# Why has the ISS changed the direction it is orbiting?

A few weeks ago it rose in the South and traveled to the North. Now I hear that this Saturday it is rising in the North and traveling South. How is this possible?

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• GeoffG
Lv 7

The ISS, like all satellites, travels above a great circle on the Earth's surface. Because of the Earth's rotation beneath this great circle, the orbit sometimes is carrying the ISS north to south and at other times is carrying it south to north, at the particular time the observer's location passes beneath the orbit. The ISS is always travelling in the same direction, but the Earth's rotation carries the observer under different parts of its orbit at different times.

 The Heavens-Above web page always shows the current position of the ISS, which may make this clearer for you.

http://www.heavens-above.com/

• Anonymous

The answer is very simple when you realize that the ISS does not orbit parallel to the equator. It orbits at a 051.6464 degree inclination to the equator. So if it starts its orbit rising north east, as it crests over the northern part of the planet it travels south east...even though it's still traveling in a circle! ref this diagram http://www.asc-csa.gc.ca/eng/educators/resources/o... to better picture what's going on.

Because our planet is rotating, sometimes you are on the side of the planet where the orbit is traveling north east, and other times you are on the side of the planet where you see the ISS traveling south east!

Here's a simple way to picture it..if you decide to walk completely around the earth and start by going directly north, eventually you cross the north pole at which point you are now walking south! (even though you never changed the 'direction' you are walking! Then you cross the south pole and are traveling north again!

• Mike
Lv 7

Remember two things - what goes North must go South - - and the Earth rotates.

If the ISS goes above the North Pole, it must come back ''down'' over the south Pole. (It makes an orbit, right?)

Now... while it is *doing* that, the Earth rotates.

So -today- you may see the ISS come from the South and travel to the North. But some days from now, thanks to the Earth rotating, and the ISS continuing to orbit (several orbits later), you may see the ISS start from the North and head South.

In a period of a few hours, our astronomy club once viewed two passages of the ISS - one to our East, the other to our far West. (Both were South to North that night). The Earth had rotated during those hours, and the ISS orbited. Someone placed properly (in China, perhaps) would have seen two passages as well - but both would have been North to South.

• Anonymous

The plane of the ISS orbit is fixed relative to the stars. The axis of Earth's rotation is also fixed relative to the stars. The sunny side of Earth always faces the sun, while the Earth orbits the sun; so the plane of the boundary between day and night rotates 360° per year. We only see ISS when it is in daylight and we are in darkness. As the seasons change the angle between the ISS orbital plane and the plane of sunrise and sunset changes. So we see it at different times, depending on the time of year.

Also, we sometimes see it after sunset moving toward the north and again before dawn moving toward the south. In winter, it is more common to see it moving west to east.

• Anonymous

The ISS orbits the Earth west to east (like most satellites). With an inclination of 50 degrees. I know of no changes to this.

You can look at the map on the chart below. It's N/S declination changes (the -s- curve you see) is because it is orbiting at that inclination, and is not a direction change.