# How do tides work?

> The Moon doesn't lift the sea. It pulls unevenly on the whole planet, and the oceans stretch.

https://learnfold.app/wait-how/how-tides-work/ · Updated 2026-10-30

Tides happen because the Moon's gravity pulls harder on the side of Earth facing it than on the far side. That difference stretches the oceans into two bulges, one toward the Moon and one away from it. Earth turns through both every day, which is why most coasts get two high tides and two low tides. The Sun does the same thing at about half the strength.
## The pull matters less than the difference in the pull

NOAA's National Ocean Service notes that the Sun's gravitational attraction on Earth is more than 177 times stronger than the Moon's. Yet the Moon is the dominant force behind the tides.

The reason is that tides aren't caused by the pull itself. They're caused by how much the pull changes from one side of Earth to the other. NOAA explains that tide-generating forces vary inversely with the cube of the distance to the body raising them, so distance counts for more than mass. The Sun has about 27 million times the Moon's mass but sits about 390 times farther away. Its tide-generating force works out to about half the Moon's. NASA puts it at a little less than half.

Gravity's change with height matters in [how GPS knows where you are](/wait-how/how-gps-works/) too. GPS satellite clocks tick faster partly because they sit higher in Earth's gravity well.

## Why is there a bulge on the far side too?

A bulge facing the Moon makes intuitive sense. A bulge on the opposite side, facing away, does not.

Think of three runners on a track, all speeding up, but the one in front speeding up the most and the one at the back the least. The group stretches out. Nobody is being pushed backward, yet the gap between front and back grows. Earth is in the same situation with respect to the Moon. NASA explains that the strongest pull is on the points closest to the Moon and the weakest on the points farthest away, but every bit of water is affected. The near-side water gets pulled toward the Moon more than the solid Earth does. The far-side water gets pulled less than the solid Earth, so it is left behind.

NOAA describes the same thing in terms of inertia. On the far side, the Moon's pull is weaker, inertia wins, and the water tends to keep going in a straight line. Either way, the result is two bulges, one where Earth and Moon are closest and one where they are farthest apart, with gravity and inertia roughly in balance everywhere else. As Earth rotates, coastlines pass through both bulges, and NASA notes that one high-tide-to-high-tide cycle takes a little over 12 hours.

## Spring tides and neap tides

Twice a month, around new moon and full moon, the Sun, Moon and Earth line up. NOAA says the Sun's pull then adds to the Moon's, so high tides are a little higher and low tides a little lower than average. These are spring tides.

About seven days later, at the first and third quarter moon, when the Moon looks half full, the two pull against each other. NOAA explains that the bulge raised by the Sun partially cancels the bulge raised by the Moon. These moderate tides are neap tides.

Drag the Moon around Earth or use the alignment buttons. Turn off the Sun to see the two lunar bulges on their own; real coastlines and ocean depths change this smooth-planet picture.

## Spring tides have nothing to do with spring

They don't. NOAA says the name has nothing to do with the season and comes from the idea of the tide "springing forth." Spring tides happen twice each lunar month, all year round.

A second myth NOAA takes on directly is that tidal range increases with latitude. It says that belief is incorrect. Some high northern latitudes do have big tides, but NOAA attributes that to continents crowding the oceans together there. At high southern latitudes, where the continents aren't pressed together, ranges don't increase.

## Why real tides ignore the simple model

The two-bulge picture describes a smooth planet with no continents in the way. NOAA points out that on such a planet every place would get two equal high tides and two equal low tides each lunar day. Ours has continents in the way. They block the bulges from sweeping around the globe, so tides set up complex patterns within each ocean basin, and those patterns can differ from neighboring basins or even from other parts of the same basin. NASA adds ocean depth to the list of local factors.

The results vary a lot. NOAA says many places on the Gulf of Mexico get only one high and one low tide a day, a diurnal tide. The US East Coast tends to get two roughly equal highs and lows, a semidiurnal tide. The West Coast tends toward a mixed semidiurnal tide, where the two highs differ in height. Ranges vary just as much. NOAA lists 30.3 feet at Sunrise on Alaska's Turnagain Arm, about 20 feet near Eastport, Maine, and only 5 to 8 feet along the coast from Cape Cod to the Florida Keys. It names coastline shape and geometry as a major factor.

