Facts and figures - invisible ISS
ISS orbits the Earth at an altitude of approximately 350 kilometers. It completes one orbit in just over 90 minutes. Its average speed is about 7.7 kilometers per second.
ISS doesn't fly exactly above Earth's equator. Nor does it fly over Earth's poles. Instead, the orbit has an inclination of just under 52 degrees. Twice per orbit, ISS crosses the equator.
No. Because of the Earth's own rotation, the ground track of ISS (the projection of its orbit on the surface of Earth) is constantly shifting eastward. In principle, ISS can fly over any point with a geographical latitude between 52 degrees north and 52 degrees south.
ISS can only be seen if it passes over your location just after sunset or just before sunrise. If a pass occurs in the middle of the day, ISS is invisible because the sky is too bright. If a pass occurs in the middle of the night, ISS is invisible because it is in the Earth's shadow.
The local time at which ISS passes above your location is slowly shifting. For a number of consecutive days, it will be visible after sunset. After some time, it will be visible before sunrise, again for a number of consecutive days. As a result, you will receive evening alerts and morning alerts from @twisst in bunches.
Depending on your geographical latitude, the season, and the precise orientation of the ISS orbit, it is possible to see more than one pass per day. In some occasions, only one pass will be visible; at other times as much as three favourable passes may be seen.
