I recently got an app that let me track the ISS. I noticed that during the time it's visible, it disappears before reaching the horizon, and sometimes reappears soon after for a bit.

Can somebody explain to me how the ISS and other satellites orbit? Specifically, what makes their visible period such a narrow one, that doesn't span all the way down to the horizon? (which is what my layman brain expected)

  • 6
    $\begingroup$ Because they've reached the edge wall. $\endgroup$
    – Valorum
    Dec 27, 2018 at 17:48
  • $\begingroup$ @Valorum this should be the accepted answer $\endgroup$
    – Mav
    Dec 28, 2018 at 8:26
  • $\begingroup$ @Valorum good timing? $\endgroup$
    – ave
    Dec 28, 2018 at 8:39

1 Answer 1


Because satellites are only visible when they are in sunlight, they are not visible when they go into the Earth's shadow. The app most likely predicts where this occurs and ends the arc.

In other words, it does not make sense for an observer to look for a satellite when it is not visible, so there is no need to draw the path when it is in the shadow.

  • 6
    $\begingroup$ So is it basically like a lunar eclipse, except because the satellite is much smaller, it happens much more frequently? $\endgroup$
    – Mav
    Dec 26, 2018 at 18:55
  • 17
    $\begingroup$ More like because it orbits so much closer to the Earth than Moon, there is an "eclipse" (I'm not sure how to call this event) during every orbit. $\endgroup$
    – NikoNyrh
    Dec 26, 2018 at 20:56
  • 26
    $\begingroup$ @NikoNyrh I like to call it "night" - it's fair because the ISS, for instance, has more in common with a airliner than the moon when it comes to this $\endgroup$
    – wedstrom
    Dec 26, 2018 at 22:02

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