How to see the ISS from your back garden

A beginners guide to  observing the International Space Station


This image, taken by Dave Walker from Crawford, shows the ISS crossing in front of the Moon.

Credit: Dave Walker

Tonight, 16 March at 7.30pm, Channel 4 will broadcast live from the International Space Station as it completes one 90 minute lap around the Earth. But you don’t need a satellite link up to see the ISS.

When the station is overhead it’s one of the brightest objects in the sky, making it easy to spot. It’s only visible for a few minutes at a time though, so it’s important to find out when it will be visible in advance.

There are many apps – such as ISS Detector Satellite Tracker and ISS? for Android, andISS Spotter andISS Finder for iOS – that use your phone’s GPS to find your location, work out when the ISS will be visible and send you an alert.

Alternatively you can use the Heavens-Above website. Here’s how to find when the ISS will pass over your location. After finding your position on the interactive map, go to the ISS section of the site to find a listing of all the times the ISS will be visible over the next 10 days. As the ISS’s orbit shifts across the globe, sightings tend to come in batches with gaps in between. If there are no observation times coming up then skip ahead a few days to find when the station is next visible.

Heavens-Above produces a table with all the upcoming visible ISS passes in your area

Credit: Heavens-Above

The table will tell you the time the ISS will appear, its altitude and azimuth, and the magnitude it is expected to have. The start azimuth tells you where the ISS will appear from, so it’s best to face in this direction a few minutes before the station is due to appear. The altitude is the angle you should be looking up: the horizon is 0° while directly above ­– the zenith – is 90°.

Magnitude tells you how bright the station will appear. For reference, the brightest star in the sky, Sirius, has a magnitude of -1.4 while the faintest object the naked eye can see is mag. +8.0. The ISS usually has a negative magnitude, which means that one of the brightest objects in the sky, so there’s usually little doubt about what you’re seeing. It will also be moving quite fast, about the speed of an aeroplane.

The light from the ISS is reasonably steady, but in the evenings it sometimes appears a little dim at first and then gradually brightens, while in the morning the opposite is true. This is caused by the station moving into and out of the Earth’s shadow, as the light is caused from the Sun’s rays bouncing of its reflective surfaces rather than being produced by the station itself.

The best way to view the ISS is with the naked eye. As the station moves so fast, it passes quickly through a telescope’s narrow field of view and tracking is difficult. However, if you have a magnification of more than 20x, you might be able to make out the shape of the ISS. Some people have even used webcams to take images, but bringing out the detail of the ISS is a big challenge.


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