Sky Arc

A sky arc (or pass arc) is the apparent trajectory of a satellite pass plotted in the observer's sky: the sequence of elevation-azimuth points from rise, through culmination at peak elevation, to set. Prediction tools render it on a sky-dome diagram — the circular chart with zenith at centre and horizon at the rim — as an arc annotated with times, direction of travel and brightness, turning orbital mechanics into a where-to-look instruction. The arc's shape encodes the pass: a shallow chord skimming one side of the dome is a low pass (long ranges, faint, brief usefulness); an arc slicing near centre is the coveted overhead pass, brightest and longest. Because LEO satellites orbit faster than Earth turns, arcs generally run west-to-east variants — northwest to southeast, southwest to northeast — with polar-orbiting spacecraft drawing north-south chords; a satellite fading mid-arc has entered Earth's shadow, and the truncated arc is drawn accordingly. Radio operators read the same geometry as an antenna-steering profile — azimuth and elevation versus time — and this site's pass tools render sky arcs for any location, from tonight's ISS crossing to any tracked object.