High tide (left) and low tide (right) in the Bay of Fundy in Canada. Image credit: Wikimedia Commons, Tttrung. Photo by Samuel Wantman.
High tides and low tides are caused by the Moon. The Moon's gravitational pull generates something called the tidal force. The tidal force causes Earth—and its water—to bulge out on the side closest to the Moon and the side farthest from the Moon. These bulges of water are high tides.
As the Earth rotates, your region of Earth passes through both of these bulges each day. When you're in one of the bulges, you experience a high tide. When you're not in one of the bulges, you experience a low tide. This cycle of two high tides and two low tides occurs most days on most of the coastlines of the world.
This animation shows the tidal force in a view of Earth from the North Pole. As regions of Earth pass through the bulges, they can experiences a high tide.
More About Tides
Tides are really all about gravity, and when we're talking about the daily tides, it's the Moon's gravity that's causing them.
As Earth rotates, the Moon's gravity pulls on different parts of our planet. Even though the Moon only has about 1/100th the mass of Earth, since it's so close to us, it has enough gravity to move things around. The Moon's gravity even pulls on the land, but not enough for anyone to tell (unless they use special, really precise instruments).
When the Moon's gravity pulls on the water in the oceans, however, someone's bound to notice. Water has a much easier time moving around, and the water wants to bulge in the direction of the Moon. This is called the tidal force.
Because of the tidal force, the water on the side of the Moon always wants to bulge out toward the Moon. This bulge is what we call a high tide. As your part of the Earth rotates into this bulge of water, you might experience a high tide.
An illustration of the tidal force, viewed from Earth's North Pole. Water bulges toward the Moon because of gravitational pull. Note: The Moon is not actually this close to Earth.
One thing to note, however, is that this is just an explanation of the tidal force—not the actual tides. In real life, the Earth isn't a global ocean, covered in an even layer of water.
There are seven continents, and that land gets in the way. The continents prevent the water from perfectly following the Moon's pull.
That's why in some places, the difference between high and low tide isn't very big, and in other places, the difference is drastic.
That explains the first high tide each day, but what about the second high tide?
The ocean also bulges out on the side of Earth opposite the Moon.
The tidal force causes water to bulge toward the Moon and on the side opposite the Moon. These bulges represent high tides.
If the Moon's gravity is pulling the oceans toward it, how can the ocean also bulge on the side of Earth away from the Moon? It does seem a little weird. It's all because the tidal force is a differential force—meaning that it comes from differences in gravity over Earth's surface. Here's how it works:
On the side of Earth that is directly facing the Moon, the Moon's gravitational pull is the strongest. The water on that side is pulled strongly in the direction of the Moon.
On the side of Earth farthest from the Moon, the Moon's gravitational pull is at its weakest. At the center of Earth is approximately the average of the Moon's gravitational pull on the whole planet.
Arrows represent the force of the Moon's gravitational pull on Earth. To get the tidal force—the force that causes the tides—we subtract this average gravitational pull on Earth from the gravitational pull at each location on Earth.
To get the tidal force—the force that causes the tides—we subtract this average gravitational pull on Earth from the gravitational pull at each location on Earth.
Tidal force = Moon's gravitational pull in a specific location on Earth —
Moon's average gravitational pull over the whole Earth
The result of the tidal force is a stretching and squashing of Earth. This is what causes the two tidal bulges.
Arrows represent the tidal force. It's what's left over after removing the Moon's average gravitational pull on the whole planet from the Moon's specific gravitational pull at each location on Earth.
These two bulges explain why in one day there are two high tides and two low tides, as the Earth's surface rotates through each of the bulges once a day.
Does anything else affect tides?
Why are there two tides a day?
It’s early January, and the Sun is blazing. Sweltering in the hottest week of the past 100 years, temperatures well above 40 degrees, I’m seeking refuge at the local beach.
The cool, calm sea provides a welcome relief. But some of the kids are disappointed, having brought their boogie boards expecting waves. These will come, but not until later in afternoon with the incoming tide. The kids are extremely confident of this fact.
I can’t resist, so out comes my favourite question. “What causes the tides?” Oh that’s too easy – everyone knows it’s the Moon. Unanimous! But my follow-up is tougher: how come there are two tides a day? Puzzlement. In most places on Earth, there are two high tides each day.
