Twilight

Twilight is the time after sunset or before sunrise when the Sun is below the horizon for an observer on the ground but still illuminates objects high above — including satellites in low Earth orbit. This creates the ideal viewing window: the ground is dark enough for a satellite's reflected sunlight to stand out, while the satellite itself remains sunlit. Astronomers divide twilight into civil (Sun 0–6° below the horizon), nautical (6–12°) and astronomical (12–18°) phases. Most bright satellite passes, including the ISS, are seen during the 1–2 hours of twilight after sunset and before sunrise.