The ISS Orbit Explained: Path, Inclination, and Period
The ISS travels along an orbit tilted about 51.6° to Earth’s equator. That tilt is why the station never flies over the poles but does reach latitudes as high as the north of Scotland or the tip of South America — and why so much of the world’s population passes underneath it.
Because the planet rotates underneath the orbit, each new lap passes over ground further west than the one before. The result is a constantly shifting, slightly diagonal ground track that never repeats exactly — which is also why "where is the ISS right now?" is different every single minute.
The basic numbers
- Period: about 93 minutes. The ISS completes roughly 15.5 orbits every 24 hours.
- Inclination: 51.6°. The orbit is tilted 51.6° to the equator, so the station swings between roughly 51.6°N and 51.6°S on each lap.
- Altitude: 400-420 km. A low, fast orbit that the station maintains with periodic reboosts.
NASA notes that because of that inclination, the station’s path passes over about 90% of the Earth’s population. It reaches high northern latitudes like Edinburgh (~56°N) or the southern tip of South America, but it never flies over the far north.
Why 51.6°, specifically
Inclination is not arbitrary: it is chosen at launch time. The station’s Russian elements launch from Baikonur Cosmodrome at about 46°N, and an orbit must be at least as tilted as its launch latitude. A 51.6° inclination is the minimum that the Russian launch sites can reach comfortably while still letting everyone reach the station — and, conveniently, that tilt also sweeps far enough north and south to cover most of the world’s people.
It is also a compromise with physics: every degree of inclination costs fuel to reach, so operators chose the steepest useful tilt that stays cheap enough to visit from two launch programmes.
Why the ground track keeps shifting west
While the station orbits, Earth rotates underneath it. One lap takes ~93 minutes, and in that time the ground below rotates east by about 23° of longitude. So by the time the station comes back around, its ground track steps about 22-25° to the west of the previous lap — a drifting, sweeping pattern rather than a fixed loop.
Over a single day the ISS draws a band of wavy ground tracks, each one offset from the last. This is why the answer to "which cities does it fly over right now" changes continuously, and why whole regions are briefly "in the path" before the track moves on.
Why sightings don’t repeat day to day
Because 15.5 orbits is not a whole number, the track also slides relative to the Sun day by day. A town that saw the station at 10 p.m. one evening will see it at slightly different times over the next weeks, working through a slow cycle of evening, then morning, opportunities.
Sighting windows therefore cluster, then quiet down, then return — normal orbital geometry, not the station disappearing. The practical pattern (and how to tell when your town is "hot") is on the why is the ISS visible page.
See it happening live
The best "diagram" is the live one. The main tracker draws the station’s current, past, and upcoming ground track in real time on the map, and the 3D view shows the orbit wrapping the whole planet. Open it and watch a lap: you’ll see the track shift west with each revolution, exactly the behaviour described above.
For the numbers behind the display, the TLE and SGP4 pages explain how this engine turns orbital elements into that moving line.
Period is set by altitude, so the orbit and height are inseparable: see the altitude page for that link.
Watch the ISS complete an orbit in real time on the live map and track its ground path.
Open the live mapSources and further reading
- NASA — Spot the Station FAQ (orbit & 90% of population)
- NASA — International Space Station Facts and Figures
