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Tides

Gravity and orbits · level 3, lesson 6 of 6

A question to start

At the seaside, the water creeps up the beach for about six hours, then slides back down for six hours, then comes up again. Twice a day. What is pulling the sea?

The idea

The Moon's gravity pulls on the Earth. But gravity is stronger close up and weaker far away, and the Earth is big, so the Moon pulls the near side of the Earth harder than the middle, and the middle harder than the far side.

The sea on the near side is pulled towards the Moon a little more than the Earth is: it heaps up into a bulge. On the far side, the Earth is pulled away from the sea a little more than the sea is: it leaves a second bulge behind. So the oceans are stretched into a slightly long shape, with two bulges.

near bulgefar bulgeMoon

The Earth spins once a day underneath the two bulges. So a beach passes through a bulge (high tide), then a dip (low tide), then the other bulge (high tide again), then a dip: two high tides a day, about 12 hours 25 minutes apart. (The extra 25 minutes is because the Moon moves on a little in its orbit while the Earth spins.)

The Sun makes tides too, about half as big as the Moon's. When the Sun and Moon line up, at new and full Moon, their tides add and the tides are extra big (spring tides). When they pull at right angles, the tides are small (neap tides).

And Mars? Mars has no oceans, so it has no sea tides. But its moon Phobos is very close, and its pull stretches the rock of Mars by a tiny amount. That small stretch pulls back on Phobos and is slowly dragging it downwards. In tens of millions of years Phobos will be torn apart or crash into Mars.

Worked examples

1. Tide times. High tide is at 8:00 am. The next one is at about 8:25 pm, and the one after at about 8:50 am the next day.

2. Bigger tides at full Moon. Fishers and sailors know the biggest tides come around new and full Moon, when the Sun, Earth and Moon are in a line.

Try it

  1. Why are there two bulges, not one?
  2. About how long is it from one high tide to the next?
  3. What are spring tides, and when do they happen?
  4. Why does Mars have no sea tides?
Answers
  1. The Moon pulls the near side harder than the middle and the middle harder than the far side, so the Earth is stretched both ways.
  2. About 12 hours 25 minutes.
  3. Extra-big tides when the Sun and Moon line up, at new Moon and full Moon.
  4. It has no oceans today. (Phobos still stretches its rock a little.)

Common mistake

"The Moon only pulls the water." The Moon pulls everything: water, rock and you. Tides happen because it pulls different parts of the Earth by different amounts. Water shows it because water can flow.

Where it leads