Earth's Shadow

Earth's shadow is the long cone of darkness the planet casts into space directly opposite the Sun. A satellite is only visible from the ground when it reflects sunlight, so the moment it crosses into Earth's shadow it stops being illuminated and fades out — sometimes within seconds — even though the sky is clear and the satellite is still above the horizon. This shadow entry (and the reverse, shadow exit) is why a bright pass can vanish in mid-sky, and why satellites in low Earth orbit are only visible during twilight: in the middle of the night the entire LEO region sits inside Earth's shadow. Higher orbits spend less time in shadow and can stay lit longer.