How tides work

Twice a day, on most coasts around the world, the sea creeps up the beach and then slides back again. Tides are so regular that harbours publish tables predicting them years ahead. The main cause is the Moon’s gravity, helped by the Sun, but the details are more interesting than “the Moon pulls the water”.

The Moon pulls unevenly

Gravity gets weaker with distance. The side of Earth facing the Moon is about 12,700 km closer to it than the far side, so the Moon pulls slightly harder on the near-side ocean, a little less on Earth’s centre, and least on the far-side ocean. It is this difference in pull across the planet, called the tidal force, that matters, not the total strength of the Moon’s gravity.

Why there are two bulges

The uneven pull stretches the oceans into two bulges. On the side facing the Moon, water is pulled toward it more strongly than Earth’s centre is. On the far side, Earth’s centre is pulled toward the Moon more strongly than the far-side water, so that water is effectively “left behind”. The result is high water on both sides of the planet at once, with low water in between.

Earth spins through both bulges each day, which is why many places get two high tides and two low tides roughly every day.

Why tides are about 50 minutes later each day

While Earth spins, the Moon also moves along its orbit in the same direction. It takes Earth about 24 hours and 50 minutes to “catch up” with the Moon again. So a tidal day is about 24 h 50 min, and high tides arrive roughly 50 minutes later each day, around 12 hours 25 minutes apart.

The Sun’s role: spring and neap tides

The Sun is far more massive than the Moon, but it is also about 390 times farther away. Because tidal force falls off steeply with distance, the Sun’s tidal effect is a little under half of the Moon’s. How the two combine depends on where they are:

Tide type	When it happens	Effect
Spring tides	New moon and full moon (Sun, Earth and Moon in a line)	Highest highs and lowest lows
Neap tides	First and last quarter moon (Sun and Moon at right angles)	Smallest difference between high and low

Spring tides have nothing to do with the season. The name comes from the idea of the water “springing up”. They happen about every two weeks, all year round.

Why tides differ so much from place to place

The two-bulge picture is a simplification. Real oceans are broken up by continents, and water sloshes around each ocean basin in complex patterns. Coastline shape and sea depth make a huge difference:

• Bay of Fundy, Canada: the funnel-shaped bay amplifies the tide, giving a range of up to about 16 metres, among the largest on Earth.
• The Mediterranean: an almost enclosed sea with a narrow entrance, so most places see tides of only a few tens of centimetres.
• Parts of the Gulf of Mexico: only one high and one low tide a day (a “diurnal” tide).

Tides are slowing Earth down

Water dragging against the sea floor creates friction, which very slowly brakes Earth’s spin. Days get longer by roughly 2 milliseconds per century. In return, the Moon gains energy and drifts away from us by about 3.8 cm a year, a figure measured by bouncing lasers off reflectors left on the Moon by Apollo astronauts.

Reading a tide table safely

Local tide tables list the time and height of each high and low water. If you walk on beaches, explore rock pools or cross to tidal islands, check the table first and allow a safety margin: incoming tides can move surprisingly fast over flat sand, and wind and air pressure can make the water higher than predicted.

In one sentence: tides happen because the Moon and Sun pull harder on the near side of Earth than the far side, stretching the oceans into bulges that the planet spins through each day.