-8.

All around the globe the atmosphere is acting to transmit a

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