Space | 12. April 2012

See the ISS with your own eyes

Langzeitbelichtung eines ISS%2dÜberflugs
Langzeitbelichtung eines ISS-Überflugs

Live webcasts have become increasingly common, enabling any Internet user to follow spaceflight events such as launches, dockings or daily life on board the International Space Station (ISS). But there is a way to experience the wonder of spaceflight even more directly – with your own eyes. This is because the Space Station is visible at certain times as a small, bright point of light moving rapidly across the morning or evening sky. Many people are fascinated when they observe the ISS in this way, especially for the first time. In this blog entry I will explain how you can observe the ISS yourself.

The ISS flies at an average altitude of around 350 kilometres, in an orbit inclined at 51.6 degrees to the Equator, and circles the Earth roughly once every 90 minutes. As the Earth rotates beneath the ISS orbit, the Space Station constantly moves over different parts of the planet. When the ISS passes over a region where the Sun has recently set – but, at an altitude of 350 kilometres, the ISS itself has not yet entered Earth's shadow – the football-field-sized Space Station and its large solar panels reflect sunlight towards the Earth, appearing as a bright point of light in the evening sky to the naked eye. And of course, the same applies in the morning before sunrise. Depending on light pollution at the viewing site and the elevation of the pass, the Station is typically visible for about one to three minutes or, under ideal conditions, up to six minutes.

The ISS is periodically visible above Germany – firstly, in the morning sky almost every day for about two weeks; then for about two weeks in the evening sky. This sequence is then repeated approximately every two months. Of course, it is necessary for the sky to be largely free of clouds. On some days two passes are visible.

Finding out when the ISS is visible

Dedicated websites and smartphone apps can advise if and when the ISS is visible from your location. For example, on this page from NASA, you can choose your country and then select the city nearest to your location from a list. A table shows when the ISS will be flying over that location in the following days, which direction it is coming from, what its maximum elevation is (90 degrees means it is flying directly overhead) and in which direction it will disappear.

The data on Heavens Above. is somewhat more detailed. Here again, you first need to select your location. If you then click on 'ISS' in the 'Satellites' section, you will see a table containing five main columns. As well as the date and brightness ('mag') of each pass, three columns give the start of visibility (time, elevation above the horizon, compass direction), the highest point in the sky and the end of visibility (with time, elevation above the horizon and compass direction data again in each case). The lower the magnitude, the brighter and therefore more visible the pass is. Magnitudes of less than -3.0 should be easily visible from brightly lit cities. Similarly, the more directly overhead the pass is (that is, with its greatest elevation reaching 90 degrees), the brighter and more easily visible it will be. Assuming a clear field of view, passes with elevations of 50 degrees should still be visible from heavily light-polluted cities.

ISS visibility table, data for Cologne, Germany. Image: Heavens Above

In addition to these websites, there are a number of smartphone apps that can determine ISS visibility for you. Use of an integrated GPS finder is even simpler than the methods described above. As an example, one platform-independent solution runs as a smartphone browser app and outputs a table from Heavens Above.

Image: Long-exposure photograph of an ISS pass over the Brandeburg Gate in Berlin. Credit: Henning Krause (CC-BY-SA 3.0).

Fascination with spaceflight – observing the Space Station with the naked eye

Try it out for yourself – all you need to do is make your way to a south-facing balcony! The ISS always appears from the (south-) west. With a tripod and a little bit of practice, you will also be able to take long-exposure photographs and videos of ISS flybys (exposure time of one minute), to share with family and friends. Here's to clear skies!


 Video snapshot of ISS flying over


Top image: Long-exposure photograph of an ISS pass. Credit: Andreas Möller (public domain). Lower images: ISS pass ground track and visibility table from Heavens Above. Video: ISS overflight. Credit: Henning Krause (CC-BY-SA 3.0).