Ocean tides are the regular rise and fall of sea level caused primarily by the gravitational influence of the Moon, with an additional contribution from the Sun.
The basic idea sounds simple, but real tides are more complicated than a pair of water bulges moving around a perfectly smooth Earth.
Quick answer: The Moon causes tides because its gravitational pull is slightly stronger on the side of Earth closest to it and slightly weaker on the far side. This difference stretches Earth and its oceans along the Earth-Moon direction. As Earth rotates through this tidal pattern, many coastlines experience alternating high and low tides.
The Sun also produces tides. Local tide height and timing are then strongly modified by coastlines, ocean depth, basin shape, and Earth's rotation.
In This Guide
What Are Tides?
Tides are periodic changes in sea level produced mainly by gravitational interactions among Earth, the Moon, and the Sun.
At many coastlines, the most familiar pattern is:
- High tide
- Falling tide
- Low tide
- Rising tide
- Another high tide
But not every location follows exactly the same schedule.
Some places experience:
- Two similar high tides and two similar low tides each day
- One high tide and one low tide each day
- Two high and low tides of noticeably different heights
These patterns are called semidiurnal, diurnal, and mixed tides.
Tides are not just an ocean phenomenon. Gravitational tides also affect the solid Earth, the atmosphere, and even water levels in some wells, although these effects are much less obvious than those of coastal ocean tides.
How Does the Moon's Gravity Cause Tides?
Every part of Earth feels the Moon's gravity.
But the Moon does not pull equally strongly on every part of Earth.
The side of Earth facing the Moon is slightly closer, so the Moon's gravitational pull there is slightly stronger.
The center of Earth feels a slightly weaker pull.
The far side of Earth is farther away and still experiences a slightly weaker lunar pull.
These differences in Earth's diameter create the tide-generating force.
In a simplified model, this differential gravity stretches Earth along the line connecting Earth and the Moon.
The oceans respond more visibly because water can move more freely than solid rock.
Why Isn't the Moon's Overall Gravitational Pull Enough to Explain Tides?
The important quantity is not simply how strongly the Moon attracts Earth as a whole.
Earth and the Moon are both accelerating toward one another as they orbit their common center of mass.
Tides arise because the Moon's gravitational acceleration is not uniform across Earth.
Tidal forces come from differences in gravity across an object—not simply from the total gravitational force acting on it.
Why Are There Two High Tides?
In the simplest equilibrium-tide model, there are two tidal bulges:
- One roughly on the side of Earth facing the Moon
- One roughly on the opposite side
Between them are regions of lower water level.
As Earth rotates, a location can move through these regions, producing alternating high and low tides.
This simplified model explains why many locations experience approximately two high tides and two low tides per lunar day.
Why Is the Tidal Day Longer Than 24 Hours?
The Moon moves eastward in its orbit while Earth rotates.
Because Earth must rotate a little farther each day to bring the same location back into alignment with the Moon, a lunar day is about 24 hours and 50 minutes.
At places with a regular semidiurnal pattern, successive high tides are therefore often separated by roughly 12 hours and 25 minutes.
Real ocean tides do not behave like perfectly fixed bulges. Continents block the flow, ocean basins vary in shape and depth, and Earth's rotation affects how tidal waves move. The two-bulge model is useful for understanding the basic force, not for predicting local tide times.
What Role Does the Sun Play in Earth's Tides?
The Sun also produces tides.
Although the Sun's gravitational pull on Earth as a whole is much stronger than the Moon's, the Sun is also vastly farther away.
Tidal force depends strongly on how much gravitational pull changes across Earth's diameter.
Because the Moon is so much closer, its tide-generating effect on Earth is stronger than the Sun's.
The solar tide is nevertheless large enough to significantly modify the lunar tide.
| Body | Role in Earth's Tides |
|---|---|
| Moon | Primary tide-generating influence |
| Sun | Secondary but important influence that strengthens or weakens the lunar tidal pattern |
| Other planets | Tidal effects on Earth's oceans are negligible compared with the Moon and Sun |
What Are Spring and Neap Tides?
The relative positions of the Sun, Moon, and Earth change throughout the lunar month.
This creates a regular cycle in tidal range.
Spring Tides
Spring tides occur around the new moon and full moon, when the Sun, Earth, and Moon are approximately aligned.
The solar and lunar tide-generating forces reinforce one another.
