MEPX
Chapter 7 of 12All chapters
  1. 1The scale of things
  2. 2The Sun
  3. 3The rocky planets
  4. 4The giant planets
  5. 5Moons
  6. 6Asteroids and comets
  7. 7Gravity and orbits
  8. 8Light and distance
  9. 9Stars and their lives
  10. 10Galaxies
  11. 11The expanding universe
  12. 12Observing for yourself

Chapter 7 of 12

Gravity and orbits

Falling forever, sideways.

What holds a planet

Gravity pulls every mass towards every other, weakening with the square of the distance. A planet in orbit is falling towards the Sun and moving sideways fast enough to keep missing it.

  • Nothing pushes a planet along. There is no friction to slow it down.
  • Orbits are ellipses, so distance from the Sun changes through the year.

Weightlessness

Astronauts float not because gravity is absent but because they and their spacecraft are falling together. Gravity at the space station is still nearly 90 percent of what it is on the ground.