Satellite Transit

A satellite transit happens when an observer, a satellite and the Sun or Moon line up, so that the satellite's silhouette crosses the bright disc. A satellite in low Earth orbit is only a few hundred kilometres away, while the Moon is about 384,000 km away and the Sun 150 million km, so the line-up holds only along a narrow band of ground called the transit corridor. Step outside that band and the satellite slips past the edge of the disc instead, a near miss called an appulse. The crossing is brief: the International Space Station takes about half a second to two seconds, depending on its elevation and on where in the corridor you stand. Its silhouette spans up to about one minute of arc against a disc of about 32 (see angular diameter), so a telescope or a long camera lens is needed to see its shape, and the position angle tells you where it enters and leaves the disc. Solar transits can happen whenever the Sun is up and must only be watched through a certified ISO 12312-2 solar filter; lunar transits need no filter and look best when the Moon is well lit. Unlike a satellite pass, which can be seen across a whole region, a transit is a precise geometric event: predictions give the time to a fraction of a second and the strip on a map, and they shift as the orbit is updated, so the final word comes the day before.