The result is typically:
- Higher-than-average high tides
- Lower-than-average low tides
- A larger tidal range
“Spring tide” has nothing to do with the season of spring. The term refers to the tide appearing to “spring forth” with a larger range.
Neap Tides
Neap tides occur around the first- and third-quarter Moon.
At these times, the Sun and Moon appear roughly at right angles as viewed from Earth.
Their tidal effects partially counteract one another.
The result is generally:
- Lower high tides
- Higher low tides
- A smaller tidal range
Why Does the Moon's Distance Matter?
The Moon's orbit is not perfectly circular.
Its distance from Earth changes over the course of each orbit.
When the Moon is closest to Earth, it is at perigee.
When it is farthest away, it is at apogee.
Because tidal force increases strongly as distance decreases, lunar tides are somewhat stronger near perigee.
When a new or full Moon occurs near perigee, the result can be an especially large tidal range sometimes called a perigean spring tide.
The Earth-Sun distance also varies over the year, but the lunar distance cycle generally has a more noticeable effect on ordinary tide variations.
Why Are Real Tides So Different From Place to Place?
If Earth were a smooth sphere covered by a uniform deep ocean, tides would be much easier to describe.
But real Earth contains:
- Continents
- Islands
- Shallow continental shelves
- Deep ocean basins
- Narrow channels
- Bays and estuaries
All of these features change how tidal waves move.
Ocean-Basin Shape
Tidal waves interact with basin boundaries and can be reflected, redirected, or amplified.
The resulting pattern can be very different from the simple equilibrium model.
Water Depth
Long shallow-water waves move more slowly in shallower water.
As tidal waves enter continental shelves and coastal areas, their speed and shape change.
Bays and Estuaries
Some bays have natural oscillation periods that interact strongly with incoming tidal forcing.
This can greatly amplify tidal range.
That is why some locations have only modest tides, while others experience dramatic differences between high and low water levels.
Earth's Rotation
Earth's rotation and the Coriolis effect help organize tides into complex rotating patterns within many ocean basins.
The Moon creates the tide-generating force, but geography largely determines what the tide looks like at a particular coastline.
What Is an Amphidromic System?
In many ocean basins, tidal waves rotate around locations known as amphidromic points.
At or near an ideal amphidromic point, the tidal range is very small.
Farther away, the tidal range generally increases.
Tide timing also changes as the tidal wave progresses around the basin.
These rotating patterns are one reason two coastlines at similar latitudes can have very different:
- High-tide times
- Tidal ranges
- Numbers of tides per day
An amphidromic point does not necessarily mean literally “no tide.” It is more accurate to describe it as a location around which the tidal wave rotates and where tidal amplitude is theoretically minimal.
Why Do Some Places Have One Tide Per Day?
The Moon's tide-generating force contains several periodic components, and local ocean basins respond differently to them.
As a result, coastal tide patterns are commonly classified as:
| Tide Pattern | Typical Daily Pattern |
|---|---|
| Semidiurnal | Two high tides and two low tides of roughly similar height |
| Diurnal | One high tide and one low tide |
| Mixed semidiurnal | Two high and two low tides with noticeable differences in height |
These differences arise from the combined influence of astronomical forcing and local ocean-basin response.
Does the Solid Earth Have Tides Too?
Yes.
The gravitational effects of the Moon and Sun slightly deform the solid Earth.
These solid Earth tides are far too small and gradual to notice directly, but sensitive scientific instruments can measure them.
The same forces can also produce small periodic effects in:
- Groundwater levels
- Earth's crust
- The atmosphere
The existence of solid Earth tides is a useful reminder that tidal forces act on the entire planet—not just the oceans.
Do Tides Slow Earth's Rotation?
Yes, very gradually.
Ocean tides interact with the seafloor and coastlines, producing friction and dissipating energy.
This process transfers angular momentum within the Earth-Moon system.
Over very long timescales:
- Earth's rotation gradually slows
- The length of the day gradually increases
- The Moon slowly moves farther away from Earth
These changes are extremely small from one human lifetime to the next but become significant over geological timescales.
Useful Facts About Tides
1. The Moon Is More Important for Tides Than the Sun
The Sun pulls much more strongly on Earth overall, but tidal force depends on the difference in gravitational pull across Earth's diameter. Because the Moon is much closer, its tide-generating effect is larger.
