Different orbits for different jobs

A satellite carrying your television signal sits 36,000 kilometres up and never moves. Your dish points at one fixed spot in the sky and stays there, bolted to the wall, for years. Meanwhile, far below it, the space station laps the whole planet every 90 minutes — sixteen times a day, a sunrise every hour and a half for the people inside. Two satellites, two completely different lives. One hangs still; one races. The only thing that sets them apart is how high they are. Height decides everything about how an orbit behaves — and it does it in a way that is exactly backwards from what feels right.

The higher you go, the slower you orbit

The last lesson on orbits ended with a strange rule: a lower orbit is faster, a higher orbit is slower. Drop down and you speed up; climb and you slow down. This lesson takes that rule to its limit.

Hold it in your head as a single fact. Down low, just above the air, a satellite must move at about 7.9 kilometres every second to stay up. That is the space station’s speed. At that pace it goes all the way around the Earth in about 90 minutes.

Now lift it higher. Gravity is gentler up there, so the satellite doesn’t need to move as fast to stay in its fall. It slows down. And it has a bigger loop to travel. Both pull the same way: a higher orbit takes longer. Slower satellite, bigger loop — the time to go around grows fast as you climb.

Period is set by height, and nothing else

The time a satellite takes to circle the Earth once is its period. The remarkable thing is what sets it. Not the satellite’s size, not its weight, not what it’s made of. Only its height. Every satellite at the same altitude takes the same time to go around, whether it’s a bus-sized weather machine or a bolt that fell off one.

There’s a clean formula behind this, but you don’t need the arithmetic to trust its shape: period depends only on the satellite’s distance from Earth’s centre, and climbs steeply with it. Pick a height, and the universe hands you the time to go around.

That means there is one special height. One altitude where the period comes out to exactly 24 hours. And 24 hours is a magic number, because that is how long the Earth itself takes to spin once.

The height where a satellite hangs still

Put a satellite at that special altitude — about 36,000 km up, above the equator — and its period is one full day. It circles the Earth once every 24 hours. But the ground beneath it is also turning once every 24 hours, in the same direction. The satellite and the spot of ground below it move in perfect step.

So from the ground, the satellite never appears to move. It rises to one point in the sky and stays there. This is a geostationary orbit — “geo” for Earth, “stationary” for standing still relative to the ground. It isn’t standing still in space; it’s racing along at thousands of kilometres an hour. It only looks still because the Earth is keeping pace underneath it.

This is why your satellite dish doesn’t need a motor. The thing it’s listening to never wanders. Television, weather watching, long-distance phone links — all of it leans on this one height, because a fixed dish can drink from a fixed point in the sky.

A worked picture

Walk it in three steps.

Start with the space station at about 400 km up. Its period is about 90 minutes. In one day it goes around about sixteen times. To anyone on the ground it streaks across the sky in minutes and is gone.

Now climb. As the altitude rises, the period stretches. A few thousand kilometres up, a satellite takes a few hours per lap. Higher still, half a day. The curve keeps bending upward; every extra kilometre buys more minutes per orbit than the last.

Keep climbing to 36,000 km. Here the period has stretched all the way to 24 hours. The satellite now turns at exactly the rate the Earth turns. From below, it has stopped. One height, and the racing satellite of the climb’s bottom has become the motionless satellite at its top. Same physics, opposite appearance — set entirely by how high you went.

Why the low ones fall and the high ones don’t

There is a price for living down low. At 400 km the sky is not quite empty. A thin haze of air still reaches up there — far too faint to breathe, but real. A satellite ploughing through it at 7.9 km/s feels a tiny, constant drag. That drag steals speed. And a slower satellite cannot hold its height; it sinks to a lower orbit, where the air is thicker, where the drag is stronger, where it sinks faster still. Left alone, a low satellite spirals down over months or years until it meets the thick air below and burns up.

This is why the station and other low satellites need an occasional shove — a reboost, a small engine burn to push them back up before the drag wins. Up at 36,000 km there is no air at all to fight. A geostationary satellite is above the drag entirely. It can hold its spot for decades on almost nothing. The low orbit is busy and short-lived; the high orbit is patient and long.

On the whole

The same word — higher — that makes a satellite slow down also makes it stop appearing to move, and also lifts it clear of the air that drags the low ones down. One choice, height, ripples through speed, through time, through how long the thing survives. Nothing here is a separate decision; they are all the same decision, seen from different angles.

It is worth noticing how thoroughly the obvious guess fails. Higher feels like faster. Up here the rule inverts, and the highest satellite of all is the one that never seems to move. We carry rules of thumb built on the ground and assume they hold everywhere. Above your head, gently and completely, they stop. The voice on your phone from across the ocean is bouncing off a thing that is falling around the planet once a day, holding still over the sea by racing at the exact speed of the world turning beneath it.