Anonymous asked in Science & MathematicsAstronomy & Space · 10 years ago

How does viewing the ISS vary with season in London?

It has an orbit at 51 deg and I am in London at 51.5 deg. I get a lot of views with it passing directly overhead in the evening. If I chose a location in New Zealand I notice it comes over in the morning.

So how will seeing it change as the year goes on in London?

2 Answers

  • 10 years ago
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    From London, the ISS comes over about 4 times every day. Whether you can see it or not depends on the time of day. If it comes over in the middle of the day you can't see it because it's broad daylight. When it comes over in the middle of the night, you can't usually see it (except in midsummer) because it's in the Earth's shadow and unlit.

    What you need is for the sky to be dark and the satellite to be sunlit. This happens when it comes over after dusk and before dawn.

    It tends to go in a two-month cycle. Two weeks of visibility every evening followed by two weeks of invisibility as it comes over in the day. Then another two weeks of visibility in the early morning followed by two weeks of invisibility as it comes over in the middle of the night. It then returns to evening visibility again.

    As I mentioned before, in midsummer the morning and evening visibility periods merge and it can be seen at any time of night.

    When you see it in the evening, an observer in New Zealand would see it in the morning and vice versa.

    You can get a good idea of what happens by going to and selecting the 10 day predictions for ISS. If you click on 'Next', you'll get the next 10 days and you can continue doing this until you can see the pattern of visibility.

  • The ISS orbits the Earth every 90 minutes or so. Each time it does, it passes further to the west than it did on the previous orbit, as the planet turns underneath it. It orbits in a north-south or south- north direction, not at a particular latitude.

    When you see the ISS, you see it because of the reflected sunlight. Because it's a couple hundred kilometres up, this means that you can see it for a couple of hours after sunset, and a couple of hours before sunrise. You can't see it at other times of night because it's in the shadow of Earth, and you can't see it during the day, of course.

    So, the satellite passes within your view fairly often, but it's only during those brief windows in the evening and early morning that you can see it. There will be times (usually several days in a row) when it happens to pass in your view during that evening time, and other times when it just misses being in your view at that time. The same for the morning viewing times.

    It doesn't depend on the season. Are you using an iPhone app to get the viewing times?

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