The Deceptive Sky

A few days after the November fireball shot across Alaska's skies, a friend called me. "Did you see it?" he asked. "I was out on my snow machine, and it went right overhead. Terrific! It looked like it landed right behind the university."

I hadn't seen that dramatic visitor from space, but my friend had generated a vivid flashback of a 23-year old memory. When the first barium rocket lifted from Poker Flat, I was standing on Cleary Summit with a bunch of cheering colleagues--who all fell quiet momentarily because it looked as if the rocket was looping up over us, destined to take out Fairbanks at our backs.

That illusion passed quickly as the rocket arched away downrange, but it's connected to the same perceptual difficulty that leads observers of streaking meteors to think they're destined to land just over the horizon. It isn't merely poetic to speak of "the dome of the sky." Our brains insist that the overhead expanse is shaped like an inverted bowl, and we mentally impose curved paths on objects that are actually flying straight. Then, too, we usually underestimate distance to a bright light, and our vision centers seem to want to believe that something bright and moving is coming toward us.

Possibly because we evolved under circumstances where dangerous predators rarely approached from above at altitudes higher than a tree branch, humans have some trouble gauging distances and altitudes when they see strange things in the sky. These difficulties in perception even apply to familiar objects; most people can't believe that a dime held at arm's length can cover the visible disk of the full moon until they perform the experiment.

Neil Davis's compendium of factual lore about the north, Alaska Science Nuggets, offered some helpful hints about understanding what you see in the sky. A tool for estimating altitude is the human hand. Make a fist with the thumb up--the hitchhiker's gesture. Place the flat of the fist on the horizon, and your thumb is about 20 degrees above the horizon. Four and a half fists plus thumbs should bring you to the zenith, no matter what size your hand. Another convenient reference is the Big Dipper constellation--the pointer stars are about five degrees apart.

Although not as satisfying as knowing a moving object's speed in miles per hour, it's possible to get some sense of how fast it is going by counting off degrees in seconds. (Some people might giggle when you tell them "it traveled a fist in One-Mississippi," but scientists would be grateful.)

So far--as I write this--no one has reported finding a piece of a satellite or a cooled-down meteorite, so exactly what the November fireball was remains unknown. The reports seem to favor a meteor, though. According to Davis, theses hunks of rock from the heavens seem to change color as they fly. White is the one usually reported, but blue, green, yellow, and orange are also often seen, as they were with this November's fireball. Some of the difference in colors comes from the burning-off of the different elements composing the object, and some comes from its changing temperature as it slows down.

And, sad to say, near its landing point a typical meteor doesn't glow at all. It burns out far above the ground. That happens because it reaches its terminal velocity, usually about 300 meters (about 1000 feet) per second, while it is miles or tens of miles high. People who have been fortunate enough to see a small meteorite land nearby and have been able to pick it up report it is warm, but not hot.