How do I know if I saw the ISS? Eight signs, and how to look up the pass

If it was one steady light that didn't blink, moved in a straight line, took a few minutes to cross and was brighter than almost every star, it was most likely the ISS. It may also have faded out partway across, or switched on partway up at dawn, as it crossed the edge of Earth's shadow. To be sure, look up the pass list for your place on Heavens-Above and match the time and direction you looked.

I ran one real pass through Orbit's engine and checked it against Heavens-Above. Over London on Tuesday 20 October 2026, the station lit up at 6:18 AM in the west-southwest, climbed to 63 degrees, about two-thirds of the way to straight overhead, and was low in the east by 6:24. At its highest it was about four times brighter than Jupiter, which was up in the southeast. My app and the website put the moment it lit up two seconds apart.

Did I just see the ISS? The eight signs

Tick off what you remember. The first seven make it likely, and the last one settles it.

How to check an ISS pass you already saw

The quickest check is Heavens-Above, a free website you can use without signing in. My links open its London pages, which show times in UTC, so add an hour for British Summer Time.

1. Choose "Change your observing location" and set your place.
2. Open the ISS. The pass list covers ten days: the date, the brightness, and the time, height and compass direction where each pass starts, peaks and ends.
3. Press the small left-arrow button beside the dates ("show previous search period") to step back ten days.
4. Match the time to the minute and the direction to the compass point.
5. If nothing matches, try its daily predictions for brighter satellites. It takes a past date, morning or evening, and lists every bright satellite that crossed.

Heavens-Above works out old passes from today's orbit data, which gets less exact the further back you go, so trust it for the last few nights, not last month. My own app, Orbit, can't help here: it only looks forward, to the next visible pass in the coming four days.

Here's how that London pass lined up in both:

Heavens-Above puts it at magnitude −3.4 at its highest; Orbit doesn't forecast brightness.

What does the ISS look like in the sky?

NASA compares it to a plane or a very bright star moving across the sky, except it has no flashing lights and doesn't change direction. It's also far quicker: about 28,000 km/h, roughly thirty times an airliner's speed.

It shines by reflected sunlight, like the Moon, and isn't bright enough to spot by day, so passes come near dawn and dusk (here's why). A high pass lasts 10 or 11 minutes from horizon to horizon, but you rarely get all of it. The 20 October pass was sunlit for only the last eight, about six of them more than 10 degrees up.

It usually travels from the western half of the sky to the eastern half. And sometimes it goes out: NASA puts short sightings down to the station "quickly moving into (or out of) the Earth's dark shadow". Over Sydney on Monday 26 October 2026, Heavens-Above lists it vanishing at 8:27 PM, still 22 degrees up in the east-southeast, and Orbit agrees to within five seconds (both worked out on 20 October; passes shift as new orbit data comes in, so check on the day).

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A closer look

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A deep blue dawn sky facing southeast with faint constellation lines. Upper left, a white point with a cyan trail pointing up and to the right. To its right, Mars and Jupiter are labelled, with Regulus, Procyon, Sirius and Adhara labelled across the sky, a faint glow low on the left, and E, SE and S marked along the dark hills.

Brightness goes by magnitude, which works like a golf score: lower is brighter, and five steps make a hundredfold difference. A high pass beats every star easily, and a low one about matches Sirius.

Was it the ISS or a plane?

Planes blink. US rules make airliners flash their anticollision lights 40 to 100 times a minute and carry a red light on the left, a green one on the right and a white one at the back. The station shines steadily, in silence, about 420 km up.

A plane flying straight at you can hang still and pass for a star, until, as Royal Museums Greenwich puts it, it seems to "veer sideways or upwards".

What was that line of lights in the sky?

A row of lights moving together is most likely a Starlink train, a fresh batch of SpaceX satellites still bunched on one track. Space.com says they show best in the days right after a launch, shortly after sunset or before sunrise, before they climb to their working orbits a few hundred kilometres up and spread out. Satellites fade as they enter Earth's shadow, and a train follows one track, so its lights go out one after another at about the same spot.

To check a train, untick "exclude Starlink passes" on Heavens-Above's daily list.

ISS or Tiangong? Can you see the Chinese space station?

Yes, and it's bright enough to pass for the ISS. Over New York from 22 to 29 October 2026, Heavens-Above shows eleven visible evening passes between 6:29 and 8:29 PM EDT, the best at magnitude −2.2. In those same days every visible ISS pass over New York falls in the morning, between 5:14 and 6:58 AM. So a bright, steady light gliding over New York on an evening that week wasn't the ISS, and was most likely Tiangong.

Its orbit is tilted 41.5 degrees against the ISS's 51.6, so from farther north it stays low: London's passes that week reach only 11 to 14 degrees, low in the south. Neither of my apps knows Tiangong is there. Both predict the ISS only, so for this one you're on Heavens-Above. How to see the ISS from India covers a Tiangong pass over Kerala.

What was that bright light in the sky last night, if not the ISS?

Go by how long it lasted. A meteor trail usually lasts less than a second and a fireball only a few seconds. A satellite takes a few minutes, and a light that keeps its place among the stars is a planet or a star. The Orionids peak on the night of 21 to 22 October 2026, and the Leonids follow in November.

Plenty of other satellites show to the naked eye, far fainter than the ISS. That same London morning, Heavens-Above listed 126 passes at magnitude 4.5 or brighter, Starlink left out. The runner-up to a space station was a spent Titan 4B rocket stage, a piece of old American rocket still going round, at +1.7, about a hundred times fainter.

A planet turns across the sky with the stars. Between the first London picture and the one below, a minute apart, the station drops well down towards the eastern hills, while Jupiter, Mars and the stars barely shift. (That pair is drifting towards its November meeting.) For which planet is which this season, see the bright star in the east before sunrise.

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The same sky one minute later. The white point with its cyan trail is now halfway down the left side, while Mars, Jupiter, Regulus, Procyon and Sirius sit where they were.

How to get an alert before the ISS flies over

All of that is detective work after the fact. A nudge beforehand is easier. On a phone, NASA's free Spot the Station app for iPhone, iPad and Android lists the passes still to come, and my free Orbit for Android warns you 12 minutes before a visible one.

On a Mac, Orbit: Live Sky Desktop, the app I make, does it too. Switch on ISS Passes under Alerts in the Moon's right-click menu, and five minutes before each pass Orbit lists you get a silent banner, "ISS rises in 5 minutes". Its alert for the 20 October pass reads "Southwest at 6:15 AM, climbing to 63°. In view for 11 minutes, very bright." The 11 minutes run from rise to set, including the three it spends in Earth's shadow. "Very bright" just means it climbs past 60 degrees. During the pass, if your desktop is in view, the station crosses it at the same moment it crosses the real sky. (One of these alerts once pulled me out of a work daze; that's in living with a real sky wallpaper.)

It has limits. Orbit plans alerts only while its desktop is on screen, so a Mac stuck behind a full-screen app for days runs out of them. It knows nothing about clouds. And Heavens-Above lists some short passes that Orbit leaves out: ones where the station is still in Earth's shadow at its highest point and lights up for only part of the way. Set-up and a week of pass times are in ISS pass alerts on a Mac.

Orbit: Live Sky Desktop is $6.99 once on the Mac App Store (₹699 in India), with no subscription and no account, for macOS 15 or later on Apple silicon and Intel. You get the real sky over your city as your desktop, tonight's Moon in the menu bar, the briefing card and the pass alert. There's no free trial, so the Mac page has screenshots to judge it by first. And if you're wondering how many years of passes are left, I wrote about whether the ISS is coming down.