To a first-order approximation, the ISS (as all satellites do), orbits in a fixed plane around the planet. As the orbital period is about 90 minutes, and observational conditions that allow viewing usually last longer than that, you might expect that if you could see it at one time, you would be able to in the future.
There are two major effects that change the relationship of the orbit with respect to twilight locations on the earth. The first is the orbit of the earth around the sun. If the plane of the orbit were fixed, this would change where the orbit encountered twilight with a yearly cycle.
The stronger effect though is precession. At the altitude of the ISS, the non-spherical mass of the earth causes the plane of rotation to move (about 5 degrees a day). So it takes a bit more than 2 months to precess all the way back around. Depending on your latitude, you should have two good periods of viewing during the full precession.
This means that for periods of weeks or so, the ISS will be passing overhead during unobservable times (middle of the day/middle of the night). As the orbit continues to precess, it will begin to pass overhead closer to twilight and you'll have opportunities for viewing based on the specifics of the orbit.
No comments:
Post a Comment