The Moon pulling
Why Does the Sea Go In and Out?
The Moon's pull raises a bulge of ocean, and Earth turns through it twice a day.
-
Step 1 of 5
The Moon pulls on everything on Earth.
Gravity reaches across 384,000 kilometres. It pulls the rock, the air and the water — but water is the only part that can visibly move.
-
Step 2 of 5
That makes a bulge of water facing the Moon.
Ocean on the Moon's side is pulled slightly towards it and heaps up. Where the bulge sits, the coast has high tide.
-
Step 3 of 5
There is a second bulge on the far side.
Earth's centre is pulled towards the Moon more strongly than the far ocean is, so that water is effectively left behind — bulging outward.
-
Step 4 of 5
Earth spins through both bulges.
The bulges stay pointed at the Moon while Earth turns underneath. Any coast passes through two of them a day — two high tides, two low.
-
Step 5 of 5
The Sun adds to it or cancels it.
When Sun, Earth and Moon line up, the pulls add and tides are extreme. At right angles they partly cancel and tides are mild.
The Bay of Fundy in Canada has a tidal range of 16 metres — the height of a five-storey building.
The short version
The Moon raises two ocean bulges and Earth spins through them, giving two high tides a day.
Try it yourself
Look up your nearest tide table. High tides arrive about 50 minutes later each day — exactly how much later the Moon rises.
Topics