Why is it so?
To answer this, let’s first turn to our usual suspects. In the mid-17th century, Galileo suggested tides were caused by the motion of water as Earth circled the around the Sun. It was one of the rare occasions that Galileo got something wrong. Johannes Kepler, his German rival, was closer to the mark.
Based upon ancient observations and correlations, Kepler thought the Moon must cause the tides. But Kepler’s theory could only explain one tide per day. Several decades later, Isaac Newton published his famous Principia.
The book was most famous for describing the laws of gravity, and these same laws finally explained the tides.
Gravity at work in the Cornish fishing village of Mousehole, but it’s not the Moon’s effect alone that causes two high tides a day. Credit: Getty Images
When it comes to the effects of gravity, there are three major players we have to consider: the Earth, the Moon and the Sun. For starters, let’s just think about the Earth and the Moon. The Earth’s gravity tugs on the Moon; the Moon’s gravity tugs on the Earth too. As a result, they end up in orbit around each other.
If they were the same mass, the centre of their mutual orbit would be half way between them. But the Earth is 81 times more massive than the Moon, so the centre of their orbit is much closer to the Earth – in fact it lies at a point inside the Earth about three-quarters the distance from the centre to the surface.
So there’s the Earth and the Moon pulling on each other and spinning around each other.
The pulling very slightly elongates the shape of both spheres. But the distortion is trivial. On the other hand the Earth is covered by a thin layer of ocean, which is very easy to distort. As the Moon pulls on the Earth, the ocean bulges towards it.
But remember the Earth is also spinning on its own axis once a day. So once a day the kids at the beach are under the Moon, the bulging high tide, and the great swell that comes with it.
But around six hours later, when the Earth has made a quarter turn away from the Moon, the ocean cover is at its thinnest: it’s a low tide and there’s little surf.
Another six hours and the Earth has turned, so the kids and the beach are directly on the opposite side to where the Moon is.
There’s another high tide. How come? This tide is also caused by gravity, but acting in a different way. Remember the Moon is in orbit around the Earth, and that orbital motion creates an outward force. Think of being in a car as it takes a turn at speed. You are pressed to the outside of the car, experiencing a centrifugal force.
That’s what our oceans experience here. They also feel the force of the Moon pulling from the opposite side of the planet, but the centrifugal force wins out ever so slightly, enough to make the oceans bulge out again on this side.
This pair of bulges is the Earth’s twin high tides, and they stay put, aligned with the Moon – it is the Earth and ocean rotating beneath them that causes the ocean to rise and fall twice a day in any given place.
It’s not just the Moon that pulls on our oceans. The Sun’s gravity affects our tides, too. For the same reasons given above it creates two bulges with an effect half as strong as that of the Moon.
We notice the effect of the Sun with the large spring tides, when the Sun and Moon are lined up.
About a week later there a the neap tides which are smaller tides that take place when the Sun is at right angles relative to the line of the Moon and the Earth.
Interestingly, the solid ground beneath our feet also rises and falls in tides, though this is much harder to see. One way scientists have observed it is through pressure variations in oil bodies deep beneath the surface, directly attributable to the Moon.
And in many high-energy accelerators in which sub-atomic particles travel at near-light speeds in tunnels beneath the ground, we need to apply corrections to the magnetic fields that guide them to compensate for the physical distortion of the ground as the Moon passes overhead.
In all of these systems, we see tides.
So what causes the tides? Gravity does.
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Several terms redirect here. For other uses, see Tide (disambiguation), Tidal (disambiguation), High Tide (disambiguation), High Water (disambiguation), Low Tide (disambiguation), Low Water (disambiguation), Ebb Tide (disambiguation) and Spring Tide (TV series).
Rise and fall of the sea level under astronomical gravitational influences
High tide, Alma, New Brunswick, Canada in the Bay of Fundy, 1972Low tide at the same fishing port in Bay of Fundy, 1972
Simplified schematic of only the lunar portion of Earth's tides, showing (exaggerated) high tides at the sublunar point and its antipode for the hypothetical case of an ocean of constant depth without land. Solar tides not shown.
