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Round planets

Gravity and orbits · level 1, lesson 4 of 6

A question to start

The Earth is round like a ball. Children in Australia live on the other side of it from children in Canada. Why don't they fall off?

The idea

Gravity does not pull towards "the bottom of the picture". It pulls every thing towards the middle of the planet. So wherever you stand, "down" means "towards the middle", and "up" means "away from the middle".

middle

A child in Australia feels the ground under their feet and the sky over their head, just like you. Their "down" is towards the middle of the Earth, and so is yours. Nobody is upside down, and nobody falls off.

That pull towards the middle is also why planets are round. Gravity pulls all the rock of a big planet towards its middle from every side, and the rock squashes into a ball. Mars is round for the same reason. Small things in space, like the little moons of Mars, Phobos and Deimos, do not pull hard enough to squash themselves round, so they are lumpy, like potatoes.

Worked examples

1. Dropping a ball in Australia. It falls towards the ground there, which is towards the middle of the Earth. Seen from space, it falls "up the page" if Australia is drawn at the bottom of the picture.

2. A tunnel through the Earth. If you could dig straight down far enough, you would be heading towards the middle. (No one has dug anywhere near it: the deepest hole ever drilled is about 12 km, and the middle is about 6,400 km down.)

Try it

  1. Where does "down" point on Mars?
  2. Why don't people on the other side of the Earth fall off?
  3. Why is Mars round, but its moon Phobos lumpy?
  4. Draw a round planet with three people standing on it, and an arrow showing "down" for each.
Answers
  1. Towards the middle of Mars.
  2. Gravity pulls them towards the middle of the Earth, so for them the ground is down too.
  3. Mars is big, and its gravity pulled its rock into a ball. Phobos is small, and its pull is too weak to do that.
  4. Each arrow should point from the person's feet towards the middle of the planet.

Common mistake

"Down is always the same direction in space." Down is only towards whatever is pulling you. Far out in space, away from planets, there is no strong pull and no real "down" at all.

Where it leads