What causes tides? The short answer
Tides are caused mainly by the Moon's gravity, with help from the Sun, and by Earth's spin.17 The Moon pulls hardest on the side of Earth facing it and most weakly on the far side. That difference stretches our planet, and the oceans (which move far more easily than rock) pile up into two bulges: one facing the Moon and one on the opposite side.45 As Earth turns, a coast passes through both bulges, so most coasts get two high tides and two low tides about every 24 hours and 50 minutes.1
Scientists call this stretching the tidal force. Isaac Newton first explained tides this way in 1687.1
How does the Moon affect the tides?
Gravity gets weaker with distance: the closer two objects are, the stronger the pull between them. So the pull on the near side of Earth is noticeably stronger than the pull on Earth's centre, and the pull on the far side is weaker still.14
What matters for tides isn't the Moon's pull itself but the difference between the pull at each spot and the average pull on the whole Earth. NASA's Space Place team puts it as a simple subtraction:4
tidal force = Moon's pull at a spot − Moon's average pull on Earth
Do that subtraction all around the globe and you get a stretch: on the near side the leftover points toward the Moon, and on the far side it points away. Along the "waist" of Earth the leftover gently squeezes inward. That's the pattern the toy in chapter 2 shows.
The same trick explains why the Moon beats the Sun at making tides. The Sun pulls on Earth far harder overall (about 177 times harder than the Moon does), but it is so far away that its pull is almost the same on both sides of Earth. The tidal force falls off with the cube of distance, not the square, so distance matters more than mass.1
Why are there two high tides a day?
Because there are two bulges, and Earth spins through both of them. NOAA describes the ocean as shaped like a football pointing at the Moon: the pointed ends are high tide and the flat sides are low tide.3
The near bulge forms because the Moon's pull is strongest there. The far bulge forms because the pull there is weakest: the rest of Earth is pulled toward the Moon a little more than the far-side water is, so that water is "left behind". NOAA explains the far bulge as inertia winning over the Moon's weaker gravity.13
The water doesn't mostly lift straight up. NASA compares it to a puddle on a table: it's much easier to slide water sideways than to lift it. The tide-producing force nudges water sideways across the ocean, heaping it into the two bulges and drawing it away from the regions in between.57
Why are tides about 50 minutes later every day?
The Moon travels around Earth in the same direction that Earth spins. So after one full turn (24 hours), Earth has to keep turning for about 50 more minutes to face the Moon again. That makes a lunar day (or tidal day) 24 hours and 50 minutes long.13
Values from NOAA's tides tutorial; real coasts vary.12 The bulges also don't sit exactly under the Moon; NASA notes they run a little ahead of it, which is one reason you can't read the tide just by looking at the Moon.5
Spring tides and neap tides
The Sun makes tides too, in exactly the same way, just smaller. The Sun has about 27 million times the Moon's mass, but it is about 390 times farther away, so its tide-making force is only about half the Moon's.15 The solar tide doesn't show up as a separate set of tides. Instead it makes the Moon's tides bigger or smaller.1
- Spring tides happen at new Moon and full Moon, when the Sun, Earth and Moon line up. The two stretches add together, giving higher high tides and lower low tides.2
- Neap tides happen about a week later, at the first and third quarter Moon, when the Sun and Moon are at right angles. The Sun's bulge partly fills in the Moon's lows, so tides are gentler: lower highs and higher lows.2
Each lunar month brings two spring tides and two neap tides.1 And despite the name, spring tides have nothing to do with the season: the word comes from the tide "springing forth", and they happen all year round.2
What is a king tide?
The Moon's orbit is an oval, so its distance from Earth changes by about 31,000 miles over a month. When it is closest (perigee), tides are bigger than usual.1 When a spring tide lines up with perigee, NOAA calls it a perigean spring tide; the popular name is king tide. This happens 6 to 8 times a year, and those high tides can be more than a foot higher than spring tides when the Moon is far away.6 Tides are also a little larger around early January, when Earth is closest to the Sun.1
Types of tides: why some places get only one
If Earth were a smooth ball covered in water, every place would get two equal high tides a day. But continents block the bulges from sweeping around the world, so the water in each ocean basin sloshes in its own complex pattern.1 In the middle of these patterns there are places where the tide hardly rises or falls at all, and the high tide travels around them.9 NOAA groups the results into three types:1
Tidal range: where are the biggest tides?
The tidal range is the height difference between high and low tide.1 It depends hugely on the shape of the coast. Islands far out in the ocean, away from continental edges, often get tides of 1 metre or less.1 But when the tide runs onto a wide, shallow shelf or into a funnel-shaped bay, it gets squeezed higher.19
- Bay of Fundy, Canada: the highest tides in the world, over 15 metres (about 50 feet), taller than a five-storey building.189
- Near Anchorage, Alaska: the highest in the US, with ranges averaging about 30 feet.8
- Eastport, Maine: about 20 feet. South of Cape Cod to the Florida Keys, the normal range is 5 to 8 feet.8
Weather matters too. Strong offshore winds push water away and make low tides lower; onshore winds pile water up. Low-pressure storms can bring tides much higher than predicted.14
Is there a sea with no tides?