In Maine (U.S.), low tide occurs roughly at moonrise and high tide with a high Moon, corresponding to the simple gravity model of two tidal bulges; at most places however, the Moon and tides have a phase shift.
Play media Tide coming in, video stops about 1-1/2 hours before high tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun, and the rotation of the Earth.
Tide tables can be used for any given locale to find the predicted times and amplitude (or “tidal range”).
The predictions are influenced by many factors including the alignment of the Sun and Moon, the phase and amplitude of the tide (pattern of tides in the deep ocean), the amphidromic systems of the oceans, and the shape of the coastline and near-shore bathymetry (see Timing).
They are however only predictions, the actual time and height of the tide is affected by wind and atmospheric pressure. Many shorelines experience semi-diurnal tides—two nearly equal high and low tides each day. Other locations have a diurnal tide—one high and low tide each day. A “mixed tide”—two uneven magnitude tides a day—is a third regular category.
Tides vary on timescales ranging from hours to years due to a number of factors, which determine the lunitidal interval. To make accurate records, tide gauges at fixed stations measure water level over time. Gauges ignore variations caused by waves with periods shorter than minutes. These data are compared to the reference (or datum) level usually called mean sea level.
While tides are usually the largest source of short-term sea-level fluctuations, sea levels are also subject to forces such as wind and barometric pressure changes, resulting in storm surges, especially in shallow seas and near coasts.
Tidal phenomena are not limited to the oceans, but can occur in other systems whenever a gravitational field that varies in time and space is present. For example, the shape of the solid part of the Earth is affected slightly by Earth tide, though this is not as easily seen as the water tidal movements.
Types of tides (See Timing (below) for coastal map)
Tide changes proceed via the following stages:
- Sea level rises over several hours, covering the intertidal zone; flood tide.
- The water rises to its highest level, reaching high tide.
- Sea level falls over several hours, revealing the intertidal zone; ebb tide.
- The water stops falling, reaching low tide.
Oscillating currents produced by tides are known as tidal streams. The moment that the tidal current ceases is called slack water or slack tide. The tide then reverses direction and is said to be turning. Slack water usually occurs near high water and low water. But there are locations where the moments of slack tide differ significantly from those of high and low water.
Tides are commonly semi-diurnal (two high waters and two low waters each day), or diurnal (one tidal cycle per day).
The two high waters on a given day are typically not the same height (the daily inequality); these are the higher high water and the lower high water in tide tables.
Similarly, the two low waters each day are the higher low water and the lower low water. The daily inequality is not consistent and is generally small when the Moon is over the Equator.
From the highest level to the lowest:
- Highest astronomical tide (HAT) – The highest tide which can be predicted to occur. Note that meteorological conditions may add extra height to the HAT.
- Mean high water springs (MHWS) – The average of the two high tides on the days of spring tides.
- Mean high water neaps (MHWN) – The average of the two high tides on the days of neap tides.
- Mean sea level (MSL) – This is the average sea level. The MSL is constant for any location over a long period.
- Mean low water neaps (MLWN) – The average of the two low tides on the days of neap tides.
- Mean low water springs (MLWS) – The average of the two low tides on the days of spring tides.
- Lowest astronomical tide (LAT) and Chart datum (CD) – The lowest tide which can be predicted to occur. Some charts use this as the chart datum. Note that under certain meteorological conditions the water may fall lower than this meaning that there is less water than shown on charts.
Illustration by the course of half a month
Further information: Theory of tides § Tidal constituents
See also: Earth tide § Tidal constituents
Tidal constituents are the net result of multiple influences impacting tidal changes over certain periods of time.
Primary constituents include the Earth's rotation, the position of the Moon and Sun relative to the Earth, the Moon's altitude (elevation) above the Earth's Equator, and bathymetry. Variations with periods of less than half a day are called harmonic constituents.
Conversely, cycles of days, months, or years are referred to as long period constituents.
Tidal forces affect the entire earth, but the movement of solid Earth occurs by mere centimeters. In contrast, the atmosphere is much more fluid and compressible so its surface moves by kilometers, in the sense of the contour level of a particular low pressure in the outer atmosphere.
