I use this to track the ISS:


EDIT: This URL no longer works. It was a couple of maps that showed the position of the ISS. One close in and one zoomed out more. It was how I spotted the ISS as it went over the UK once. Yes - on occasion the UK does have clear skies!

What it doesn't show though is the diameter of the circle on Earth that can see it as it passes over head. For example as it passes over southern England how much of England will be able to see it?

  • $\begingroup$ If you got a smartphone you could try by searching for ISS at your applications store. ISS detector by RunaR works great. $\endgroup$
    – Alchimista
    Apr 11, 2018 at 15:36
  • 1
    $\begingroup$ On a scale of smart phones - dumb phones my phone is less intelligent than a brick. An exceptionally stupid brick, It was really cheap though. And I think I overpaid. $\endgroup$
    – Richard
    Apr 11, 2018 at 19:42
  • $\begingroup$ Not so important :) As far one has the curiosity of spotting the station NASA email service can even warn you in time. $\endgroup$
    – Alchimista
    Apr 11, 2018 at 19:57
  • $\begingroup$ There was at least one other clear night in the UK in recent history as well: What actually happened here with a satellite, the ISS and the moon? $\endgroup$
    – uhoh
    Oct 13, 2018 at 18:32

2 Answers 2


Take a look at https://en.wikipedia.org/wiki/Horizon

Basically you're asking about distance s: https://upload.wikimedia.org/wikipedia/commons/2/21/GeometricDistanceToHorizon.png

There is a formula to calculate it: There is a formula to calculate it

Inserting the numbers for the radius of the Earth and the height of the IIS orbit: 6378 * acos (6378 / (6378 + 408)) = 2223 [km]

I guess it will be an absolute upper bound because atmospheric conditions would limit it.

  • 1
    $\begingroup$ 2223km? So if the ISS passes over the English channel, the whole of the UK should be able to see it? On a clear night of course. I think the next clear night for the UK is scheduled for August 2026 - so fingers crossed I don't forget about it! $\endgroup$
    – Richard
    Mar 20, 2018 at 20:34
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    $\begingroup$ If by any random chance there is a clear night before that date, you can check when and where to look here (the official ISS tracker website for London): spotthestation.nasa.gov/sightings/… , for all other locations: spotthestation.nasa.gov/sightings/index.cfm $\endgroup$ Mar 20, 2018 at 20:41
  • 4
    $\begingroup$ Realistically, though, a more practical criterion for sighting ISS would be to specify that it be approximately 10 degrees above the horizon. In this case, you can apply the law of sines and the knowledge that three angles of a triangle sum to 180 degrees. Using the notation above, $\gamma = \pi - \frac{100\pi}{180} - \sin^{-1}\left(\frac{R}{R + h}\cdot\sin\frac{100\pi}{180}\right)$ and $s = R \cdot \gamma$, which gives $\gamma$ around 12.6 degrees, giving $s \approx 1360$ km. $\endgroup$
    – Tristan
    Mar 21, 2018 at 14:54

NASA provides a website with interactive maps that can help you determine viewing times:


My wife and I enjoy watching the station and we get emails and text alerts from this website.

Watch the International Space Station pass overhead from several thousand worldwide locations. It is the third brightest object in the sky and easy to spot if you know when to look up. Visible to the naked eye, it looks like a fast-moving plane only much higher and traveling thousands of miles an hour faster!


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