Why Satellites Vanish Mid-Pass: Earth's Shadow Explained

It's one of the most commonly reported 'anomalies' in satellite watching: a bright, steady point of light tracks smoothly across the stars, then dims and vanishes in the space of a few seconds — mid-sky, nowhere near the horizon. No flash, no debris, just gone. What you witnessed is the single most important concept in satellite visibility: eclipse entry, the moment a satellite flies into Earth's shadow.

Satellites don't produce light

A Starlink satellite has no lights. Every photon you see from the ground is reflected sunlight, bounced off its solar array and body panels. That makes satellite visibility a pure geometry problem involving three actors: the Sun, the satellite, and you.

To see a satellite, two conditions must hold at the same time. The satellite must be in sunlight — and you must be in darkness. If the satellite is in Earth's shadow, there's nothing to reflect. If your sky is still bright, the reflection is drowned out. The overlap of 'satellite lit, observer dark' is a narrow window, and it explains almost everything about when and where you can spot Starlink.

The shadow is a cylinder you can't see

Earth casts a shadow roughly the shape of a giant cylinder (tapering to a cone) stretching away from the Sun, more than a million kilometres long. A satellite at 550 km altitude spends a large fraction of every orbit inside it. When your satellite faded mid-pass, it crossed that invisible boundary — still flying, still functioning, just no longer illuminated.

You can even predict the direction: in the evening, satellites vanish as they move east, away from the sunset, deeper over the night side of Earth. Before dawn the show runs in reverse — satellites materialize mid-sky as they exit the shadow, which looks even more startling.

Why passes cluster around twilight

This same geometry explains why visible passes bunch into the hours after sunset and before sunrise. In the middle of the night, almost everything overhead is inside the shadow with you — the satellites are still passing, but none are lit. In the middle of the day, plenty of satellites are sunlit, but so is your sky. Only in twilight are you in the dark while objects 550 km above you still catch the Sun.

There's a seasonal wrinkle: at high latitudes in summer, the Sun stays shallow below the horizon all night, so satellites overhead can stay sunlit for hours. Late June nights in places like London or Seattle can deliver pass after pass all night long — the same effect that makes those latitudes best for spotting noctilucent clouds.

How our pass predictions handle this

The pass forecasts on this site model the full geometry, not just 'is the satellite above your horizon'. For every minute of the prediction window we compute whether the satellite is sunlit using an Earth-shadow model, whether your Sun is far enough below the horizon (civil twilight, −6°), and whether the satellite clears 10° elevation. Only when all three hold do we call a pass visible — which is why our list shows a handful of genuinely watchable passes instead of hundreds of technically-overhead-but-invisible ones.