These tweets about how sometimes the sun doesn't set on the International Space Station for multiple days made me realize I still have trouble visualizing solar beta angles.

Does anyone have a projection of the ISS ground track, with the shadowed/unshadowed bits of Earth, during a high beta angle time? Or perhaps an animation?

The tweets in question:

@ISSarchaeology at 3:16 PM CDT - 20 May 2018

ISS Archaeology Retweeted Ed Van Cise

Another interesting similarity between life on a space station and life in Antarctica (often used as an analog for reasons of isolation and confinement): sometimes the sun doesn't set.


Ed Van Cise (@Carbon_Flight) at 3:13 PM - 20 May 2018

A couple of times a year our orbit line precesses to align with the solar terminator. When that happens, the sun appears off our port or starboard side and never sets. This lasts for 3-4 days and puts significant thermal stress on the spacecraft (constant heat or constant cold).

  • $\begingroup$ This is a great question! It's not the ground track alone that will give insight, it's the addition of the day/night to the world map that makes it clear. Or a 3D representation. $\endgroup$
    – uhoh
    May 21 '18 at 4:57

I haven't found a way to make Heavens-above draw satellite orbit images for other time periods. (It will easily draw start chart pass information for them)

But APEX right now is in an orbit with a very high beta angle. So the images for its orbit should be useful.

https://www.heavens-above.com/orbit.aspx?satid=23191&lat=0&lng=0&loc=Unspecified&alt=0&tz=UCT enter image description here

The "View from above satellite" is a nice image. If we imagine that the view is from some observer on a distant star, the observer would see:

  • The earth turns beneath the track about once a day
  • The terminator rotates around the globe once a year
  • The orbit precesses around the globe about six times a year

The ISS precession is around the earth's N-S axis, while the sun/terminator move along the ecliptic. So the two don't line up exactly every period. With the ISS inclination to the earth's axis at a constant 51.6 degrees and the sun's maximum inclination to the axis of 23.4 degrees, the maximum beta angle between the sun and the ISS is 75 degrees. At the current orbit altitude, that's just enough for it to keep a glimpse of the sun.

Note that the ground track is a bit of an odd image. The terminator is drawn at a single point in time, while all the bits of the ground track away from the satellite are draw where it will be at a different point in time. So the track diverges from the terminator in this rendering.

  • $\begingroup$ Thanks for emphasizing the time discrepancy with the terminator and the ground track. That's vital. $\endgroup$
    – Erin Anne
    May 21 '18 at 7:03

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