2. Most Tide Schedules Follow the Moon, Not the 24-Hour Solar Day
A lunar day lasts about 24 hours and 50 minutes. This is why high tides at many locations occur roughly 50 minutes later from one day to the next.
3. Spring Tides Happen About Twice Each Lunar Month
They occur near new and full moons, when lunar and solar tidal forces reinforce each other.
4. Neap Tides Also Happen About Twice Each Lunar Month
They occur near the quarter moons, when the Sun's and Moon's tide-generating effects partially oppose each other.
5. Tide Height Is Strongly Local
The same astronomical forcing can produce very different tide ranges at different coastlines because water depth, basin shape, channels, and resonance alter the tidal response.
6. Solid Rock Moves With the Tides Too
The Moon and Sun also slightly deform the solid Earth. Scientists can measure these Earth tides with sensitive geophysical instruments.
Are Tides the Same as Tsunamis?
No.
Tides and tsunamis are completely different phenomena.
Tides are predictable, periodic changes produced by gravitational interactions involving the Moon and Sun.
Tsunamis are long ocean waves most commonly generated by sudden disturbances such as:
- Undersea earthquakes
- Landslides
- Volcanic activity
The older phrase “tidal wave” is therefore misleading when used to describe a tsunami.
Frequently Asked Questions
Does the Moon pull ocean water toward it?
Yes, but the important tidal effect arises from the Moon's gravity being slightly stronger on the near side of Earth and slightly weaker on the far side. This difference creates the tide-generating force.
Why is there a high tide on the side opposite the Moon?
The far-side high tide results from the differential gravitational field across Earth. Relative to Earth's center, the far side experiences a smaller lunar acceleration, producing stretching along the Earth-Moon direction.
Why doesn't the Sun cause bigger tides than the Moon?
Although the Sun's total gravitational attraction on Earth is stronger, it is much farther away. Tidal force depends strongly on distance, so the closer Moon produces a larger tide-generating effect.
Why are there usually two high tides each day?
In the simplified equilibrium model, Earth has two tidal bulges. As Earth rotates through them, many locations experience two high and two low tides during a lunar day.
Why are tides about 50 minutes later each day?
Because the Moon moves eastward in its orbit while Earth rotates, Earth must rotate slightly more than 360 degrees to bring a location back into alignment with the Moon. A lunar day is about 24 hours and 50 minutes.
What is a spring tide?
A spring tide is a period of larger-than-average tidal range near a new or full Moon, when the solar and lunar tide-generating effects reinforce one another.
What is a neap tide?
A neap tide occurs near the quarter Moon, when the Sun and Moon are roughly at right angles, and their tidal effects partially counteract each other.
Does a full moon always cause the highest tide?
Full moons are associated with spring tides, but the actual highest tide depends on lunar distance, local geography, weather, atmospheric pressure, winds, and other ocean conditions.
Does a supermoon cause stronger tides?
When a new or full Moon occurs near perigee, tidal ranges can be somewhat larger than usual. NOAA commonly refers to this as a perigean spring tide.
Do lakes have tides?
Large lakes can experience gravitational tides, but the changes are usually much smaller than ocean tides and can be masked by wind, atmospheric pressure, and other water-level variations.
Does the solid Earth experience tides?
Yes. The Moon and Sun deform Earth's crust slightly in predictable cycles called solid Earth tides.
Do other planets affect Earth's tides?
Their tide-generating effects on Earth are negligible compared with those of the Moon and Sun.
Why are tides different in different places?
Continents, ocean depth, basin shape, channels, resonance, Earth's rotation, and local coastline geometry all modify the tide-generating forces and tidal waves.
Can tides be predicted?
Yes. Because the astronomical components of tides are highly regular, tide predictions can be calculated far in advance. Weather and other local conditions can still cause actual water levels to differ from predicted astronomical tides.
Final Thoughts
The Moon does not simply “pull the oceans upward.”
Tides arise because the Moon's gravitational pull changes slightly across Earth's diameter.
That differential force stretches Earth and its oceans, while the Sun adds its own smaller tidal influence.
The positions and distances of the Moon and Sun create predictable cycles such as spring and neap tides.
From there, Earth's real geography takes over.
Continents, basin shape, ocean depth, resonance, and rotation transform the simple astronomical forcing into the complex tide patterns observed at coastlines around the world.
The Moon provides most of the tide-generating force, but the shape of Earth's oceans determines how that force appears at your local shore.