From one moon to another

From one moon to another

From one moon to another

Bad Astronomy
The entire universe in blog form
June 28 2009 7:00 AM

From one moon to another


The Big Picture once again does the International Space Station. My favorite picture? No contest:


Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

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Oh how I love this picture.

Of course I love shots of the Moon, but this speaks volumes. Note the Earth just below the Moon; the ISS was seeing the Moon through the top of Earth's atmosphere. As you may know, light bends when it passes from one medium to another, like from water to air, which is why a spoon in a glass looks bent. The same is true when light passes from a vacuum through air; it bends. In fact, the amount the light bends depends on the angle it intercepts the boundary; so that light coming in from one direction may get bent more than if it comes in from another.

So here comes the cool part: the Earth's atmosphere follows the curve of the Earth, so you can picture it as a thick shell of air around us. Here's a diagram:

ISS Moon line of sight diagram

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The Earth's surface is the lower arc, and the air above the upper arc. The Moon is to the left, the ISS to the right.

The red lines indicate the line-of-sight view to the Moon. When an astronaut looks at the bottom of the Moon, the angle of the air/space boundary is a bit different than it is when he or she looks at the top of the Moon. In my diagram that angle is close to being 45 degrees for the bottom line, but is more like 30 degrees for the top line. That means the light coming from the bottom of the Moon gets bent more than the top. As it happens, the light from the Moon gets bent upward as it passes through our air... so the bottom of the Moon looks like it's getting pushed into the top.

This squashes the view of the Moon! All of the light is getting bent, but by different amounts; the upper part of the Moon is closer to being a circle but is still distorted significantly. Making it worse, the Moon was not quite full in this picture, so the "left" side looks off, too.

What a mess! But it's an explainable mess, and one that's not even all that hard to do. The math is really just a bit of trig and a bit of algebra. In detail it gets more complicated, because the Earth's air gets thinner with altitude, and I didn't account for that. And I bet there are a hundred other variables as well.

But making some quick assumptions explains the gross characteristics of this picture just fine. And to me, that adds to my amazement of such a shot. Knowing more about it doesn't detract from its beauty and its wonder; it enhances them.

I really love that about science. It's easy to be awed when you don't know how something works, but when you get a glimpse into the machinery behind it, get an idea of how it really works, what you see becomes that much more beautiful.