Facts and figures - spotting ISS

ISS is big. Its huge solar arrays measure some 110 by 73 meters. But it's also far away. In the sky, it just looks like a bright star, slowly moving against the background of real stars.

ISS circles the Earth at a speed of some 8 kilometers per second. In the sky, it appears to move about as fast as an aeroplane. A single ISS pass usually lasts a few minutes.

ISS doesn't emit its own light. It's visible because it is illuminated by the sun. What you see is in fact reflected sunlight. Incidentally, the same is true for the moon, the planets and all other Earth-orbiting satellites!

Quite often, ISS may pass over your observing location during the daytime. However, because of the bright daylight, you won't be able to see it. That's why visible passes only occur during twilight.

ISS circles the Earth at an altitude of a few hundred kilometers. After sunset, when it's already dark at your observing location, ISS still captures sunlight for a while. Likewise, it can already be illuminated by the sun before sunrise, when it's still dark outside.

During an evening pass, ISS rises above the horizon more or less in the west. It then climbs higher in the sky, moving eastward, until it disappears in the Earth's shadow. During a morning pass, ISS emerges from the Earth's shadow, then moves eastward and lower in the sky, until it sets more or less in the east.

You certainly have to go outside to see ISS. Usually, it can be easily seen from big cities, despite the light pollution, but the view is much more impressive from darker, rural areas. However, in general your garden or balcony will suffice.

When the altitude of ISS is lower than 10 degrees, it is usually difficult to observe. Twisst generally doesn't mention any ISS passes lower than 15 degrees.

 

Because it's so big, ISS is quite bright compared to normal stars. During a favorable pass, it is usually the brightest object in the sky (apart from the moon and some bright planets). You can't really miss it!

 

ISS passes are considered more favorable when ISS is brighter, higher in the sky, and visible for a longer time. Less favorable passes have a shorter duration, while ISS is lower in the sky and less bright.
 

Using a pair of binoculars, you will clearly notice that ISS is not really a point of light. Instead, it may look elongated or squarish. You won't be able to see much detail, though. Moreover, it may be hard to get and keep ISS in the field of view.

This might be a good idea. When you're traveling to another part of the world, you may be able to see ISS when it is invisible from your hometown.
Or, if you're traveling shorter distances, ISS passes may occur on different times. However, in general, you do not need to change anything when you stay within about 100 kilometers from your default location.

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