Passes
When to see the ISS
Every pass of the International Space Station over your sky for the next five days — the ones you can see, and why you can’t see the others.
Why you can’t always see it
The station makes no light of its own: what you see is sunlight bouncing off its solar wings and radiators. So three things have to hold at once. The station has to be in sunlight, your sky has to be dark — the Sun at least six degrees below your horizon, the end of civil twilight — and it has to be high enough to clear the trees and the haze, which here means ten degrees.
That is why the good passes come in the hour or two after dusk and before dawn. In the middle of the night the station is usually in the Earth’s shadow, overhead but dark. In daytime it is lit, but so is the sky. And for weeks at a time, depending on how the orbit sits against the Sun, a city gets no visible pass at all. The list says which of these rules each pass out.
One more rule is the page’s own: a pass has to be visible for at least a minute. Near the edge of the Earth’s shadow the three conditions can hold for a few seconds at ten degrees, and a faint point that vanishes before you have found it is not worth going outside for. The list calls those “seen for under a minute”.
How bright is “very bright”
At its best the station outshines every star and every planet but Venus. The estimate here starts from its standard magnitude of −1.8 — its brightness at 1,000 km with half of it lit, the figure most often quoted — and corrects for how far it is from you and the angle between you, the station and the Sun, treating it as a matt sphere.
It is not a sphere. It is a cross of flat panels whose attitude decides how much light comes your way, and its real brightness can differ from the estimate by a whole magnitude either way. So the list says it in words — very bright (brighter than −2.5), bright (brighter than −1), visible but faint — and keeps the number for whoever hovers over the word.
Your location stays here
Nothing is sent anywhere. The passes are computed in this page, from the station’s orbital elements and the position you give it. If you use your location, it is rounded to a tenth of a degree — about ten kilometres, which moves a pass by seconds — before anything uses it, and it is never put in a link. A link you share names a city, not a place.
The city list is fetched a letter at a time, only the letter you type. The page remembers your choice in your own browser, and “change” forgets it.
Why five days
The station’s orbit decays a little every day and is raised by engine burns that are not announced in advance. The orbital elements the page uses are usually less than a day old, and five days ahead their prediction is still good to well under a minute — unless a reboost happens in between, which can move a pass by several minutes. The age of the elements is shown under the list. For a pass you care about, check again on the day.
Sources
Orbital elements from Celestrak, NORAD object 25544, propagated with SGP4 — the same computation as the live console. City names and time zones from GeoNames, under CC BY 4.0. The times are checked against an independent implementation, Skyfield, to within ten seconds; how it works says more.