ISS

[peg]

Hi, I'm just an elementary teacher so please excuse this question,but, last night at our monthly meeting a friend said the ISS was pass over so we all went outside to watch. It was cool, but then someone said that next month we would have to move the meeting to see it again. Why? The ISS doesn't follow the same orbit path? Will I be able to see it again tonight? Thanks-

[Chris Peterson]

Hi, I'm just an elementary teacher so please excuse this question,but, last night at our monthly meeting a friend said the ISS was pass over so we all went outside to watch. It was cool, but then someone said that next month we would have to move the meeting to see it again. Why? The ISS doesn't follow the same orbit path? Will I be able to see it again tonight? Thanks- :D

The ISS is in an orbit with an inclination of 52°. Start by imagining it orbiting over the equator, and then tip that orbit 52°, and you'll see what that looks like. At its furthest north the ISS reaches latitude N 52°, then a quarter of the way around its orbit it will cross the equator going south, halfway around its orbit it will be at S 52°, and three quarters of the way around it will cross the equator again, this time going north. If you looked at this orbit on a flat map, it would look very like a sine wave- you've probably seen such orbits mapped out on the screens at NASA or JPL during televised launches.

The ISS is about 200 miles above the Earth, which is very low. At that height, it orbits about once every 90 minutes. So in one complete orbit, the Earth has rotated beneath it by about 1/16 its circumference (23°). That means that from orbit to orbit, the ISS finds itself over a different part of the Earth- a different longitude for any given latitude. Thinking about the flat map again, that sine wave describing the ground path doesn't have an even number of cycles from one side to the other, so when it goes off one side and picks up on the other, it isn't lined up with the previous track.

This drifting orbit pattern creates repeated cycles of ISS visibility from any given location. You get a few days with evening passes (usually a day or two also where you have two passes), then a few days of morning passes, then a few days with no passes at all. Then that cycle repeats. You might like the Heavens-Above website, which always shows where the ISS is located, and allows you to enter your location and find out about upcoming passes.

[peg]

Thanks Chris, that was a great explanation. I get it now. Thanks for the website too, Im sure my kids would love to keep up with the ISS.

[hankpac]

On the same site, "Heavens Above" they also have the schedules for "Iridium Flares".
These are reflections from the solar panels on the Iridium comm satellites.
Great fun.