35,786 km up: why a satellite can hang still in the sky
Published
A geostationary orbit is a circle 35,786 km above Earth’s equator, 42,164 km from Earth’s centre[1]. A satellite there takes exactly as long to go round as the Earth takes to turn once, one sidereal day, so from the ground it seems not to move at all[2].
Why it hangs still
The higher an orbit, the slower it goes round. At one height the time for one orbit matches one turn of the Earth. A satellite at that height, over the equator and moving east, keeps pace with the ground beneath it. It travels at 3.07 km a second[3], but always over the same spot.
Arthur C. Clarke popularised the idea in a 1945 magazine paper about worldwide radio coverage from relay stations in orbit[4]. Today there are hundreds of geostationary satellites, used for remote sensing and communications[5].
From that height Earth fills about 17° of the sky (derived from the radius and distance above), and the polar caps beyond about 81° latitude are never in view.
Three points in the game
In game, geostationary orbit is at the same 35,786 km, around an Earth at its true size. There are three points on it, 120° apart:
| Point | Looks down on |
|---|---|
| 100° E | Asia, India, Australia, the west Pacific |
| 140° W | the Pacific, Hawaii, the west coast of the Americas |
| 20° W | Africa, Europe, South America, the Atlantic |
Together the three see the whole globe except the poles. The Sun in game is fixed at a June solstice morning, so 100° E is nearly full daylight and 140° W is mostly night.
- The three points link to each other, about 73,000 km apart: about 1.5 hours at 1 g.
- Each links down to the low-orbit points it can see, and up to Earth–Moon L1.
- Ships at a point stay over its spot. The game does not model orbital motion at any of its places; each is a fixed circle 800 km across.
More on how the places connect: Travel nodes. Lower down: low Earth orbit.
Related
- 7.8 km a second: what low Earth orbit means
- 326,400 km out: the Earth-Moon L1 point
- 11 places, 14 links: how travel works in Hostile Flagship
- 384,400 km apart: the Earth and the Moon at true scale
Sources
- Geostationary orbit, Wikipedia.
a circular geosynchronous orbit 35,786 km (22,236 mi) in altitude above Earth's equator, 42,164 km (26,199 mi) in radius from Earth's center
- Geostationary orbit, Wikipedia.
An object in such an orbit has an orbital period equal to Earth's rotational period, one sidereal day, and so to ground observers it appears motionless, in a fixed position in the sky
- Geostationary orbit, Wikipedia.
This equates to an orbital speed of 3.07 kilometres per second
- Geostationary orbit, Wikipedia.
Clarke popularised and expanded the concept in a 1945 paper entitled Extra-Terrestrial Relays – Can Rocket Stations Give Worldwide Radio Coverage?, published in Wireless World magazine
- Geostationary orbit, Wikipedia.
Today there are hundreds of geostationary satellites providing remote sensing and communications