Appulse

Appulse is an old astronomical word for the closest apparent approach of two objects in the sky, such as the Moon passing near a planet or a star, when they come close but do not overlap. When one does pass in front of the other it becomes a transit or an occultation instead. For satellites the idea maps neatly onto the transit corridor: inside the strip a satellite transit happens, and just outside it the satellite passes beside the disc of the Sun or the Moon, its minimum separation growing steadily with your distance from the edge of the strip. Predictions express the miss as an angle, usually in minutes of arc from the edge of the disc. A near miss is not a failure: a satellite skimming the limb of the Moon, or passing just outside a bright solar limb with prominences, can make a striking photograph, and it still demands the same timing to the second. Do not confuse an appulse with a conjunction in orbital safety, which is a close approach between two objects in space measured in kilometres and timed by the time of closest approach; an appulse is purely a line-of-sight effect seen from one place on the ground.