Predicting the ISS and other satellites (including Starlink)

You can view the International Space Station (ISS) when it passes over your location very easily – in fact, it can be one of the brightest objects in the night sky other than the Moon. It moves faster through the sky than most planes, and usually appears brighter as well. Because the ISS is in an orbit of inclination 51° to the equator, it passes directly over southern England.

Being comparatively close to Earth (about 200 miles up), its exact location in the sky will vary depending on your location. From the north of England and Scotland, it will appear lower in the sky. In order to spot the ISS you will need to get predictions for your own location, though a few miles either way won’t make a significant difference. Follow the steps below to get your own predictions online.

These instructions also work for predicting other satellites mentioned on Heavens Above, notably Starlinks. See the note at the end about these.

How bright?

The ISS and all other satellites shine only by reflected sunlight, and not because they carry lights (unlike planes). So they are only visible from Earth when they are still in sunlight, although the Sun has set at the surface of the Earth. As a result, they are only visible during certain times. In winter, most satellites are only visible during a short period after sunset and before sunrise, but in summer they are visible for a longer time during the hours of darkness.

The ISS is usually one of the brightest objects in the sky, and is easily visible, at up to –4 on the astronomical magnitude scale, where typical stars are magnitude 0 and faint stars magnitude 5 or 6.  Most other satellites are smaller and therefore appear less bright, although you may well see some at around magnitude 2, about the same as the stars of the Plough (Big Dipper).

A satellite is likely to pass into Earth’s shadow as it nears the eastern horizon (for evening passes). Most satellites travel from the west to the east or in a polar orbit, from north to south or south to north. Anything moving from east to west is not a satellite!

Satellites may also vary in brightness if they are spinning, with a sudden flare or dip. Occasionally one may flare from invisibility to being easily visible in a few seconds, then just as quickly fade again. They do this much more slowly than typical meteors (shooting stars) although some very slow meteors or fireballs can be similar. Until 2019 the first generation of the Iridium series of satellites produced bright and regularly predictable flares, but these have now all been removed from orbit (by crashing them into the ocean).

Geostationary satellites, used particular for meteorology or communications, are generally too faint to be seen, although they regularly appear on amateur astronomers’ photographs of the night sky around the region of the celestial equator as faint trails on guided exposures at about 15th magnitude.

Step 1
Go to the website (not operated by the SPA). Choose your location from their list of cities. Just choose your nearest one, unless you want to enter your precise latitude and longitude manually. You can register online (free of charge) to allow you to save your location for future use.

Step 2
Once you’ve chosen your location, at the Heavens-Above home page, for ISS timings click on ’10-day predictions for ISS’. This takes you to a list of predictions like the one below. Other satellites are also available.

Date Mag Starts Max.altitude Ends
Time Alt. Az. Time Alt. Az. Time Alt. Az.
24 Jun -1.4 00:21:33 10 WSW 00:22:32 14 SW 00:22:32 14 SW
24 Jun -2.7 23:07:46 10 W 23:10:22 31 SSW 23:11:45 20 SSE
25 Jun -3.5 21:54:22 10 W 21:57:13 58 SSW 22:00:03 10 ESE
25 Jun -1.6 23:29:59 10 WSW 23:31:49 16 SW 23:32:15 15 SSW
26 Jun -2.6 22:16:09 10 W 22:18:46 32 SSW 22:21:23 10 SE
27 Jun -1.6 22:38:12 10 WSW 22:40:12 17 SW 22:41:58 11 S
29 Jun -1.5 21:46:15 10 W 21:48:16 17 SW 21:50:17 10 S

For each date, the table lists the start, maximum altitude and end of the appearance. Each one is given as a time, altitude and azimuth. Altitude means height above the horizon in degrees, where 0 deg is the horizon and 90 deg is overhead. Azimuth is given in compass direction. But ISS is so bright that there is usually little doubt that you are seeing it. The magnitude column specifies the brightness compared with stars. Magnitude 0 stars are bright, magnitude 4 stars are faint. Usually, ISS has a negative magnitude, meaning that it is one of the brightest objects in the night sky apart from the Moon.

Clicking on the date (when on the Heavens-Above site) will give you an all-sky star map. Remember that ISS always moves from the western horizon towards the eastern horizon, as seen from the UK. And because you’re looking up at the sky rather down at the ground, as with a conventional map, east and west are the other way round from what you’d expect!

If there are no passes predicted, you can get predictions for subsequent weeks by going to the ‘Next’ link at the top right of the page. Or go back to the home page to check for Iridium flares – brief but bright flares from the Iridium series of satellites.

Step 3
From your observing site, look in the direction specified and you should spot ISS easily, moving noticeably, about as fast as a fairly close aircraft but of course without the additional flashing wingtip lights! As a guide, 20 degrees altitude is represented approximately by your outstretched hand at arm’s length. 45 deg is halfway up the sky, and 80 deg is almost overhead. ISS may fade out as it progresses towards the east, as it enters Earth’s shadow.

Track of ISS

A typical path of the ISS, rising in the west and fading out as it enters Earth’s shadow in the east. The precise track will vary from night to night.

Using your camera
As an alternative to viewing, try taking a time exposure of the region using a digital SLR camera on a tripod with an exposure time of a few seconds and a speed setting of ISO 100. The exact speed setting and exposure will depend on how dark your skies are at the time. Late in the evening you may be able to give a 10-second exposure at ISO 400.

Photo of ISS
Martin Lewis’s snap of the ISS as it passed over St Albans, Herts

Can I use my binoculars or telescope?
You can view the ISS through ordinary binoculars, but it will probably only look like a brilliant, moving point of light. But when it’s at its closest to you (right overhead) the ISS appears about as large as Jupiter does in the sky, so in theory if you have a magnification, on either binoculars or a telescope, of more than about 20 you should be able to see some details of the station, particularly its solar arrays. It is possible to use a telescope to view the ISS, but as it moves quite quickly through the sky the problem is keeping it in the field of view. You will need to have a freely moving mounted telescope, and you will probably only catch fleeting glimpses of it because of the difficulty of tracking it.

Some amateur astronomers have actually photographed the ISS in detail, using large telescopes equipped with webcams to record a video sequence. Here’s a news story about how SPA member Martin Lewis snapped the Space Station from St Albans.

Starlink satellites

These are launched regularly in streams. Depending on the circumstances of the launch, these streams may be visible within a few nights of launch. However, we have found that the brightness predictions are not always accurate and sometimes what should be easily visible according to the expected brightness is not visible at all. And the greatest brightness may not be occur until the satellite is some way through its track, particularly during evening passes, when each satellite is backlit by the Sun during the early parts of its pass, so only a small area of it shines. Remember that the satellites only shine by reflected sunlight.

When you look at the predictions, you can choose which particular launch you want predictions for. Satellites from several separate launches may be visible over the course of one night. The most recent launches are most likely to have satellites closely separated in a train so that many appear in the sky at once clearly following each other. In these cases, the individual appearance times will be separated by only a few seconds.

What does ‘placeholder’ mean?

In the case of Starlink satellites, the page you arrive at on Heavens Above may refer to a ‘placeholder’. This term is used shortly before a launch is due, and uses the expected launch details to give predictions of where the satellites may appear shortly after launch. If no predictions are shown, it means that the satellites from this launch are not expected to be visible from your chosen location. However, you can use the downward arrow in the Launch box to get predictions of Starlink satellites from previous launches, or use the forward arrow to the right of the search period details to look for predictions on subsequent nights.