L14. Tides: The Moon Pulls the Ocean

Unit 3 · Gravity and Motion

R-report

L14. Tides: The Moon Pulls the Ocean

Twice a day, every day, the sea rises and falls along most coastlines — in places by more than ten metres. The cause is an object 384,000 kilometres away that you can cover with your thumb.

Why there are two bulges

The Moon’s gravity pulls on Earth, but it does not pull on every part equally. Gravity weakens with distance, so the side of Earth facing the Moon is pulled hardest, the centre of Earth is pulled a bit less, and the far side is pulled least of all. That difference is what creates tides. - On the near side, the ocean is pulled toward the Moon more strongly than the solid Earth beneath it, so water piles up into a bulge. - On the far side, the solid Earth is pulled toward the Moon more strongly than the ocean there, so Earth is effectively pulled out from under that water, leaving a second bulge behind. So there are two high-tide bulges, on opposite sides of the planet, with low water in between. As Earth rotates once a day, most coastlines pass through both bulges — which is why they get roughly two high tides and two low tides daily. The cycle is about 24 hours 50 minutes rather than 24, because the Moon has moved along its orbit and Earth has to turn a little extra to catch up.

Spring tides and neap tides

The Sun raises tides too. It is vastly more massive than the Moon but also far more distant, and since the tidal effect depends on the difference in pull across Earth, the Sun’s contribution is only about 45% as strong as the Moon’s. What matters is how the two line up. - Spring tides: at new Moon and full Moon, the Sun and Moon are lined up with Earth. Their effects add together, so high tides are unusually high and low tides unusually low. - Neap tides: at the first and last quarter, the Sun and Moon pull at right angles. They partly cancel, so the difference between high and low tide is at its smallest. The word "spring" here means to leap up, not the season — spring tides happen twice a month all year round. Local geography matters enormously too. The Bay of Fundy in Canada funnels water into a narrowing channel and sees a tidal range above 16 metres, while parts of the Mediterranean barely change at all.

Tides change the Earth-Moon system

Tides are not just a coastal curiosity. They slowly reshape the system that makes them. As Earth rotates, the tidal bulges are dragged slightly ahead of the Moon. Friction between the moving water and the sea floor steals a little of Earth’s rotational energy, so our planet’s spin is gradually slowing. Days are getting longer by roughly 1.8 milliseconds per century. Fossil evidence supports this: growth bands in corals from 400 million years ago suggest a year then contained about 400 shorter days. The energy is not lost — it is transferred to the Moon, which is drifting away from Earth at about 3.8 centimetres per year. This has been measured directly by bouncing lasers off reflectors that Apollo astronauts left on the lunar surface. The same process, running much more strongly in the past, is what locked the Moon’s rotation to its orbit so that it now keeps one face permanently toward us.

Putting the investigation together

Tides come from the difference in the Moon’s pull across Earth, not from its overall strength. That difference raises two bulges, Earth’s rotation carries coastlines through both each day, and the Sun’s smaller contribution makes tides larger or smaller depending on alignment.

The wider point is that a very small effect, applied constantly over a very long time, adds up. A few milliseconds per century sounds negligible until you extend it across hundreds of millions of years and find that days were once noticeably shorter. Astronomy is full of tiny effects with enormous cumulative results.