Why Did the Satellite Disappear Mid-Sky?
No warning, no horizon, no blinking out -- just gone, while it was still climbing or high overhead. Here's exactly what happened, in the real geometry.
Why did the satellite I was watching just disappear?
You watched what's called the Fade: the satellite flying out of the Sun and into the cone-shaped shadow the Earth casts into space -- the exact same shadow that, from the Moon's side of things, produces a lunar eclipse. A satellite doesn't produce any light of its own; every one you can see with the naked eye is visible for exactly one reason, sunlight bouncing off it hundreds of kilometers over your head. Cross into the Earth's shadow and that light source is simply gone, with no warning and no horizon to explain it.
It isn't instantaneous, either, even though it can look that way. The crossing itself -- the instant SpacePings calls as the fade or ignition -- is computed against Earth's umbra, the region of full shadow, modeled as a cone-shaped boundary with a hard edge, not a softly graduated one. That computed instant is accurate to something like ±10 seconds in the real world, once you stack up the small modeling choices behind it (more on that below). What you actually watch happen in the sky, though, isn't a single instant: the visible dimming plays out over roughly 22 seconds around that crossing -- fast enough to feel sudden the first time, not fast enough to be a true instant. That gap between the moment the crossing is computed and the several seconds you actually watch is exactly why SpacePings calls a window around the fade, not a promise of a precise tick. Run the same event in the other direction -- a satellite flying out of shadow into sunlight, appearing from nothing -- and it's called an ignition, typical of morning passes for the same reason fades are typical of evening ones: whichever side of the planet is currently facing the Sun.
Is that normal, or did something go wrong?
Entirely normal, and common. Nothing about the satellite changed, broke, or reentered. It flew into the same shadow the Earth casts on every single orbit, on a totally ordinary schedule -- the only thing unusual is that you happened to be watching at the moment it crossed the boundary, mid-pass, instead of the boundary happening to fall before rise or after set, where you'd never notice it at all.
Will it come back?
Not on that pass. Once a satellite is fully in shadow it stays dark until it flies back into sunlight on a different, later part of its orbit -- typically well past the point where it's still above your horizon. If you want to see it again, that means waiting for its next pass over your location, not standing there expecting it to relight a few seconds later.
How is this different from it just setting?
Setting is a horizon event: the satellite drops below the physical line where the curve of the Earth blocks your line of sight, always happening at the edge of the sky, low down, gradually. A fade is not a horizon event at all -- the satellite can still be climbing, or sitting near the top of its arc, well clear of any horizon, and simply stop being lit. That's what makes it feel eerie the first time you see it: something that was clearly, unmistakably still up there just goes out.
Predicting the exact instant this happens is genuinely harder than the geometry alone suggests. Several modeling choices -- which Earth radius defines the shadow cone, which coordinate frame the Sun's position is computed in, whether atmospheric bending at the shadow's edge is accounted for -- each nudge the computed crossing by a handful of seconds. Stacked 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 countdown to a window, not a promise of an exact tick. The full physics writeup is here.
This is the entire reason SpacePings' notification design exists: not "it's up there," but "look now -- it's about to go out." A live countdown ticks down to the crossing, with a haptic buzz ten seconds before it happens, so the moment you just watched happen by surprise is, the next time, one you saw coming. Free on iOS -- no account needed to check tonight's sky.