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The Moon goes round

Gravity and orbits · level 1, lesson 6 of 6

A question to start

The Earth pulls everything towards it: balls, spoons, raindrops. It pulls the Moon too. So why doesn't the Moon fall down and crash into us?

The idea

Tie a soft ball to a string and swing it round your head (outside, away from people). The string pulls the ball towards your hand all the time, but the ball does not come to your hand. It goes round and round. If you let go of the string, the ball flies off in a straight line.

moving sidewayspulled in

The Moon is like the ball, and gravity is like the string. The Moon is moving sideways very fast, about 1 km every second. The Earth's gravity pulls it in towards us all the time, so instead of flying off in a straight line, it bends round in a circle. It is falling towards us all the time, and all the time it is moving sideways so fast that it keeps missing.

That going round is called an orbit. The Moon takes about a month to go round once. The Earth orbits the Sun the same way, once a year. Mars has two small moons, Phobos and Deimos, and they orbit Mars. Phobos goes round so fast that it rises in the west and crosses the sky of Mars about twice a day.

Worked examples

1. Spinning a bucket. Swing a bucket of water round in a big circle, fast, and the water stays in, even upside down. The bucket keeps pulling it round. (Try it outside, with a little water.)

2. If the Moon stopped. If the Moon somehow stopped moving sideways, nothing would keep it from falling, and it would fall straight towards the Earth. Moving sideways is what keeps it up.

Try it

  1. What is the "string" that keeps the Moon going round the Earth?
  2. What would happen to a swinging ball if you let go of the string?
  3. What does the Earth go round, and how long does it take?
  4. Name the two moons of Mars.
Answers
  1. Gravity: the Earth's pull.
  2. It would fly off in a straight line (and then fall to the ground).
  3. The Sun, once a year.
  4. Phobos and Deimos.

Common mistake

"There is no gravity up where the Moon is." There is: that is what keeps it going round. If there were no gravity, the Moon would fly away in a straight line.

Where it leads