Principal lunar semi-diurnal constituent
In most locations, the largest constituent is the “principal lunar semi-diurnal”, also known as the M2 (or M2) tidal constituent. Its period is about 12 hours and 25.
2 minutes, exactly half a tidal lunar day, which is the average time separating one lunar zenith from the next, and thus is the time required for the Earth to rotate once relative to the Moon. Simple tide clocks track this constituent.
The lunar day is longer than the Earth day because the Moon orbits in the same direction the Earth spins. This is analogous to the minute hand on a watch crossing the hour hand at 12:00 and then again at about 1:05½ (not at 1:00).
The Moon orbits the Earth in the same direction as the Earth rotates on its axis, so it takes slightly more than a day—about 24 hours and 50 minutes—for the Moon to return to the same location in the sky.
During this time, it has passed overhead (culmination) once and underfoot once (at an hour angle of 00:00 and 12:00 respectively), so in many places the period of strongest tidal forcing is the above-mentioned, about 12 hours and 25 minutes.
The moment of highest tide is not necessarily when the Moon is nearest to zenith or nadir, but the period of the forcing still determines the time between high tides.
Because the gravitational field created by the Moon weakens with distance from the Moon, it exerts a slightly stronger than average force on the side of the Earth facing the Moon, and a slightly weaker force on the opposite side.
The Moon thus tends to “stretch” the Earth slightly along the line connecting the two bodies. The solid Earth deforms a bit, but ocean water, being fluid, is free to move much more in response to the tidal force, particularly horizontally.
As the Earth rotates, the magnitude and direction of the tidal force at any particular point on the Earth's surface change constantly; although the ocean never reaches equilibrium—there is never time for the fluid to “catch up” to the state it would eventually reach if the tidal force were constant—the changing tidal force nonetheless causes rhythmic changes in sea surface height.
When there are two high tides each day with different heights (and two low tides also of different heights), the pattern is called a mixed semi-diurnal tide.
Range variation: springs and neaps
The types of tides
Animation of tides as the Moon goes round the Earth with the Sun on the right
Main article: Tidal range
The semi-diurnal range (the difference in height between high and low waters over about half a day) varies in a two-week cycle. Approximately twice a month, around new moon and full moon when the Sun, Moon, and Earth form a line (a configuration known as a syzygy), the tidal force due to the Sun reinforces that due to the Moon. The tide's range is then at its maximum; this is called the spring tide
At this point, we strongly urge you to read, or at least review, a document explaining centripetal force.
Why can't they be consistent?
This curious example shows the earth-moon system as seen looking up toward the Southern hemisphere of the earth, or else it has the moon going the “wrong way”. The accompanying text with this picture was no help at all. The (almost) universal textbook convention is to show these pictures as seen looking down on the Northern hemisphere of the earth, in which case the earth rotates counter-clockwise, and the moon orbits counterclockwise as well. It's getting so you can't trust pretty diagrams from any internet or textbook source.
Many textbook pictures show the moon abnormally close to the earth. Therefore the arrows representing the moon's gravitational forces on the earth are clearly non-parallel. But in the actual situation, drawn to scale, the moon is so far away relative to the size of the earth that those arrows in the diagram would be indistinguishable (to the eye) from parallel.
Misconceptions lead to false conclusions
These pictures, and their accompanying discussions, would lead a student to think that tides are somehow dependent on the rotation of the earth-moon system, and that this rotation is the “cause” of the tides.
We shall argue that the “tidal bulges”, which are the focus of attention in many textbooks, are in fact not due to rotation, but are simply due to the combined gravitational fields of the earth and moon, and the fact that the gravittional field due to the moon has varying direction and strength over the volume of the earth.
These bulges distort the shape of the solid earth, and also distort the oceans. If the oceans covered the entire earth uniformly, this would almost be the end of the story. But there are land masses, and ocean basins in which the water is mostly confined as the earth rotates.
This is where rotation does come into play in ocean tides, but not because of inertial effects, as textbooks would have you think.
Variations in ocean level reflect from continental shelves, setting up standing waves that cause more complicated water level variations superimposed on the tidal bulges, and in many places, these are of greater amplitude than the tidal bulge variations.
