GUIDES / BASICS
Comparing the ISS and Tiangong
By Jeroen Wouda · 2 November 2026 · Basics
Two crewed space stations are in low orbit at the same time, and from the ground they look much alike: a bright point crossing the sky in a few minutes. They are in quite different orbits, though, and that difference decides who can see them, how high they get and how bright they appear.
Size and orbit, side by side
- The International Space Station orbits at roughly 400 km, inclined 51.6° to the equator, and completes one lap in about 92 to 93 minutes — around 15.5 orbits a day.
- Tiangong orbits lower, broadly in the 340 to 450 km band, inclined about 41.5°, with a period of about 90 minutes.
Both travel at close to orbital velocity, and both are visible for the same reason: sunlight reflected off their structure while the observer stands in darkness. Neither carries lights you could pick out from the ground.
Inclination is the number that matters most
Inclination is the tilt of an orbit relative to the equator, and it sets a hard limit on the ground the satellite can pass over. A satellite inclined at 41.5° never travels north of 41.5° north latitude, and never south of 41.5° south. Its ground track stays inside a band around the equator, and the edge of that band is as far as it goes.
That has a direct effect on what you can see:
- Between about 41.5° and 51.6° latitude, the ISS can pass directly overhead but Tiangong cannot. Tiangong is still visible there, but always as a lower arc to the south in the northern hemisphere, or to the north in the southern hemisphere.
- South of 41.5°, both can pass overhead.
- North of 51.6°, neither passes overhead. Both appear in the southern sky, and the ISS — with the higher orbit and the higher inclination — generally gives the better show.
The closer your latitude is to a limit, the higher the best pass you will get. An observer just inside Tiangong's band sees it pass almost overhead; an observer well beyond it sees only the top of a distant arc.
Brightness: size, and the angle of the light
The ISS is the largest artificial object in orbit — a long truss, large solar arrays, a stack of modules — and it reflects a great deal of light. At a comparable elevation it is generally the brighter of the two, and a high ISS pass can be striking: brighter than any star in the sky, briefly.
Tiangong is much smaller. A good pass is still a clear naked-eye object, easy to see and unmistakably moving, but it does not usually rival the ISS at its best. Brightness also depends on the angle at which sunlight strikes the structure and how that light comes back towards you, so the ranking is a general rule rather than a promise for any given night.
Telling them apart
From the ground, both are steady points of light with no blinking, moving in a smooth line across the sky. There is no colour difference you can rely on, and no shape to see. If you have not checked which one is due, you cannot reasonably tell them apart by eye.
The reliable method is the table. If both are listed at the same time in the same part of the sky — uncommon, but not impossible — the higher and brighter one is likely to be the ISS, but the table is what settles it. A flashing light is an aircraft, and a streak that is gone in a second is a meteor.
Which one to look for where you live
- Mid-latitude Europe and North America: the ISS is the main event, and gets high in the sky. Tiangong is visible on good nights, low in the south.
- Southern Europe, North Africa, the Middle East, southern China, the southern United States: both are worth checking, and both can ride high.
- Above 51.6°: the ISS, always to the south, and worth going out for whenever the geometry is right.
- Tropical and southern latitudes: both can pass overhead, and Tiangong is a far more serious proposition there than it is in northern Europe.
Pass length
Passes of both stations last a few minutes, and both cross the sky at much the same apparent pace. Tiangong, being lower, tends to move a little more quickly overhead and to give slightly shorter passes. Neither is difficult to follow: a slow sweep of the head is enough.
Where to go next
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.