What is an ISS pass and how do I spot it?

The International Space Station is about the size of a football pitch, including the solar panels. It orbits at 400 kilometres up, going roughly seven and a half kilometres per second, which is fast enough to circle the planet every ninety minutes. When the geometry is right, it’s the brightest thing in the night sky after the Moon and Venus, and it’s not even close.

If you’ve never seen it, the experience is strange. You’re expecting a satellite, which most people imagine as a faint slow dot. Instead a brilliant white light comes up over your western horizon, climbs across the sky in a perfectly straight line for four or five minutes, and disappears into the east. No blinking, no sound, no trail. It looks artificial in a way that’s hard to put into words. The first time my partner saw one I had to convince her it wasn’t a low-flying plane with one bulb burnt out.

Why some passes work and others don’t

Three conditions have to line up before you can see the station. The orbit has to bring it over your part of the world, which happens often. The sky above you has to be dark, which restricts useful viewing to the hours around sunrise and sunset. And the station itself has to be in sunlight, high enough above the curve of the Earth that the Sun still hits its solar panels even though the ground below is dark.

That third condition is what makes the sweet spot so narrow. In the middle of the night, the station is in Earth’s shadow and reflects nothing. In broad daylight, the sky outshines it. The window that works is the ninety minutes or so after your sunset, and again before your sunrise, when the ground is dark but the station is still catching the Sun overhead. Within that window, depending on the orbit, you might get one good pass per evening or nothing at all.

Reading a pass

A “good” pass for naked-eye watching has three numbers worth checking. Maximum elevation, which is how high above the horizon the station climbs at its peak. Anything under twenty degrees and you’ll lose it behind trees and buildings, anything over thirty is comfortable, and the rare ninety-degree passes go directly overhead and look like the station is falling on you. Duration, which is how long it’s visible from horizon to horizon — usually four to six minutes for a high pass, less for a low one. And brightness, given as a magnitude where lower numbers are brighter; the station averages around magnitude minus three, which puts it brighter than Sirius, the brightest star.

SkyMinute filters the marginal passes for you. We only show ones that climb above twenty degrees and last long enough to actually find before they’re gone.

Doing it without an app

NASA runs Spot the Station at spotthestation.nasa.gov, which lets you enter your city and get the next few passes by email. It’s the official source. Set a reminder for two minutes before the pass starts, walk outside, give your eyes thirty seconds to adjust to the dark, and watch the part of the horizon the alert names as the start. You don’t need to scan, because once the station is up, you can’t miss it. It is by a wide margin the brightest thing moving across your sky.

What you’re actually looking at

When you wave at the ISS, the astronauts can’t see you. From four hundred kilometres up, your house is half a pixel. But the geometry of sunset works both ways. The reason the station is bright is that it’s still in daylight while you’re in the dark, which means the people on board are watching the Sun set just as you’re watching it appear in their windows. For those few minutes you’re sharing the same sunset from very different sides.

There has been a human on board the station continuously since November 2000. As of this year, that’s more than a quarter of a century of uninterrupted human presence in low Earth orbit. Whatever else has happened in the world over those twenty-five years, every night you’ve been outside, there have been seven people overhead.