Why the sea breathes

Tides are not caused by the Moon simply “pulling the water up.” The Moon pulls on the entire Earth. What raises tides is the difference in that pull across the planet — stronger on the side nearest the Moon, weaker on the far side. This tide-raising force falls off with the cube of distance, which is why the nearby Moon out-pulls the far more massive but distant Sun.

That difference stretches the ocean into two bulges— one directly under the Moon, and one on the exact opposite side of the Earth, where the Moon's pull is weakest. Because there are two, most coastlines experience two high tides and two low tides every day as the Earth spins through both bulges. And since the Moon keeps moving along its orbit, the cycle arrives about 50 minutes later each day.

The Sun adds its own, smaller tide — about 46%as strong as the Moon's. When the Sun and Moon align at new and full Moon, their bulges combine into the big spring tides; near the quarter Moons they work against each other and the tides go slack — the weaker neap tides.

Everything above is the classic equilibrium model — and it predicts tides of only about half a metre, everywhere. The real ocean is far wilder. Continents block the travelling bulges, and water sloshes back and forth in ocean basins that can resonate like a bell. Where a basin's natural rhythm matches the tide, the range is amplified enormously; where the water is nearly enclosed, it barely stirs.

Same Moon, same physics — utterly different seas. That gap between the tidy model and the real coastline is exactly where the science of tides gets interesting.

The Earth spins faster than the Moon orbits, and friction drags the tidal bulge slightly ahead of the Moon. That off-centre bulge exerts a tiny forward tug on the Moon, nudging it into a higher orbit: the Moon is drifting away from us by about 3.8 centimetres a year. The same exchange slowly brakes Earth's rotation, so our days are gradually growing longer — a direct, measurable consequence of the tides.

Why does the Moon cause tides on Earth?

The Moon pulls on the whole Earth, but slightly harder on the side facing it and slightly weaker on the far side. Tides come from that difference in pull across the Earth — the tidal force — not from the Moon's gravity alone. The difference stretches the ocean into two bulges.

Why are there two high tides a day?

The tidal force raises a bulge on the side of Earth nearest the Moon and an equal bulge on the opposite side. As the Earth rotates, any coastline passes through both bulges each day, giving two high tides and two low tides — roughly every 12 hours and 25 minutes.

Why do the tides arrive about 50 minutes later each day?

While the Earth spins once, the Moon moves along its orbit, so the Earth must turn a little further to bring the Moon overhead again. That 'lunar day' is about 24 hours and 50 minutes, so the whole tide cycle slips roughly 50 minutes later each day.

What are spring and neap tides?

The Sun raises tides too — about 46% as strongly as the Moon. When the Sun, Earth and Moon line up at new and full Moon, the solar and lunar bulges add together for large 'spring' tides. Near the quarter Moons they sit at right angles and partly cancel, giving weaker 'neap' tides.

If the Sun is so much heavier, why does the Moon dominate the tides?

Tides depend on the difference in gravity across the Earth, which falls off with the cube of distance (1/d³), not the square. The Sun is about 27 million times heavier than the Moon but nearly 400 times farther away — and 400³ beats 27 million — so the Moon's tide wins, and the Sun's is only ~46% as strong.

Why is the Bay of Fundy's tide the highest on Earth?

The simple model predicts tides of only about half a metre everywhere. Real tides depend on how ocean basins respond. The Bay of Fundy's natural sloshing period nearly matches the ~12.4-hour tide, so each pulse reinforces the last — resonance amplifies the tide to about 16 metres. Enclosed seas like the Mediterranean barely respond, with tides of only tens of centimetres.

Is the Moon really drifting away from Earth?

Yes. Friction means the tidal bulge is dragged slightly ahead of the Moon. That off-centre bulge tugs the Moon forward, nudging it into a higher orbit — the Moon recedes about 3.8 centimetres a year — while Earth's rotation slowly brakes, lengthening the day over geological time.