Predicting the ISS and other satellites

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 much 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.

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.