My city is a bit to the north of ISS orbit's northmost point (inclination is 51°, my city is N55°). I observed ISS around 23:xx in summer at its heighest point over horizon, 45°.

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I supposed that the orbit northern point (lets call it ONP) is stable and with season it changes it's side: it may be on the lit side, half year later it's in shadow.

An observer like me sees ONP pass every day (Earth rotates "inside" the orbit), and with season it should be later every day. Apparently, in my winter it passes at noontime, I don't see it in the bright sky. In summer it passes around midnight.

At least this observation concords with my theory: it's a bit more than 1/4 of year till summer solstice, and it's visible 1/4 of day till midnight.

Am I right with the conclusion? Is ISS orbital plane stable as I describe?

This image shows the opposite picture. ONP is on the far side of this image, and with seasons Sun should move in counter-clockwise manner (in geocentric system). So, with season, the time I pass near this point should move to earlier, to daytime rather than to midnight.

enter image description here

Wikipedia illustration: same side (for example, a distant star) in inertial reference system should be seen earlier and earlier with seasons changing.

enter image description here

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    $\begingroup$ I'm not entirely sure what you're asking, but if you're asking about ISS' orbital node, then no, it is changing because of nodal precession. Mind that the station's altitude isn't constant, which also slightly changes its precession rate. If you're asking about the north-most boundary of its ground track, then yes, those are at nearly exact latitude to its orbital inclination, and those are stable (inclination of 51.65°). $\endgroup$ – TildalWave Feb 8 '16 at 22:18
  • $\begingroup$ I was asking about orbital node. In my case not ascending/descending node but the "middle" between them. I read about this precession, so this means the orbit rotates a bit together with Earth? $\endgroup$ – culebrón Feb 8 '16 at 22:27
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    $\begingroup$ Yes. See e.g. How often does the ISS orbit align with the day/night terminator? Mind that the ISS isn't in a sun-synchronous orbit, so what you observe is partially also due to its orbital period of roughly 92 minutes, making over 15 orbits per day. You would see it cross nearly the same path at similar solar time over your local horizon about once ever 2 months, but you might also see its pass in two consecutive orbits. See here for more info on ground observations. $\endgroup$ – TildalWave Feb 8 '16 at 22:34
  • $\begingroup$ @TidalWave yes, I'm aware of orbital mechanics and that ISS passes nearby at different sides. I'm more interested in a more theoretical question: at what time does one Earth meridian cross the ascending node. I supposed orbit plane were stable like distant stars and behave accordingly. If orbital plane precesses to the west, then I guess it should be earlier and earlier every day. (Opposite of what I thought.) $\endgroup$ – culebrón Feb 8 '16 at 22:45
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    $\begingroup$ Sorry, I didn't mean to suggest that you don't, I was just having problems understanding your question. I guess a similar plot than the solar beta angle one could be done for ISS' longitude of the ascending node / synodic period in STK. I currently don't have access to it, but I'm sure someone has. $\endgroup$ – TildalWave Feb 8 '16 at 23:06

No. The ISS has a cycle of when it rises and sets, that is about 2 months. The following plot shows an example of the beta cycle. The movement is the precession around the Earth. Note that the cycle time is about 2 months. That means that about every 2 months, the satellite will be overhead at the same time. Note that there is some variation with the day, but the overall trend is such that it repeats every 2 months.

enter image description here

From Heavens Above, I have found the 10 day orbital predictions for the ISS at 55N 0E, UTC. There are many passes per day, but the best prediction comes from the first pass in a cycle. The first is the day in February 2016.

  • 9- 16:45
  • 10- 15:53
  • 11- 16:35
  • 12- 15:43
  • 13- 14:51
  • 14- 15:34
  • 15- 14:41
  • 16- 13:49
  • 17- 12:58

Note that in 10 days, the satellite went from its first appearance at about 1700 to about 1300. That's about 1/6th of the day, which 10 days is about 1/6th of 2 months.


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