Tidal bulges move around the earth in synchronism with the moon and sun. But do not think of these as vast oceans of water moving with respect to continents. It is only the variations in water level—the surface profile of water—that follows the positions of the moon and sun in the sky.
Too often textbooks try to dismiss the tides question with a superficial analysis that ignores some things that are absolutely essential for a proper understanding. These include:
- Failure to define the specific meaning of “tide”.
- Failure to properly define and properly use the terms “centripetal” and “centrifugal”.
- Failure to say whether the analysis is being done in a non-inertial rotating system.
- Failure to warn the student that the force diagrams are different depending on whether the plane of the diagram is parallel to, or perpendicular to, the plane of the moon's orbit. If continents are shown on the earth, that's a clue. If part of the orbit of the moon is shown, that tells you that the diagram is in its orbital plane. But do students always notice these details?
- Neglect of tensile properties of solid and liquid materials.
- Neglecting to mention that liquids under stress physically move toward a lower-stress configuration.
- Failure to specify the baseline earth shape against which a tide height is measured.
They are trying to get by “on the cheap”.
So why are there tidal bulges on opposite sides of earth?
For a while we will set aside the complications of the actual earth, with continents, and look a the simpler case of an initially nearly spherical earth entirely covered with an ocean of water.
If this Earth rotates on its axis there's equatorial bulge of both earth and water, but we will treat this as a “baseline” shape upon which tidal bulges due to the earth and sun are superimposed. The ocean's shape is produced by the Earth's gravitation and its axial rotation.
The distortions of this baseline shape are called tidal effects and are entirely due to the gravitational forces of the moon and sun acting upon the earth.
The distortion of water and earth that we call a “tidal bulge” is the result of deformation of earth and water materials at different places on earth in response to the combined gravitational effects of moon and sun.
It is not simply the size of the force of attraction of these bodies at a certain point on earth that determines this. It is the variation of force over the volumes of materials (water and earth) of which the earth is composed.
Some books call this variation the differential force or tide-generating force (TGF) or simply tidal force.
Let's concentrate on the larger effect of the moon on the earth. To find how it distorts shapes of material bodies on earth we must do the calculus operation of finding the gradient of the moon's gravitational potential (a differentiation with respect to length) upon each part of the earth.
What Causes the Tides?
Tides may seem simple on the surface, but the ins and outs of tides confounded great scientific thinkers for centuries they even led Galileo astray into a bunk theory.
Today people know that the gravitational pulls between the earth, moon and sun dictate the tides. The moon, however, influences tides the most.
The moon's gravitational pull on the earth is strong enough to tug the oceans into bulge. If no other forces were at play, shores would experience one high tide a day as the earth rotated on its axis and coasts ran into the oceans' bulge facing the moon.
However, inertia — the tendency of a moving object to keep moving — affects the earth's oceans too. As the moon circles the earth, the earth moves in a very slight circle too, and this movement is enough to cause a centrifugal force on the oceans. (It's centrifugal force that holds water in a bucket when you swing the bucket in an overhead arc.)
This inertia, or centrifugal force, causes the oceans to bulge on the opposite side facing the moon.
While the moon's gravitational pull is strong enough to attract oceans into a bulge on the side of the earth facing the moon, it is not strong enough to overcome the inertia on the opposite side of the earth.
As a result, the world's oceans bulge twice once when they are on the side of Earth closest to the moon, and once when they are on the side farthest from the moon, according to the Wood's Hole Oceanographic Institution in Wood's Hole, Mass.
Geography complicates the tides, but many places on Earth experience just two high and two low tides every 24 hours and 50 minutes. (The extra 50 minutes is caused by the distance the moon moves each day as it orbits Earth).
The sun and the tides
“Solar tides” are caused by the sun's gravitational pull and are weaker than lunar tides.
The sun is 27 million times more massive than the moon, but it is also 390 times farther away. As a result, the sun has 46 percent of the tide-generating forces (TGFs) that the moon has, according to the National Oceanic and Atmospheric Administration (NOAA).
Solar tides are therefore often considered just variations on lunar tides.
Local geography can vary tide strength as well.
