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ScienceWise - Sep/Oct 2007

The Editor's Corner -

Article Illustration
During a lunar eclipse the shadow of the earth falls across the face of the moon. It’s a beautiful sight but it’s also filled with interesting physics. At the start of the eclipse the earth blocks one side of the sun’s disc but not the other. This results in a partial shadow, known as the penumbra. As the earth moves further across, it begins to block all the sunlight creating a full dark shadow - the umbra. The shape of the umbra is graphic evidence that the earth is round.

Another interesting phenomenon is the changing colour of the moon during the various stages of the eclipse. The Earth’s atmosphere scatters blue light far more than it does red light. During the day, the scattered blue photons enter our eyes from every possible angle, which is why the sky appears bright blue. However all this blue light lost to scattering results in the remaining sunlight appearing more red. This effect is at its greatest at sunrise or sunset because when the sun is on the horizon, its light has to pass through far more air than when it’s directly overhead. The exact same thing happens to moonlight, often causing the rising moon to appear orange. Notice how in this time-lapse of the eclipse, the moon appears orange then yellow as it’s rising behind the gum trees.

Another less obvious effect comes into play due to the curvature of the earth and the changing density of air with altitude. This causes the atmosphere to act like a very weak lens bending any light that passes through it. As a consequence of this, there is a slight apparent flattening of the sun or moon when near the horizon. Lensing also results in the refraction of a small amount of sunlight into the earth’s shadow. Because this refracted light has passed through the atmosphere at very shallow grazing angles, the scattering effect is especially strong, colouring the light red. During a total eclipse, the moon’s face is partially lit by all the sunrises and sunsets on earth put together.

The Infinite Atmosphere Theory

During a total lunar eclipse, the moon can turn a deep coppery red at totality. In the middle ages, this “blood moon” was seen as an omen of disaster. Of course in more enlightened times we have a far better explanation for the reddening of the lunar disc at totality. We understand it in terms of atmospheric lensing and scattering of the blue component of sunlight by the dust and gasses in the air. It’s all about the earth’s atmosphere and nothing to do with prophesy.

As science makes more and more progress towards understanding natural things like the earth’s atmosphere, I think there’s a tendency to forget the awesome power that they posses. It’s almost as though they become specimens in jars, understood, catalogued and for the most part, totally harmless. This is largely how we’ve treated the atmosphere over the last century. Ten kilometres up multiplied the surface area of the earth makes a gargantuan volume of air. At the human scale, essentially an infinite volume. So if you pour a few cubic metres of this or that into it, the effect is a few divided by infinity - in other words absolutely nothing at all. The problem with this arithmetic is of course that the number of humans on the planet is also becoming quasi-infinite, and as any good mathematician will tell you, infinity over infinity is an undefined quantity.

I don’t think there’s a simple answer to climate change but what I am sure of is that science and technology will have to play a central role in both fully understanding the problem and creating appropriate solutions. It would be terrible to see superstition proved right about the “blood moon” - even though it would be for all the wrong reasons. And perhaps equally terrible to see our response to the situation driven by modern superstitions rather than well grounded scientific reasoning.

Dr Tim Wetherell - ScienceWise Editor

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