What Causes Tides? Spring and Neap Tides Explained

The question “how do tides work?” is answered here step by step, starting with the pull of the Moon. The guide also explains why there is a high tide on the far side, why high tides come about 12 hours 25 minutes apart, and what spring and neap tides are. It ends with a real example, the tidal range at the Bay of Fundy, and how tides can make electricity.

What causes tides?

Gravity gets weaker with distance. The side of the Earth nearest the Moon is about 6,400 km closer to it than the Earth’s centre, so it is pulled a little harder than the centre. The far side is pulled a little less. The solid Earth moves as a whole, but the water is free to slide, so it stretches into two bulges along the line joining the Earth and the Moon.

The Sun does the same thing, but the Sun is about 400 times further away, so its tidal pull is only about 46% of the Moon’s.

Why is there a high tide on the far side?

This surprises many students, because the Moon is pulling on the near side only. The answer is that the Earth is pulled toward the Moon more than the far-side water is. Relative to the solid Earth, the near-side water moves toward the Moon and the far-side water is left behind, which is the same as being pulled away from the Earth. So there are two bulges, one on each side, and both are high tides.

• Near-side bulge: the Moon pulls the water most strongly, so it piles up toward the Moon.
• Far-side bulge: the Earth is pulled toward the Moon more than the water on the far side, so the water is left behind.
• Between the bulges: the sea is pulled sideways toward the bulges, so the water level is low. These are low tides.

Why are high tides about 12 hours 25 minutes apart?

The Earth turns once every 24 hours, but the two bulges stay pointed at the Moon. A fixed coast therefore passes through a bulge, then a low, then the other bulge, then the other low, in one tidal day of about 24 hours 50 minutes.

Two high tides are separated by half of that:

1. The Earth turns once in about 24 h.
2. The Moon moves on in its orbit, so a point on Earth must turn an extra 50 minutes to meet the Moon again.
3. The time between high tides is half of 24 h 50 min, which is 12 h 25 min.

So most coasts have two high tides and two low tides a day, and each high tide is about 50 minutes later than the one the day before. Some places have one high tide a day, or a mixed pattern. The shape of the coastline and the sea floor decides which.

What are spring and neap tides?

The Moon and Sun both raise tides, and their effects add or cancel depending on where they are. The Moon goes round the Earth in about 29.5 days, so the pattern repeats every two weeks.

Spring tides

Spring tides happen when the Sun, Earth and Moon are in a straight line, at a new Moon or a full Moon. The Sun’s and the Moon’s bulges line up and add together. High tides are higher and low tides are lower, so the tidal range (the height between high and low water) is at its largest. The name has nothing to do with the season. It comes from the old word “spring”, meaning to rise up.

Neap tides

Neap tides happen at the first and last quarter of the Moon, when the Sun and Moon are at right angles as seen from the Earth. Now the Sun’s bulges sit where the Moon’s lows are, and the two partly cancel. High tides are lower and low tides are higher, so the tidal range is at its smallest.

A spring tide is about one and a half times the range of a neap tide in many places, though the exact figure depends on the coast.

Tidal range explained with a real example

The tidal range is the vertical difference between high water and the low water before or after it. Most coasts have a range of a metre or two. The Bay of Fundy, between Nova Scotia and New Brunswick in Canada, has some of the largest tides in the world. At its head, the range can reach about 16 m on a spring tide, which is roughly the height of a five-storey building.

Why is the range so big there? Two things help. The bay is long and narrow and shaped like a funnel, so the incoming tide is squeezed as it moves in. Also, the natural period of water sloshing in the bay is close to 12 hours 25 minutes, so each tide adds to the one before, a bit like pushing a swing at the right moment. The result is mudflats that empty for kilometres at low tide.

How tidal power works

Tides are a steady, predictable source of energy, because the Moon’s motion can be calculated years ahead. There are two main ways to use them:

• Tidal barrage: a dam across an estuary holds water at high tide, then releases it through turbines as it flows back out. The La Rance barrage in Brittany, France, has been generating since 1966, with a capacity of about 240 MW.
• Tidal stream turbines: underwater turbines sit in fast tidal currents and turn like wind turbines.

Tidal power is limited to places with a large tidal range or strong currents, and building a barrage changes the estuary’s wildlife, so it is far from everywhere.

Common mistakes

• Saying the Sun causes the tides alone. The Moon causes most of the tide. The Sun adds about 46% of the Moon’s effect.
• Thinking the far-side bulge is caused by the Moon pulling the water away. It comes from the Earth being pulled toward the Moon more than the far-side water.
• Expecting high tide at the same time every day. It comes about 50 minutes later each day.
• Confusing spring tides with the spring season. Spring tides are named after the rise of the water, not the time of year.

Frequently asked questions

What causes tides?

Tides are mainly caused by the Moon’s gravity. It pulls the sea toward it, making a bulge on the near side of the Earth. A second bulge forms on the far side because the Earth is pulled toward the Moon more strongly than the water there. The Sun also raises tides, about 46% as strong as the Moon’s. As the Earth turns through these two bulges, most coasts have two high tides a day.

Why are there two high tides a day?

The Earth turns once every 24 hours, but the two tidal bulges stay pointed at the Moon. A fixed coast passes through both bulges in about 24 hours 50 minutes. Two high tides are therefore about 12 hours 25 minutes apart.

What is the difference between spring and neap tides?

Spring tides happen at new and full Moon, when the Sun, Earth and Moon are in a line. Their effects add together, giving the largest tidal range. Neap tides happen at the first and last quarter, when the Sun and Moon are at right angles. Their effects partly cancel, giving the smallest tidal range.

Why does the tidal range vary?

The tidal range changes with the Moon’s phase, the distance between the Earth and Moon, and the shape of the coast. Bays and estuaries can amplify the range. The Bay of Fundy in Canada can reach about 16 m on a spring tide because of its shape.

Try it yourself

Open the Tides & the Moon simulation. Drag the Moon round the Earth and watch the two bulges follow it. Then move the Sun into line with the Moon at new and full Moon to see a spring tide, and at a quarter Moon to see a neap tide. Compare the tide height at the coastal town for each one. For the bulges on a larger scale, try the Gravity & Orbits tool, and for the Earth’s tilt and the year, read about why we have seasons. For the Moon’s position in its orbit, see what happens in a lunar eclipse.