Earth's shadow is the cone of darkness the planet casts away from the Sun. Satellites crossing it go dark — the reason most passes happen near twilight — and spacecraft inside it run on batteries. The same umbra, reaching the Moon, is a lunar eclipse.
Understanding Earth's Shadow
The shadow in mission design
Eclipse seasons structure spacecraft life. Power engineers integrate worst-case umbra durations into battery depth-of-discharge budgets and array sizing; thermal design bounds the cold soak; solar observatories choose orbits minimising interruptions (a reason dawn-dusk SSO and L1 halo orbits are prized); and GEO operators calendar equinox eclipse seasons, when satellites cross the umbra nightly and — sharing geometry — ground antennas also stare into the Sun's radio glare on the other side of the year's symmetry.
Predicting entries and exits
Shadow crossings compute cleanly: model the umbra/penumbra cones from solar and terrestrial geometry, intersect with the propagated orbit, and timestamp entry and exit — standard output in operations tools and the "satellite enters shadow" markers on pass predictions. Refinements add atmospheric refraction (the shadow's edge is soft and slightly enlarged), matching the observed slow amber fade of a satellite rather than a switch-flick disappearance.