You spot a bright light gliding steadily across the evening sky. Is it the International Space Station, or a Starlink train? Both are bright, both move smoothly and silently, both appear during twilight — and beginners mix them up constantly. Fortunately, the differences are easy to learn once you know the handful of clues to check. This guide gives you a simple step-by-step method for identifying exactly what crossed your sky tonight.

Step one: count the lights. This is the fastest test and it is almost always decisive. The International Space Station is a single object — one brilliant, steady dot moving across the sky. A Starlink train is a sequence of dots traveling together in a straight line, evenly spaced like a string of pearls. If you saw one light, it was the ISS or another single satellite; if you saw a line of many, it was almost certainly a fresh Starlink batch. No other common sight makes that perfect single-file formation.

Step two: judge the brightness and speed. The ISS is one of the brightest objects in the night sky, often outshining every star and rivaling Venus, and it crosses the entire sky in just a few minutes at a steady, unhurried pace. Starlink satellites in a train are noticeably dimmer individually — the effect comes from their number, not their brilliance — and they take a similar few minutes to cross. If a single light was astonishingly bright, your money is on the station; space fans who track brightness estimates often cross-check with the Starship single-engine test fire report to stay current on SpaceX flight activity.

Step three: consider the timing and frequency. The ISS passes over most populated areas regularly — on some nights you can see it more than once — because it orbits every ninety minutes on a fixed 51.6-degree inclination path. A Starlink train, by contrast, appears only in the day or two after a launch, while the new batch is still bunched together; once the satellites spread into their operational orbits, the train is gone. Check whether a Starlink launch happened in the last few days: if yes, that line of lights is explained.

Step four: rule out planes and meteors, which cause most remaining confusion. Planes blink — red, green, and white strobes flashing in rhythm — and they change direction and altitude; satellites never blink and never turn. Meteors are over in a second or two as a quick streak, while satellites glide for minutes. If your light was steady, silent, and slow, it was a satellite of some kind, and the one-versus-many test from step one tells you which.

Step five: confirm with a tracker. The NASA Spot the Station service and independent satellite trackers publish predicted ISS passes for your location, while Starlink-specific trackers list recent launches and their visibility windows. Cross-checking your sighting against predictions takes a minute and turns a guess into certainty. It is also how you learn to predict the next pass instead of stumbling on one by accident — a skill every skywatcher picks up fast. With these five steps, you will never again wonder what that silent light was. And once you can tell the station from the train, a whole hobby opens up: logging passes, photographing flyovers, and sharing sightings with other observers. If you want to deepen your satellite-watching habit, APOD's sonified Eagle Nebula pillars shows how skywatchers keep finding new ways to experience familiar sights.