Every body of water feels the tidal force, but some barely show it. In the Mediterranean, Black and Baltic seas the tidal range is only a matter of centimetres.9 The Great Lakes do have true tides, but even their spring tides are less than 2 inches (5 cm), hidden by changes from wind and air pressure, so NOAA treats them as non-tidal.6
Where does the water go at low tide?
Nowhere far. It hasn't disappeared: it has moved across the ocean toward wherever high tide is happening right now.10 The incoming (rising) tide is called the flood and the outgoing (falling) tide the ebb. The sideways flow is a tidal current, and the quiet pause between them is slack water.1
How do ocean tides affect the Earth?
Tides don't only move the oceans. The Moon stretches the solid Earth too, very slightly, into a football shape. It's far too small to feel, but scientific instruments can measure it.54 Earth returns the favour: its gravity raises tides on the Moon's surface of about 4 to 6 inches (10 to 15 cm).5
For people, tides matter every day: for ships entering harbours, for building bridges and docks, and for fishing, boating and surfing.7 For sea life, the daily rise and fall shapes whole habitats; animals in rock pools and on shores have special ways of surviving being covered and uncovered.11 And because the Moon and Sun move so predictably, tides can be forecast years ahead. Forecasters combine many repeating patterns, often around 40 for a single place, measured from years of observations.910
Stay safe on tidal flats: a rising tide can come in fast across flat mud or sand and cut you off. Check the local tide times before you walk out, and keep an eye on the water.
Want more? See how the Moon changes shape, how gravity works, and why the wind makes waves (tides are waves too, just very long ones).
Common misconceptions
"There's one bulge, facing the Moon." There are two. The far bulge forms where the Moon's pull is weakest.
14
"Spring tides happen in spring." They happen twice a month, all year, at new and full Moon.
2
"The Sun is too far away to matter." Its tidal effect is about half the Moon's, which is enough to make spring and neap tides.
1
"High tide is at the same time every day." Tides come about 12 hours 25 minutes apart, so they shift roughly 50 minutes later each day, and some coasts get only one a day.
1
"Tides get bigger toward the poles." NOAA says that's wrong: big tides come from the shape and crowding of continents, not latitude.
8
FAQ
What is the main cause of tides?
The Moon's gravity. It pulls harder on the side of Earth facing it than on the far side, which stretches the oceans into two bulges. As Earth spins, coasts move through those bulges and get high tides. The Sun adds a smaller effect, about half as strong as the Moon's.
Why is there high tide twice a day?
There are two bulges of water: one facing the Moon and one on the opposite side of Earth. Earth turns through both of them every lunar day (24 hours 50 minutes), so most coasts get two high tides about 12 hours 25 minutes apart.
Where does the sea go when the tide goes out?
It doesn't disappear. The water moves across the ocean toward the places that are having high tide. Low tide at your beach means the water is heaped up in a bulge somewhere else.
Is there a sea with no tides?
Every sea feels the tidal force, but some barely show it. The Mediterranean, Black and Baltic seas have tides of only centimetres, and the Great Lakes have true tides of less than 5 cm, so they are treated as non-tidal.
What is the difference between spring tides and neap tides?
Spring tides happen at new and full Moon, when the Sun, Earth and Moon line up and their tidal effects (stretches) add together: higher highs and lower lows. Neap tides happen at first and third quarter Moon, when the Sun and Moon are at right angles and partly cancel: gentler tides. Each happens twice a month.
Why are high tides about 50 minutes later each day?
The Moon moves along its orbit in the same direction Earth spins. After one full turn, Earth needs about 50 more minutes to face the Moon again, so a lunar day is 24 hours 50 minutes and the tides slide later each day.
What is a king tide?
"King tide" is a popular name for a perigean spring tide: a spring tide that happens when the Moon is at its closest to Earth. NOAA says these happen 6 to 8 times a year and can push high tides noticeably higher than usual.
Sources
- NOAA National Ocean Service, Tides and Water Levels tutorial
- NOAA National Ocean Service, What are spring and neap tides?
- NOAA National Ocean Service, Why are there two high tides a day?
- NOAA NESDIS and NASA Space Place, What Causes Tides?
- NASA Science, Tides
- NOAA Tides & Currents, Frequently Asked Questions
- NOAA Tides & Currents, Our Restless Tides
- NOAA National Ocean Service, Where are the highest tides?
- Encyclopaedia Britannica, Tide
- National Oceanography Centre (UK), Tides explained
- National Geographic Education, Tide