Spot the ISS: It’s easy!

The International Space Station seen from Montreal. Automatic 5-second exposures created the dashed line effect.
Credit: Espace pour la vie (Marc Jobin)
La Station spatiale internationale vue depuis Montréal. Des expositions automatisées de cinq secondes ont créé l’effet de ligne pointillée.
Spot the ISS: It’s easy!

One hundred years ago powered flight was in its infancy: aircraft sightings were exciting and rare, but by the mid-twentieth century, commercial airliners were commonplace in the sky. Today, we’re in the space age: we hardly notice when a passenger jet flies by, but what about the International Space Station?

If you watch the night sky, odds are within 15 minutes you’ll see a satellite pass by: not surprising considering there are currently over 35,000 manmade objects orbiting Earth. Most of them are space debris, but when they are large enough and orbit low enough you can easily see them reflecting sunlight down to Earth.

Size matters

The International Space Station, or ISS, isn’t space debris: it’s by far the largest man made object in orbit, measuring 108 by 88 meters with nearly half a hectare of highly-reflective solar panels. It circles the Earth at 27,700 km/h, at an average altitude of 400 km, and orbits about once every 90 minutes. In fact, on favourable passes, the ISS can take over six minutes to cross the sky and appear as bright as Venus (magnitude -4.5).

Spotting the space station

Finding the ISS is easy: you don’t need a very dark sky, you just have to know when and where to look. Here’s a website that will guide you: Just select your preferred language in the box to the upper right, and enter your observing location under "configuration" at the top of the column on the left. Clicking "update" will generate a table of visible ISS passes. 

For example, here’s a line of data from the table of visible ISS passes over Montreal for October 6 to 15, 2016.

14 oct. -3,5 19:41:08 10°  WNW 19:44:25 85° NNE 19:45:03 55° ESE visible

Interpreting it is simple:

  • The first column gives the date of the pass and the second column gives the brightness magnitude (-3.5 is brighter than -2.5, and so on).
  • The next group of three columns tells when the ISS will first appear, at what elevation above the horizon and where;
  • The following three columns give the time and direction of the space station’s maximum elevation;
  • The next three columns tell when and where the ISS will disappear from view.
  • The last column indicates this is a visible pass, since the Sun will have already set.
  • Clicking on the date opens a sky map with the space station’s trajectory and further details.
  • Note that local times are corrected for standard or daylight time; just bear in mind that the space station’s orbit constantly changes, so predictions may be off by a few seconds, especially for more distant dates. It’s best to check the table on the day of your observation, and be ready a few minutes in advance.

Good luck… and have fun spotting the International Space Station!

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