A Starlink train is the line of bright dots seen gliding across the night sky in the days after a Starlink launch — dozens of freshly deployed satellites still travelling close together. Best seen in the first evenings after deployment, it is among the most-reported "UFO" sightings in the world.
Understanding Starlink Train
Why a line, and why so even
The geometry that spooks first-time viewers is orbital mechanics at its most literal. All the batch's satellites share one orbit, so from the ground they follow one another along an identical arc — a bead chain threaded on an invisible wire. The even spacing appears as deployment drift and early phasing separate members at near-uniform rates; small brightness differences ripple down the line as each satellite's solar-panel angle and attitude differ slightly. Sometimes the procession glints in sequence — a wave of flares travelling down the train as satellite after satellite crosses the same mirror-angle with the Sun — the single most otherworldly-looking behaviour in the modern sky, and pure geometry. Later in a batch's life the train loosens into a "pearls on a longer string" look, then into stragglers, each stage a readout of how far the deployment schedule has progressed.
The train and the telescope
What delights the naked eye troubles the observatory. A train crossing a wide survey field during twilight imaging leaves parallel streaks through the exposure, and the launch cadence of the mega-constellation era made such crossings statistically unavoidable for certain programmes. The response has been a negotiated middle: operators darken satellites (visors, dielectric films, off-pointing during the raise), publish orbits so telescopes can schedule around passes, and participate in dark-sky working groups; astronomers in turn quantify impacts honestly — severe for a few twilight-hunting programmes, manageable for most others. The train phase remains the worst of it: satellites at their lowest, brightest and most clustered. The same freshness that makes a train worth running outside for makes it the thing observatory schedulers most want to avoid.