In examining the upcoming schedule for viewing the ISS, why does it seem like the AM/PM is repetitive. It will appear in the AM several days in a row, then it will switch to the PM several days in a row. What is it about the ISS orbit that causes AM/PM repetition? Why wouldn't the schedule alternate more often between AM and PM?

Here are some current examples. Notice the repetition of PM or AM based on the four locations. enter image description here


To be visible from the ground, a satellite (including the ISS) must satisfy several conditions:

  1. It must be above the horizon
  2. It must be illuminated by the sun
  3. The observer's sky must be sufficiently dark to not overwhelm the reflected light from the satellite.

Those three conditions basically mean that with the exception of a few extremely bright Iridium flares, all visual satellite observations must take place in fairly narrow windows before sunrise or after sunset (depending on the satellite's reflectiveness) such that the sun is visible to the satellite but not the observer.

The reason successive passes are clustered as all-morning or all-evening has to do with the inclination of the orbit, its orbital period, and the rate of precession of the orbital plane (which also depends on the inclination). The ISS orbits very nearly divides a 24-hour period, so the timing of overhead passes does not change very much from day to day. The orientation of its orbit does rotate (completing a full rotation over a span of about two months or so), however, so what you end up with is several days of morning passes (with occasional interruptions), followed by several days of no visible passes, followed by several days of evening passes, etc.

This is further complicated by the changing seasons altering how the terminator between illumination and shadow is laid out across the planet.

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  • $\begingroup$ Interesting, so the cycle repeats about every two months? Is there a general pattern? For example, two weeks of AM, then two weeks of PM, etc.? $\endgroup$ – James Lawruk Oct 16 '15 at 16:47
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    $\begingroup$ There's a two month cycle overlaid on top of a one year cycle (seasonal changes in sun angle), except that the orbital period doesn't quite evenly divide 24 hours, so there's about a ten minute skew per day as well. There's also a couple times a year when the angle of the orbit matches up almost perfectly with the terminator, meaning that ISS never goes into shadow. $\endgroup$ – Tristan Oct 16 '15 at 17:23
  • $\begingroup$ @Tristan: Sidereal days, I'm guessing? $\endgroup$ – Lightness Races in Orbit Oct 16 '15 at 19:05
  • $\begingroup$ @LightnessRacesinOrbit Not really -- the orbital period is strictly a function of the semimajor axis of the orbit (i.e., the arithmetic mean of the apoapsis and periapsis ) and the gravitational parameter of the body being orbited. That the ISS orbit comes close to evenly dividing a solar day is pretty much a pure coincidence. $\endgroup$ – Tristan Oct 16 '15 at 20:44
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    $\begingroup$ @Tristan: Okay. So it's a pure coincidence that the orbital period of GPS satellites is one sidereal day? $\endgroup$ – Lightness Races in Orbit Oct 16 '15 at 23:54

Satellites are easiest to see if the satellite is in light, and the sky is dark. This happens most often in evening/morning. In the middle of the night, the satellite is in darkness, and is difficult to see. In the day, the sky is too bright, and again, you can't see it. The satellite, being overhead, has a shorter time without seeing the sun than those of us on the Earth do, thus, it is the best time of day to see a satellite.

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  • $\begingroup$ Sure, but why wouldn't the schedule alternate between AM and PM. Why does the schedule contain a streak of AMs in a row, or PMs? $\endgroup$ – James Lawruk Oct 16 '15 at 16:39
  • $\begingroup$ Tristan's answer explains that, basically it has to do with orbital dynamics, a typical path will roughly repeat itself every day. $\endgroup$ – PearsonArtPhoto Oct 16 '15 at 16:46
  • $\begingroup$ To clarify what @PearsonArtPhoto is saying -- the conditions that I described occur with the ISS orbit, but that is not necessarily the case with other orbits. Different altitudes result in different orbital periods, and different inclinations (coupled to some extent with different altitudes) result in different precession rates. In many cases, a particular orbit may be selected for a satellite because of the characteristics of its ground passes (see e.g. sun-synchronous orbits). That is mission-dependent, however, and should not be taken as a rule for all satellites. $\endgroup$ – Tristan Oct 20 '15 at 15:49

It is due to orbital period time of earth and ISS. Earth orbits it self each 24 hour however ISS orbits around the earth each 1.5 hour approximately. It would be possible to see ISS once each 12 hour if earth and ISS were at same orbital period. However now coinsident of two phenomena is needed for visibility. This coinsident is is not in a second. It is in several hours.

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