The ISS is often the brightest thing in the evening sky, and its appearances can be predicted to the minute. Here's why the schedule works.
A city-sized mirror
The Station carries no beacon bright enough to see — what you're watching is sunlight reflecting off an object the size of a football field, 400 kilometers up. That geometry sets the rules: you need the sky dark where you stand, but the Station high enough to still be in sunlight. Those conditions overlap in the hours after sunset and before sunrise, which is why passes cluster at dusk and dawn.
Ninety minutes around the world
Orbiting at about 7.7 km/s, the ISS circles the Earth every 90 minutes or so, its path shifted westward each time as the planet turns beneath it. That's why predictions are so precise: the orbit is measured continuously, and where it will be over the next days is straightforward mechanics. Our pass predictions scan the real ephemeris for windows when the Station climbs above 10° from your horizon — low passes hide behind buildings and haze.
What you'll see
A steady, bright, fast-moving star — no blinking (aircraft blink; the Station doesn't), crossing the sky in two to six minutes. Sometimes it vanishes mid-flight as it slips into Earth's shadow: you have just watched sunset happen 400 km overhead. Wave if you like; there have been people aboard continuously since November 2000.