Transit Corridor

Extend the straight line from the centre of the Sun or the Moon through a satellite until it meets the ground: that point sees the satellite exactly on the centre of the disc. As the satellite moves, the point traces the centreline of the transit corridor. Around it lies the band from which the satellite appears anywhere on the disc. Its width follows from simple geometry: the satellite's distance multiplied by the disc's angular radius plus the satellite's own (see angular diameter), stretched where the line of sight meets the ground at a slant. A high satellite is close, so the strip is narrow: the ISS overhead gives a strip about 4 km wide. A low satellite is far away and the line of sight grazes the ground, so at 15 to 20° of elevation the strip widens to 20 km or more. Within the strip the crossing is longest on the centreline and shortens towards the edges, where the satellite clips the edge of the disc; just outside it the satellite misses the disc, an appulse. The corridor sweeps along at several kilometres per second, so the time changes by about a second for every few kilometres along it, while moving across it changes what you see but not when.