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Lessons / Space around Earth

Space around Earth · 5 min · 4 steps

Why the ISS track moves west

Plays here

Goal. Explain what keeps a satellite in orbit and why its ground track shifts with each orbit.

Steps

  1. The ISS orbits about 400 km up at about 28,000 km/h. The solid line is where it has been, the dashed line where it is going.

  2. Predict

    After one full orbit of about 93 minutes, the ISS crosses the equator again, heading the same way. Where?

    • At the same place
    • About 2,600 km further west
    • About 2,600 km further east
    Show the answer

    About 2,600 km further west. About 2,600 km further west: in 93 minutes the Earth turns about 23° east beneath the orbit.

    Play speeds up time. Follow the station round one full orbit and watch where the new track falls.

  3. The orbit stays almost fixed in space while the Earth turns east beneath it at 15° an hour. So each new track crosses the equator about 2,600 km further west.

  4. Himawari-9 orbits 35,786 km up, once every 23 hours 56 minutes, exactly as fast as the Earth turns. So it stays over the same point above the equator and can watch East Asia's weather all the time.

The common mistake

Myth

Astronauts float because there is no gravity in space.

Fact. At the height of the ISS gravity is still about 90 % as strong as on the ground. The station and everyone in it are falling all the time; they move sideways so fast that they keep missing the Earth.

Check yourself

Question 1

Why does each new ISS track lie further west?

  • The ISS slows down
  • The Earth turns east beneath the orbit
  • The wind pushes it
Show the answer

The Earth turns east beneath the orbit. The orbit stays fixed in space while the Earth rotates 15° an hour to the east underneath it.

Question 2

How strong is gravity at the ISS compared with the ground?

  • Zero
  • About 90 %
  • About 10 %
Show the answer

About 90 %. Only 400 km up, gravity is still about 90 % of its surface strength. Astronauts float because they are in free fall.

Question 3

Why does a geostationary satellite stay above one spot?

  • It is not moving
  • It orbits once a day, as fast as the Earth turns
  • It hangs from a cable
Show the answer

It orbits once a day, as fast as the Earth turns. It moves at about 3 km per second, but completes one orbit in exactly the time the Earth takes to turn once.

Words in this lesson

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Curriculum

Taiwan 12-year curriculum (108 課綱), natural science
Kb-Ⅳ-1
US Next Generation Science Standards
MS-ESS1-2 · HS-ESS1-4
England national curriculum
KS3 Physics: gravity force, weight = mass × gravitational field strength