If the ISS orbits say 16 times a day, averaging 90 minutes, it only crosses the Greenwich Meridian (0° longitude) 15 times in a day, or averaging 96 minutes.

A kid asked me how often it comes over a day. Which is the right answer? He was presumably meaning coming over our country.

(Sometimes we can see consecutive passes, but of course you can't take timings from them, it depends when it is in sunlight.)

  • $\begingroup$ Is the question how many times ISS passes over a line of longitude, how many circuits of the Earth, how many times it passes of some part of your country (this will vary day to day), or how many times it passes over a point within a couple hundred miles of your house (also varies)? $\endgroup$ – Russell Borogove Oct 29 '16 at 10:50

With reference to what?

With respect to Earth-centered inertial rather than Earth-centered, Earth-fixed. Consider the extreme case of a satellite in a geostationary orbit. From the perspective of an Earth-centered inertial frame, such a satellite orbits the Earth once per sidereal day. From the perspective of an Earth-centered, Earth-fixed frame, the orbital period of a geostationary satellite is infinite.


The official ISS orbit count increments when the station crosses the equator (presumably in the southern direction, although I haven't turned up a specific reference to this detail). On May 16, 2016 this count reached 100,000.

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    $\begingroup$ Presumable in the northern direction. Crossing the equator from the north to the south marks the descending node, while crossing from the south to the north marks the ascending node. It is the latter (ascending node) that counts. This is of course completely arbitrary. $\endgroup$ – David Hammen Oct 29 '16 at 16:34
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    $\begingroup$ I assumed that the first complete orbit would have been marked by the 2nd equatorial crossing in the southern direction after launch from Kazakhstan - since the first crossing would have been in the southern direction after launch. But yeah, arbitrary. $\endgroup$ – Organic Marble Oct 29 '16 at 17:01

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