Part 6 — The Skimspeed and Weight of Earth

Out in the garden at night, maybe an hour and a half past sunset, we sometimes see man-made satellites — starlike dots moving slowly on an approximately north-south line. We only see the low satellites and they all seem to move at about the same speed, which is pretty close to the Earth's skimspeed. They would be moving exactly at the skimspeed if they were in circular orbit down here at the surface, but they need to be a few percent farther out to get clear of the air and farther means slower.

Those little dots of light are going about 250,000 dimes. If I square that twice in succession I get 4×10²¹ quartic dimes, each of which is worth a ton in weight. This tells me the earth's weight (in gravity equal to that at its surface) is: 4×10²¹ tons.

My weight at the earth's surface is a fairly modest fraction (about 1/14) of a ton. So at least in an order-of-magnitude sense, watching a satellite move gives me a way of comparing my weight, in surface gravity, with that of earth.