What causes tides, and why are there two a day?

The moon pulls harder on the water that’s closer

Gravity weakens with distance. The moon pulls on the whole Earth, but not equally: it pulls a little harder on the side facing it, a bit less on the planet’s center, and less still on the far side, which is almost 8,000 miles farther away.

Solid ground barely responds to that difference. It does rise and fall a little, but nobody notices. Ocean water, on the other hand, moves freely and slowly piles up where the pull pushes it.

What matters is not how hard the moon pulls on the Earth, but the difference between its pull on one side and the other. Scientists call that difference the tidal force, and it explains something that surprises many people: even though the sun is vastly bigger, the moon moves the ocean more.

The sun pulls on the Earth far more strongly than the moon does, but it’s about 390 times farther away. At that distance, the difference between the near and far sides of the planet is small. As a result, the sun’s effect on the tides is about half of the moon’s.

Why there are two bulges of water, not one

If only the moon’s pull mattered, you’d expect a single heap of water facing it. Instead there are two, and the second sits on the opposite side of the planet.

Picture three points in a line: the water on the side facing the moon, the Earth’s center, and the water on the far side. The moon pulls on all three, but harder on the first than the second, and harder on the second than the third:

• the near-side water gets pulled ahead toward the moon;
• the Earth’s center gets pulled slightly ahead of the far-side water, which is left behind.

The planet is stretched along the line toward the moon, like a rubber ball pulled from both ends. The water forms two bulges, one toward the moon and one on the far side. Halfway between them, a quarter turn away, the water is lower. Those are the low tides.

The Earth turns once a day, so any point on the coast passes through both bulges and both troughs. That’s why most of the Atlantic coast, from Maine to Florida, gets two high tides and two low tides a day, a little over six hours apart.

Why the tide comes about 50 minutes later every day

While the Earth spins, the moon also moves along its orbit in the same direction. To bring the moon back to the same place in the sky, the Earth has to turn a bit more than one full rotation. That lunar day lasts about 24 hours and 50 minutes.

Tides keep the moon’s clock, not the sun’s. In practice:

• two high tides in a row are, on average, about 12 hours and 25 minutes apart;
• tomorrow’s high tide comes about 50 minutes later than today’s (depending on the day and place, the shift can be quite a bit shorter or longer);
• in about two weeks, the tides work their way around the clock: if high tide is at 8 a.m. today, it will be back near 8 a.m. roughly 15 days from now.

It’s a handy rule for planning: if you know today’s high tide, you can estimate the next few days by adding about 50 minutes per day. For exact times, always check the tide table for your spot.

The sun’s role: spring and neap tides

The sun stretches the ocean too, just less. What matters is how its effect combines with the moon’s:

• At new moon and full moon, the sun, Earth and moon line up. The two effects add together: high tides run higher and low tides run lower. These are spring tides, a name that has nothing to do with the season.
• At the first and last quarter moons, the sun and moon pull at right angles. Their effects partly cancel, and the difference between high and low tide shrinks. These are neap tides.

That cycle happens twice every lunar month of about 29.5 days, so spring tides come roughly every two weeks. They often peak a day or two after the new or full moon, because the ocean takes time to respond. In France and Spain, the size of each tide is summed up in a single number, the tide coefficient, which we explain in our guide to spring and neap tides.

Distance matters too. The moon’s orbit isn’t a circle. When the moon is closest to the Earth (perigee), its pull is stronger, and if that lines up with a new or full moon, spring tides get noticeably bigger. Those are the perigean spring tides that, along with seasonal effects, many coastal towns call king tides.

On real coasts, the shape of the sea is in charge

The two-bulge model explains the rhythm of the tides, but not their size. On a planet covered by a deep ocean with no continents, the difference between high and low tide would be small everywhere. Real coasts are all over the map:

• in Canada’s Bay of Fundy, the range can top 15 meters, about 50 feet;
• in Boston, it averages around 9 to 10 feet;
• in Charleston, South Carolina, it averages a bit over 5 feet;
• along much of the Gulf Coast, it’s often a foot or two, and in the Mediterranean it can be just a few inches.

The reason is that the continents block the bulges from sweeping around the planet. Instead, the tide behaves like an enormously long wave that enters each ocean basin and travels around it, rotating around points where the water level barely changes at all. The shape and depth of each basin, the width of the continental shelf, and the bays and estuaries along the coast amplify or dampen it. When a bay is just the right size, the water resonates, like water sloshing in a bathtub when you push it at the right rhythm. That’s what happens in the Bay of Fundy.

For the same reason, high tide doesn’t arrive when the moon is directly overhead. It comes with a delay that depends on the place and can be several hours. Even towns a few miles apart can have different tides: inside a sound or a tidal river, high tide may come well after it does at the inlet. That’s why every harbor has its own table. Coasts with one tide a day or with uneven tides are covered in our guide to types of tides, and the seas with hardly any tide in why some seas have almost no tide.

Weather moves the water too

A tide table gives the astronomical tide: the one calculated from the moon and the sun. Weather nudges it:

• Air pressure. Under a low-pressure system, the sea rises; under high pressure, it drops. A rough rule is 1 centimeter of water per millibar, a bit over an inch for every 0.1 inch the barometer drops. A deep storm can lift the water a foot above an ordinary day.
• Wind. Hours of onshore wind pile water against the coast, so high tide runs higher and sometimes earlier. A strong offshore wind leaves the water lower than predicted. Along the Gulf Coast, where tides are small, wind can matter more than the tide itself.
• Rivers. In estuaries, high river flow delays and flattens the low tide.

Usually it’s a matter of inches, but on a spring high tide during a nor’easter, those inches can decide whether a coastal road stays dry.

What it all means when you head to the coast

• On most of the East Coast, plan for two high tides and two low tides a day, a little over six hours apart.
• The tide comes about 50 minutes later each day, so a morning high tide drifts into the afternoon over the week.
• Roughly every two weeks you get spring tides (higher highs, lower lows) and, in between, neap tides.
• Every spot has its own tide, even nearby ones. Check the table for your beach or harbor, not the closest big city.
• During a storm, leave a margin: the water can come in higher than the table says.

If you fish, dig clams or walk tide pools, the next step is learning how to read a tide chart: what the times, heights and minus tides really mean.