# From one moon to another

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

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:

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.