What is a satellite fade?
The moment a satellite flies out of sunlight and into Earth's own shadow, mid-sky — explained with the actual geometry, in the actual numbers.
A satellite doesn't produce its own light. Every one you can see with the naked eye — the ISS, Starlink, Hubble, the lot — is visible for exactly one reason: sunlight is bouncing off it, hundreds of kilometers over your head. Which means there's a second condition hiding behind "is it dark where I'm standing": is the satellite itself still in daylight, up there?
Partway through some passes, the answer flips. The satellite flies out of the Sun and into the cone-shaped shadow the Earth casts into space — the same shadow that, from the Moon's side of things, makes a lunar eclipse. From the ground, it looks like the satellite simply goes out, mid-sky, with no warning and no horizon to blame. That's a fade. Run it in the other direction — a satellite emerging from the Earth's shadow into sunlight, appearing from nothing — and it's an ignition. Fades are typical of evening passes, ignitions of morning ones, for the obvious reason: which side of the planet is currently facing the Sun.
The actual shadow
Earth's umbra — full shadow — is a cone, not a cylinder. It starts at Earth's own radius (about 6,378 km) and narrows as it stretches away from the Sun, converging to a point roughly 1.38 million kilometers downrange. A satellite in low Earth orbit, a few hundred kilometers up, is nowhere near that far out — it's crossing a shadow that's still nearly as wide as the Earth itself, which is why the transition, when it happens, happens fast relative to the length of the pass, but not instantaneously.
Not a cliff — a window
It's tempting to model the shadow's edge as a hard line: lit on one side, dark on the other, done. Real sunsets don't work that way from orbit either. At the depth a satellite like the ISS orbits, the dimming takes roughly 22 seconds from noticeably fading to gone — the partial-shadow (penumbral) region has real width, and a satellite takes a measurable few seconds to cross each kilometer of it. That's the number this matters for: not "when does it disappear" as a single instant, but "when does the ~22-second window start."
Predicting that start time honestly is harder than the geometry alone suggests. Several modeling choices each nudge the computed instant by a handful of seconds — which coordinate frame the Sun's position is computed in versus the satellite's, which Earth radius defines the shadow cone, whether the atmosphere's own bending of sunlight at the shadow's edge is accounted for (it usually isn't). Added up, a careful estimate is honestly accurate to something like ±10 seconds, not fractions of a second — which is exactly why the right way to call a fade is a window, not a countdown to a single tick.
Why it's the moment worth calling
Most pass predictors give you a rise time and a set time and stop. Neither of those is the moment people actually remember watching. Read about the ISS's own dawn-and-dusk visibility windows for how this plays out for a specific, very bright, very findable satellite — or see what SpacePings actually pings for on the app itself.
SpacePings' whole notification design is built around this one event: not "it's up there," but "look now — it's about to go out."
Just watched one happen and want the short version of what you saw? See why did the satellite disappear.