The pass list gives direction and maximum elevation in degrees, and degrees are easier to measure than you might think. Hold your arm out: your fist covers about ten degrees, the span from thumb to little finger about twenty. The horizon is zero and straight up is ninety.
A pass peaking at ten degrees is therefore only one fist above the horizon and hides easily behind houses, trees and hills. At forty degrees and above it barely matters where you stand.
A satellite shines with a steady white light, never blinks and makes no sound. An aircraft blinks red and green. The clearest giveaway is that a satellite can go out mid-sky as it enters the Earth's shadow, which no aircraft does.
If instead you see a line of dots following each other like beads on a string, that is a recently launched Starlink train. They spread apart over a few weeks and are then no longer visible as a line.
The ISS orbit is inclined 51.6 degrees to the equator and stays fixed relative to the stars while the Earth turns beneath it. The ground track shifts west on every orbit, so passes over your particular town arrive in bunches: several good evenings in a row, then a couple of weeks with nothing, until the pattern drifts back.
On top of that, the station is slowed by the outermost atmosphere and sinks a couple of hundred metres a week. Periodically the orbit is raised again with engine burns, and every future pass time then jumps by a minute or so. That is why forecasts more than a few days ahead become approximate.
Orbits are computed in your browser using SGP4, the standard model for near-Earth objects, from orbital data refreshed daily. The model accounts for the Earth being flattened at the poles and for drag in the upper atmosphere.
Times within the next day are usually accurate to under a minute. Further out the error grows, because drag depends on solar activity and that cannot be predicted exactly. Direction is steadier than the clock time, so if you are outside early and looking the right way, you rarely miss a pass.
There is no shape to make out, just a point of light gliding along. The ISS is about a hundred metres across and four hundred kilometres away, equivalent to a tennis ball at five kilometres. What makes it so bright is the large solar arrays, which is why brightness varies between passes depending on how they happen to be angled towards the Sun.
The longer treatment is in the guide to seeing the space station from Sweden.
Written for Nollan.se. Knowledge base · About Nollan · Nollan.se