A Red Moon

By Allan Ko, Grade 11, Fremont, CA

In light of the recent lunar eclipse on Saturday, December 10, here’s a thought: why would the moon turn red during an eclipse as it moves into the Earth’s shadow? One might expect instead that the moon simply turns black and disappears briefly, since there wouldn’t be any light to illuminate it.

This reddening of the moon, it turns out, is due to the effects of the Earth’s atmosphere. This blanket of air actually bends sunlight around the Earth during a lunar eclipse so that some light still strikes the moon to illuminate it, somewhat like a huge lens.

So then, why isn’t the moon white during the eclipse if it’s still illuminated by the sun? This is due to something called Rayleigh scattering. Rayleigh scattering is caused by the individual particles in the atmosphere. When sunlight, which is white light, travels through the atmosphere, these particles tend to scatter the shorter wavelengths of light (such as violet, blue, or green) while allowing the longer wavelengths (like red, orange, and yellow) to pass through relatively unhindered.

Thus, when sunlight bends around the Earth through the atmosphere during an eclipse, the atmosphere scatters to the sides the short-wavelength light and leaves only the red and some orange light on its path to the moon. It is this light that strikes the moon and illuminates it in a beautiful rusty red color.

Incidentally, it’s worth noting that this same Rayleigh scattering is what causes the sky to look blue: some of the blue light scattered away is scattered towards the ground, and we see a wonderful blue sky. This is also the same reason that sunsets are red and orange; the light from the sun has to travel through a lot of atmosphere to reach our eyes (since at sunset, sunlight strikes the Earth at an angle), so most of the other colors of light have been scattered away to leave the picturesque red of the sunset.

(INSPIRE would like the Online Learning Community to comment on blog posts by using the link below to the Discussion Board. If you can't find a thread, you may create a new one. Please observe all rules on etiquette.)
Comment on this blog:http://tinyurl.com/bloginspire