How the Moon Affects Ocean Tides: A Plain Guide

The Moon causes ocean tides because its gravity pulls hardest on the side of Earth nearest to it and weakest on the side farthest away. That difference stretches the oceans into two bulges, one facing the Moon and one on the opposite side, and as Earth turns through them, most coasts get two high tides and two low tides about every 24 hours and 50 minutes. The Sun adds a tide of its own about half as strong: when the Sun and Moon line up at new and full moon, the tides are bigger, and when they pull at right angles, at the quarter moons, the tides are smaller.

Why there are two bulges, not one

The first bulge is easy to picture: the Moon pulls the ocean on the near side of Earth toward it. The second, on the far side, is the part that puzzles people. Why would the ocean pile up on the side facing away from the Moon?

The answer is that tides come from differences in gravity, not from gravity alone. The Moon pulls on the whole Earth, but not evenly. It pulls the near-side ocean a little more strongly than it pulls Earth's center, and it pulls Earth's center a little more strongly than the far-side ocean. Measured from Earth's center, the near-side water is drawn toward the Moon, and the far-side water is, in effect, left behind. The result is an ocean stretched along the line toward the Moon, with high water at both ends and low water in between.

Distance matters enormously for this kind of stretching. NOAA explains that tide-generating forces fall off with the cube of distance, so being close counts for far more than being massive. The Sun is vastly more massive than the Moon but so much farther away that its tidal effect is only about half the Moon's. That's why the Moon, not the Sun, runs the tide tables.

Why high tide comes about 50 minutes later each day

If the bulges stayed put, every coast would get high tides at the same times each day. They don't, because the Moon is moving too. As Earth spins, the Moon travels a little farther along its orbit in the same direction, so Earth has to turn for an extra 50 minutes or so to bring a given coast back under the Moon. NOAA calls that 24-hour-50-minute period a lunar day.

Two bulges in each lunar day means high tides arrive about 12 hours and 25 minutes apart on a typical coast, with low tides in between. It's also why the Moon rises about 50 minutes later each night: the same extra turn of the Earth explains both. If you know when the Moon crosses the sky at your local beach, you have a rough feel for when the tides will come, though local geography shifts the timing, as we'll see below.

Spring tides, neap tides and the phases of the Moon

Because the Sun raises its own, smaller tides, the size of the tide depends on how the Sun and Moon are arranged:

Moon phase	Sun and Moon	Tide
New moon	Lined up, same side of Earth	Spring tide: higher highs, lower lows
First quarter	At right angles	Neap tide: smaller range
Full moon	Lined up, opposite sides of Earth	Spring tide: higher highs, lower lows
Third quarter	At right angles	Neap tide: smaller range

At new and full moon, the solar tide adds to the lunar one, producing extra-high high tides and very low low tides. A week later, at the quarter moons, the Sun's tide partly cancels the Moon's, and the result is the moderate tides called neap tides. So there are two spring tides and two neap tides in every cycle of phases, about a week apart.

The name confuses everyone. NOAA notes that a spring tide "has nothing to do with the season of spring." The word comes from the idea of the tide springing forth, and spring tides happen twice a month all year round. Our guide to moon phases in order shows when each phase falls in the cycle, and the full moon calendar and new moon calendar give the next dates.

Perigee, apogee and king tides

The Moon's distance changes during the month too, by about 31,000 miles between its closest and farthest points according to NOAA. When the Moon is near perigee, its closest approach, tide-generating forces are higher than usual. About two weeks later, near apogee, tidal ranges are smaller than average.

When perigee happens to coincide with a new or full moon, the two effects stack, giving the biggest spring tides of the year. These are sometimes called perigean spring tides, and in many coastal towns people call the highest of them king tides, a popular rather than scientific name. A full moon near perigee is also what headlines call a supermoon, and a full moon near apogee is a micromoon, with slightly gentler tides.

On their own, perigean spring tides are only modestly higher than usual. Combined with a storm surge or strong onshore winds, they can push water into low-lying streets, which is why coastal flood warnings sometimes mention them. Earth's distance from the Sun matters a little as well: NOAA notes slightly larger tidal ranges around early January, when Earth is closest to the Sun, and slightly smaller ones around early July.

Why tides differ so much from coast to coast

If Earth were covered in a smooth, deep ocean, tides would follow the Moon neatly everywhere. The real planet has continents in the way. As NOAA puts it, the large continents block the westward passage of the tidal bulges as Earth rotates, so the tides slosh around within each ocean basin in complex patterns of their own.

That's why different coasts get different kinds of tide:

Semidiurnal tides, two highs and two lows of about the same height each day, are typical of the U.S. East Coast.
Mixed tides, two highs and two lows of unequal height, are typical of the U.S. West Coast.
Diurnal tides, just one high and one low a day, occur in places such as parts of the Gulf of Mexico.

The shape of a bay can amplify the tide enormously. In the Bay of Fundy, between New Brunswick and Nova Scotia, the funnel shape and the natural rhythm of the water sloshing in and out combine so well that the largest tides can rise and fall by around 16 meters, more than 50 feet. In enclosed seas and lakes, by contrast, there's too little water for the Moon to stretch. NOAA says the spring tide in the Great Lakes is less than five centimeters, about two inches, easily hidden by the effects of wind and air pressure.

The Moon, the tides and the length of the day

Tides don't just move water; they move energy. As Earth spins beneath the tidal bulges, the friction of water moving over the seabed acts as a very gentle brake on Earth's rotation, lengthening the day by a tiny amount over centuries. In return, the Moon is nudged into a slightly wider orbit, drifting away from Earth by about an inch a year.

The same kind of tidal braking, acting on the Moon itself long ago, is why the Moon now keeps one face toward us. We explain that in why we only see one side of the moon. And if you've heard that the Moon pulls on the water in our bodies the way it pulls on the oceans, our look at whether the moon affects mood explains why that idea doesn't hold up: tides need an ocean-sized body of water to work on.

Questions readers ask

Does the full moon cause higher tides?

Yes, a little. At full moon, and at new moon, the Sun and Moon line up, so their tides add together into spring tides, with higher high tides and lower low tides than average. The effect is the same at new moon, even though you can't see the Moon.

Why are there two high tides a day?

Because the Moon stretches the oceans into two bulges, one on the side of Earth facing it and one on the opposite side. As Earth rotates, most coasts pass through both bulges every 24 hours and 50 minutes, so high tides come roughly twelve and a half hours apart.

Does the Moon affect lakes?

Technically yes, but the effect is tiny. Even the Great Lakes have spring tides of less than five centimeters, according to NOAA, far smaller than the changes caused by wind and air pressure. Ponds and swimming pools have no tide worth measuring.

What would tides be like without the Moon?

Much smaller. The Sun would still raise tides, but they would be only about half the size of the Moon's, and they would arrive at the same times each day, following the Sun instead of the Moon.