Just north of the coast of Maine in Canada, the Bay of Fundy has a unique funnel shape at just the right position to creates the largest tides in the world. Water in the bay can rise more than 49 feet (15 meters) or about as high as a 4-story house.
- FORCE, the Fundy Ocean Research Center for Energy, estimates the Bay of Fundy pushes110 billion tons (100 billion metric tons) of water with every tide.
- Recently, local leaders have moved to take advantage of the tides.
- In July, Maine's Governor John Baldacci and Nova Scotia's Premier Darrell Dexter signed a Memorandum of Understanding to share research and ideas in tidal and offshore wind sources of renewable energy, according to Business Weekly.
- Understanding tides: then and now
What Causes Tides?
Tides refer to the rise and fall of our oceans’ surfaces. It is caused by the attractive forces of the Moon and Sun’s gravitational fields as well as the centrifugal force due to the Earth’s spin.
As the positions of these celestial bodies change, so do the surfaces’ heights.
For example, when the Sun and Moon are aligned with the Earth, water levels in ocean surfaces fronting them are pulled and subsequently rise.
The Moon, although much smaller than the Sun, is much closer. Now, gravitational forces decrease rapidly as the distance between two masses widen. Thus, the Moon’s gravity has a larger effect on tides than the Sun. In fact, the Sun’s effect is only about half that of the Moon’s.
Since the total mass of the oceans does not change when this happens, part of it that was added to the high water regions must have come from somewhere. These mass-depleted regions then experience low water levels. Hence, if water on a beach near you is advancing, you can be sure that in other parts of the world, it is receding.
Most illustrations containing the Sun, Moon, Earth and tides depict tides to be most pronounced in regions near or at the equator. On the contrary, it is actually in these regions where the difference in high tide and low tide are not as great as those in other places in the world.
This is because the bulging of the oceans’ surface follows the Moon’s orbital plane. Now, this plane is not in line with the Earth’s equatorial plane.
Instead, it actually makes a 23-degree angle relative to it.
This essentially allows the water levels at the equator to seesaw within a relatively smaller range (compared to the ranges in other places) as the orbiting moon pulls the oceans’ water.
What Are Tides | How Do Tides Work?
They roll in and roll out. They're one of the Earth's steadiest forces, moving water from the ocean onto the land and then taking it back. Ocean tides are one of the oldest fields of scientific inquiry, dating back to approximately 330 B.C., when Greek astronomer and explorer Pytheas traveling via boat from his home in Massalia, modern day France, to the British Isles.
Tides weren't noticeable in Massalia, but Pytheas detected them on his voyage. Pytheas published a book titled On the Ocean, which discussed, among other things, the moon’s clear influence on the tides. Ever since, tides have fascinated scientific minds throughout history, including Isaac Newton.
What Are Tides?
Tides are the rise and fall of sea levels. At some parts of the day there will be more water in one location and at other parts of the day there will be less.
The tidal effect, as its known, doesn't just affect water. There's an Earth tide as well, where the solid Earth changes its shape directly due to the pressures of the Sun and the Moon.
But that's not as noticeable as what happens in the ocean.
How Does The Tidal Effect Work?
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Anything in the universe that has mass also has its own gravitational field. Sometimes, in the case of humans, that gravitational field is so tiny that they're irrelevant to our everyday lives.
But when the mass starts increasing, changes start to take place. The Earth, for example, has enough of a gravitational field to keep things on the ground, and to keep the Moon rotating around the planet.
The Moon, in turn has its own gravitational field. This field is strong enough to create a tug on the Earth's oceans, and because the Moon is in rotation around the Earth, the strength of this tug varies by location and time of day. The Moon is mostly responsible for high tide, when there's more water in areas, and low tide, when there's less.
What Causes Tides Beside the Moon?
The Moon is the biggest player in creating tides, but it's not the only planetary body involved. There's also the body with the biggest gravitational pull in the solar system, the Sun. Even though its closeness to Earth means the Moon has the bigger impact, the Sun's affect on tides is noticeable.
During new, or full, moons, the Earth, Moon, and Sun are all in alignment. That alignment allows all of those gravitational forces to join together, creating stronger tides known as spring tides. They're not associated with the Spring season at all as they occur every month.