A satellite is a steady, silent point that glides across the sky in a few minutes; planes blink red, green and white, and meteors flash past in seconds.
You look up after sunset and see a ‘star’ moving. Three things are likely: a satellite, an aircraft or a meteor. You can usually tell which within a few seconds.
1. Does it blink? Large aircraft must carry a red light on the left wing and a green on the right, a white light at the back, and flashing anticollision lights — 40 to 100 flashes a minute, in red or white. Satellites have no lights you can see. The Planetary Society notes that, unlike a plane, the space station's light won't blink, though it may briefly flare brighter.
2. How fast and how long? A meteor is a streak of light from a speck of dust or grit — sand-grain to pebble size — burning up roughly 80 to 120 km up. Even a very bright fireball usually lasts only a few seconds. A low satellite like the ISS usually takes a few minutes to cross the sky.
3. Does it follow a straight, smooth path? A satellite moves in a steady line at a steady speed. It does not turn. It may fade out mid-sky when it enters Earth's shadow.
The surest test is to compare with a prediction. Open the passes for your city on this site. If a card says ISS — 7:12 pm IST, Look W → SE, Max height 64°, and you see a steady bright point rising in the west at 7:12, it is almost certainly the ISS.
If you saw something that matches no pass, it may be a fainter satellite this site does not list — the site leaves out objects whose brightness it cannot back up — or a rocket body. The Remind me button on a pass card adds the pass to your calendar, so next time you can be outside and looking the right way before it starts.
Match what you saw against a pass card: the start time, the Look direction (e.g. W → SE) and how high it climbed. See tonight's passes over Delhi
You can tell the three apart from angular speed — how many degrees per second the object crosses.
Satellite. The ISS moves at about 7.66 km/s. Overhead, at 420 km range, its angular speed is roughly v / ρ = 7.66 / 420 ≈ 0.018 rad/s ≈ 1.0° per second (a little less in practice, because the observer turns with the Earth). Near the horizon, at about 1,490 km range and with motion partly towards you, it drops to well under 0.3° per second. It is fastest when highest.
Meteor. Meteoroids enter at 11–72 km/s at heights of about 80–120 km. A meteor at 100 km range moving sideways at 30 km/s sweeps 0.3 rad/s ≈ 17° per second — more than ten times faster than the ISS — and burns out before you can turn your head.
Aircraft. A jet cruising near 250 m/s at 10 km height overhead moves 0.25 / 10 = 0.025 rad/s ≈ 1.4° per second — similar to the ISS overhead. That is why speed alone can mislead, and why the lights are the better test: aircraft must flash their anticollision lights 40 to 100 times a minute.
Duration. A satellite pass on this site is the time between crossing 10° elevation (or leaving Earth's shadow) and dropping below 10° (or entering shadow). For a high ISS pass that is several minutes; for a fireball, the American Meteor Society says, usually only a few seconds. The American Meteor Society's rule of thumb: if you had time to take out your phone and photograph it, it was almost certainly not a fireball.
A satellite. Meteors last only seconds, and aircraft carry blinking red, green and white lights.
About 80 to 120 km above the ground.
An airliner overhead and the ISS overhead can cross the sky at similar angular speeds, about 1° per second; the blinking lights are a more reliable clue.