Apparent Magnitude

Apparent magnitude measures how bright an object appears from Earth on the ancient-but-formalised logarithmic scale: five magnitudes equal exactly 100× in brightness, so one magnitude is ×2.512, and the zero-point ties to standard stars. Brighter is lower — Sirius −1.5, Venus around −4, the full Moon −12.7 — while +6 marks the naked-eye limit under dark skies (urban skies manage +3 to +4). Satellite watching runs on it: the ISS ranges from −2 to −4 on favourable passes — often the brightest thing in the sky — Tiangong reaches −2, the old Iridium flares hit an astonishing −8, and everyday LEO objects ride between +2 and +7 depending on size, elevation and Sun angle. A satellite's magnitude is dynamic: it brightens toward pass peak (range shrinking), varies with phase angle exactly like a tiny moon, flares when flat panels catch the Sun, and tumbling rocket bodies pulse rhythmically — a diagnostic observers use. Predictions fold size, distance and geometry into per-pass magnitude estimates, and the same numbers feed the astronomy community's concern with constellation brightness, where operators now design and operate against magnitude targets (fainter than +7) to protect the night sky.