GUIDES / BASICS
The ISS orbit, and why 51.6 degrees decides what you can see
By Jeroen Wouda · 2 November 2026 · Basics
The International Space Station is often described as being in orbit, which is true but not very useful. What matters is the altitude, the period and the inclination, and of those three the inclination decides whether the station ever appears in your sky.
A low orbit, and what low means
The station orbits at roughly 400 kilometres above the surface. That sounds high, and compared with an airliner at 10 kilometres it is, but on the scale of the planet it is a thin shell. The Earth's radius is about 6,370 kilometres, so the station is only around six per cent further from the centre of the planet than you are.
That closeness has consequences. It means the station moves fast, about 7.7 kilometres per second, and completes a full orbit in roughly 92 to 93 minutes. It also means the horizon behind which it hides is close, so a pass only lasts a few minutes.
Because the orbit takes about 92 minutes, the station goes round the Earth a little over 15 times in a day. Every one of those orbits is a chance for a pass over some part of the world. Whether it is a pass over you depends on geometry rather than luck.
Inclination in plain terms
Inclination is the angle between the orbital plane and the equator. Zero degrees would be an orbit directly over the equator, always heading east. Ninety degrees would be a polar orbit, passing over the poles.
The station's inclination is 51.6 degrees. It travels north-east, crosses the equator at 51.6 degrees of longitude at a time, keeps climbing until it reaches 51.6 degrees north latitude, then swings south and repeats the exercise in the southern hemisphere.
The practical result is a band. The station's ground track never goes further north than 51.6 degrees north or further south than 51.6 degrees south. That band contains most of Europe, all of Africa, most of Asia, both Americas down to the tip of Argentina, and Australia. It excludes the Arctic, Antarctica, and the northern parts of Scandinavia, Canada and Russia.
If you live inside the band, the station can, at some point in its cycle, pass directly over you. If you live outside it, it never will. From Edinburgh, at 56 degrees north, the station never reaches the zenith; the best you can hope for is a pass low in the southern sky. From Rome, at 42 degrees north, it passes overhead regularly.
Why 51.6 and not something rounder
The figure was fixed early in the programme and it was not chosen for elegance. It is close to the latitude of the Russian launch site at Baikonur, and a station inclined at about 51.6 degrees can be reached by launches from there without a costly plane change manoeuvre once in orbit. A station in a lower inclination would have been awkward for the Russian modules; a much higher one would have cost more energy to reach. The number is a compromise between launch access, the desire to overfly as much of the inhabited world as possible, and the physics of getting there.
Once set, it is effectively permanent. Changing the inclination of something as massive as the station would require an enormous amount of propellant, far more than any resupply ship could deliver. The orbit is what it is.
What it means from the ground
Inclination shapes every pass you will ever see. For an observer at mid-northern latitudes, the station almost always appears in the west or south-west, climbs towards the south or overhead depending on the pass, and sets towards the east or north-east. It never appears in the north, because its orbit does not take it there.
The maximum elevation of a pass is a measure of how close the ground track comes to your position. A pass with the station overhead has an elevation near 90 degrees. A pass where the track runs a few hundred kilometres away might peak at 30 degrees. A pass on the far side of the band could peak at 10 degrees and be barely visible above the rooftops.
This is why two cities a few hundred kilometres apart get different schedules. The orbit is the same. The offset is not.
The ground track, drawn out
Because the Earth turns underneath while the station orbits, each successive pass is shifted west. In the 92 minutes of one orbit the planet rotates through roughly 23 degrees of longitude, so the next ground track runs about 23 degrees to the west of the previous one.
The upshot for observing is that the station does not return to the same part of the sky at the same time each evening. It comes back into a good position over your location every few days, in a pattern rather than a rhythm.
Why the altitude drifts
The 400 kilometre figure is an average, not a constant. The station is faintly slowed by the thin remains of the atmosphere at that height, and it loses altitude slowly as a result. Periodic boosts from docked cargo vehicles and from the station's own thrusters push it back up. The exact altitude moves within a band of a few tens of kilometres, which is why prediction services refresh their orbital data constantly.
A small altitude change shifts the period slightly, and over days that adds up to a shift in pass times. This is one of the reasons a pass prediction is only trustworthy for a limited window ahead.
Where to go next
- ISS passes shows the next passes for your city, with elevation and direction for each one.
- How often does the ISS pass over my city? turns the orbit into an answer about your own sky.
- Why passes cluster at dusk and dawn explains the sunlight side of the same geometry.
More guides
- Best time of year to watch the ISS — There is no single best month. The good weeks depend on the angle between the orbit and the Sun, and here is how that cycle works.
- How often does the ISS pass over my city? — The station circles Earth 15 times a day, yet you can go weeks without a visible pass. Here is why the schedule behaves that way.
- How to photograph the ISS — A moving point of light, a dark sky and a camera, and what it takes to turn a pass into a photograph worth keeping.