## Why the Bay of Fundy goes wild

NOAA says the highest tide in the world is in Canada's Bay of Fundy, which separates New Brunswick from Nova Scotia. NASA's Jet Propulsion Laboratory says the highest tides on Earth occur in the Minas Basin at its eastern end, where the range can reach 16 meters when the various factors line up.

JPL calls resonance the primary cause. The Bay of Fundy and Gulf of Maine form one system with a natural sloshing period of about 13 hours, close to the 12 hours 25 minutes of the dominant lunar tide in the Atlantic. Think of pushing a child on a swing. Small pushes timed to the swing's own rhythm build a big motion. The Atlantic tide nudges the basin at nearly that ideal rhythm, and the biggest slosh happens at the head of the system, in the northeast. JPL notes the system is effectively bounded at its outer end by the edge of the continental shelf, where the depth increases about 40-fold.

## Why your local high tide keeps moving

If high tide was at noon today, it won't be at noon tomorrow. NOAA explains that a lunar day lasts 24 hours and 50 minutes, because the Moon moves along its orbit in the same direction Earth rotates, so Earth needs extra time to catch up. High tides come 12 hours and 25 minutes apart, and the water takes about six hours and 12.5 minutes to go from high to low. That's why tide times slide roughly 50 minutes later each day.

The exact time and height at any spot depend on local conditions, which is why tide tables are made port by port.

## One tide a day, and a Moon drifting away

The Gulf of Mexico's single daily tide is the natural next puzzle, and further out there is the question of whether tides are slowly pushing the Moon away from Earth. In Learnfold, either one starts a short course you approve, and each follow-up you pick becomes a page linked to the last, so a [curiosity rabbit hole](/use-cases/follow-your-curiosity-rabbit-hole/) stays on one course map. If the sky is more your thing, try [why stars twinkle](/wait-how/why-stars-twinkle/).

## Quick answers

**Why are there two high tides a day?**

The Moon's uneven pull stretches the oceans into two bulges, one facing the Moon and one on the far side. As Earth rotates, most coastlines pass through both, so they get two high tides and two low tides.

**What is the difference between spring and neap tides?**

Spring tides happen around new and full moon, when the Sun's tidal pull adds to the Moon's and the range is larger. Neap tides come about a week later, at the quarter moons, when the two partly cancel.

**Why does the Moon affect tides more than the Sun?**

Tidal force depends on how much a body's pull changes across Earth, and that falls off with the cube of distance. The Sun is about 390 times farther away than the Moon, so its tide-raising force is only about half the Moon's.

**Why is high tide about 50 minutes later each day?**

The Moon moves along its orbit in the same direction Earth spins, so Earth takes about 24 hours and 50 minutes to face the Moon again. High tides follow that lunar day rather than the 24-hour clock.

**Where are the biggest tides in the world?**

In the Bay of Fundy in Canada, between New Brunswick and Nova Scotia. NASA's JPL says the range in its Minas Basin can reach 16 meters, largely because the bay resonates with the Atlantic tide.

## Sources

1. [What Causes Tides](https://oceanservice.noaa.gov/education/tutorial_tides/tides02_cause.html), NOAA National Ocean Service
2. [Gravity, Inertia, and the Two Bulges](https://oceanservice.noaa.gov/education/tutorial_tides/tides03_gravity.html), NOAA National Ocean Service
3. [Frequency of Tides, The Lunar Day](https://oceanservice.noaa.gov/education/tutorial_tides/tides05_lunarday.html), NOAA National Ocean Service
4. [Types and Causes of Tidal Cycles](https://oceanservice.noaa.gov/education/tutorial_tides/tides07_cycles.html), NOAA National Ocean Service
5. [What are spring and neap tides?](https://oceanservice.noaa.gov/facts/springtide.html), NOAA National Ocean Service
6. [Where is the highest tide?](https://oceanservice.noaa.gov/facts/highesttide.html), NOAA National Ocean Service
7. [Tides](https://science.nasa.gov/moon/tides/), NASA Science
8. [Bay of Fundy (PIA01912)](https://www.jpl.nasa.gov/images/pia01912-bay-of-fundy/), NASA Jet Propulsion Laboratory