If Astropotamus looks at the moon with his eyes, it looks beautiful. Just like is has for billions of years. Bright, crisp, full of dark splotches and light splatters. If I use my Time Machine and look at what the moon looked like 8 minutes and 14 seconds ago, I’m sometimes disappointed. I can’t see the moon as clearly through the eyepiece of the Time Machine as I can with my naked eye. Very frustrating! What’s up with that?
Astropotamuses refer to the seeing of a particular night as being good or bad. One potamus will say to another, “how’s the seeing?” The response is usually one of three things: “it’s okay,” “it’s bad,” or “it’s really bad.” It’s almost never “really good.” So what is this seeing that they’re talking about? It’s the general clarity and steadiness of an image as seen through the Time Machine.
When you look at something with your eyes, you have a built-in filter that helps you fill in the blanks when things aren’t quite right. It’s called your brain. That big, energy consuming brain of yours sees a scene and notices that the color isn’t quite right or the brightness isn’t quite right or that there is a chunk of visual information missing, and it fills in the blanks or corrects the scene with what it thinks should be there, based on past experience and subconscious expectations. This is great for watching movies, but not so great for looking at the stars.
Movies, prior to 2000, anyway, were mostly individual frames (pictures) that were shown at 24 frames (or pictures) per second. Even though this means each image is only on the screen for about 0.04 seconds and there is a bit of darkness in between each frame, your brain never notices, because it fills in the blanks. In the end, it looks like ET is smoothly flying across the moon because your eyes and your brain work together to give you the sense of motion that you experience in the real world when you watch something moving, even though you’re only seeing quick moments in time capturing individual images during a movie.
This same thing happens when you look at the moon and stars. You see a continuous source of light that’s fairly steady because that’s what you’re expecting to see. Look through the high magnification of a Time Machine, however, and you will see all sorts of weird movements and blurry spots and times where stars all but disappear as if they’ve sunk into a pool of murky oil. In general, this is because of the air we breathe.
Air is warm or cool. Warm air contains more moisture than cool air. Water vapor causes light that passes through it to bend slightly, just like a rainbow. Warm air also moves faster than cool air. This is what causes you to see ripples above the roadway in the distance. Combine all this with the wind and rain and clouds and dust and other things that happen in the 100 kilometer thick blanket of air that we call our atmosphere, and you can be looking through a turbulent soup of air to see the moon. What’s worse, this 100km thick blanket is only 100km if the moon is straight overhead. If you have to look downwards towards the horizon, you’re looking through more air than if you’re looking straight up, which makes things even worse!
The air warms and cools for lots of reasons. Assuming that 100% seeing is perfectly calm, even temperature air with no dust or water in it, here are some of the reasons that the seeing can get worse:
So this messy air thing can get in the way of a crisp, clear, bright moon (or star, or even our Sun!) and make things look like they’re wavy – like you’re looking at something in the distance across a hot desert and you get that wavy line thing you always see in the movies. Actually, it’s exactly like that, but you’re looking up and it’s dark. The less this happens, the easier it is to see things. This is where the term seeing comes from. The easier it is to see something through atmosphere, the better the seeing.