The Science Behind Tides: Causes, Types, and Theories

Every day, coastal communities around the world witness a phenomenon that has shaped human life for millennia-the rhythmic rise and fall of ocean waters we call tides. These movements are far from random. They represent one of nature’s most predictable patterns, driven by the gravitational dance between Earth and its celestial neighbors.

How tides originate 🔗

The story of tides begins with gravity. In 1687, Sir Isaac Newton first explained that ocean tides result from the gravitational attraction of the sun and moon on Earth’s oceans. While this might seem straightforward, the mechanics behind it reveal something fascinating about how forces work across distance.

The moon, despite being far smaller than the sun, exerts the dominant tidal force on Earth. This happens because tidal forces don’t simply depend on mass-they vary inversely with the cube of distance. The sun is 27 million times more massive than the moon, but it sits 390 times farther from Earth. When you account for distance, the moon’s tide-generating force turns out to be approximately twice that of the sun.

But why do we experience two high tides each day rather than just one when the moon is overhead? The answer lies in understanding two opposing forces. On the side of Earth facing the moon, gravitational pull draws ocean waters toward it, creating a bulge. On the opposite side, something equally important happens: the Earth-moon system rotates around a common center of gravity called the barycenter. This rotation generates centrifugal force that pushes water away from the moon, creating a second bulge on the far side of Earth.

As Earth rotates through these two bulges during each day, most coastal locations experience two high tides and two low tides. This explains the basic tidal pattern, though the actual timing and height of tides vary significantly based on location and other factors.

Understanding tidal variations 🔗

Tides don’t present themselves uniformly across the globe. Based on how frequently high and low tides occur, oceanographers classify them into three main types.

Semidiurnal tides 🔗

Most areas experience two high tides and two low tides each day of approximately equal size-a pattern called semidiurnal. The eastern coast of North America typically experiences this type, where the two daily high tides reach similar heights, as do the two low tides.

Diurnal tides 🔗

Some regions experience only one high tide and one low tide per day. The Gulf of Mexico provides a classic example of diurnal tides, where the daily tidal cycle is compressed into a single rise and fall.

Mixed semidiurnal tides 🔗

Areas with mixed semidiurnal tides experience two high and two low tides daily, but the heights differ significantly. The U.S. West Coast demonstrates this pattern clearly-one high tide reaches considerably higher than the other, and the same variation appears in the low tides.

Spring and neap tides 🔗

Beyond daily patterns, tides also vary throughout the lunar month. During new and full moons, when the sun, moon, and Earth align, their combined gravitational forces create spring tides with the greatest difference between high and low water. The term “spring” has nothing to do with the season-it comes from the concept of water “springing forth.”

Conversely, when the moon is in its first or third quarter, the sun and moon pull at right angles to each other. Their gravitational forces partially cancel each other out, producing neap tides with smaller tidal ranges.

Distance-related tidal variations 🔗

The elliptical orbits of both the moon around Earth and Earth around the sun introduce additional variations. When the moon reaches perigee (its closest point to Earth), tide-generating forces increase, producing above-average tidal ranges. About two weeks later at apogee (the farthest point), tidal ranges decrease. Similar patterns occur based on Earth’s distance from the sun, with enhanced tides around January when Earth is closest to the sun (perihelion) and reduced tides around July when Earth is farthest (aphelion).

Scientific theories explaining tidal behavior 🔗

Understanding what causes tides is one thing; predicting their actual behavior in the world’s oceans is quite another. Scientists have developed several theories over the centuries to explain tidal phenomena.

Newton’s equilibrium theory 🔗

Isaac Newton’s equilibrium theory provided the foundation for understanding tides. Newton envisioned an ideal ocean of uniform depth covering the entire Earth, where gravitational forces would create two static bulges-one facing the moon and one on the opposite side. This model successfully explains basic tidal phenomena like the 12-hour tidal period and the spring-neap cycle.

However, Newton’s theory assumes the ocean can instantly adjust to gravitational forces and ignores the effects of continents, ocean depth variations, and Earth’s rotation. While useful for understanding fundamental principles, it fails to predict the actual tides observed at coastal locations.

Laplace’s dynamic theory 🔗

Nearly a century after Newton, Pierre-Simon Laplace revolutionized tidal science with his dynamic theory in 1775. Rather than treating tides as static bulges, Laplace described them as waves moving through the ocean in response to tidal forces.

Laplace’s theory incorporates friction, resonance, ocean basin geometry, and Earth’s rotation. It explains why tidal ranges vary dramatically across the globe-from just 10 centimeters in the Mediterranean Sea to 17 meters in Canada’s Bay of Fundy.

The theory also accounts for the Coriolis effect, which causes tidal waves to follow circular patterns around ocean basins rather than moving in straight lines. This creates amphidromic systems-large-scale circulation patterns where tidal crests rotate around central points of minimal tidal variation.

Progressive wave theory 🔗

Building on Laplace’s work, William Whewell developed the progressive wave theory in the 1830s. He recognized that tides propagate across ocean basins like waves, interrupted and redirected by continents. Whewell created cotidal charts showing lines connecting points experiencing high tide simultaneously, revealing how tidal waves move across oceans. This theory helped explain why tidal timing varies along coastlines and why enclosed seas show different tidal characteristics than open oceans.

Stationary wave theory 🔗

In the early 20th century, George Darwin and Rollin Harris proposed that ocean basins respond to tidal forcing like resonant systems. Just as a guitar string vibrates at certain natural frequencies, ocean basins have natural periods of oscillation. When tidal forcing frequencies match these natural periods, tidal ranges can be dramatically amplified through resonance. This explains extreme tidal ranges in certain locations, like the Bay of Fundy, where the basin’s natural oscillation period aligns closely with the semidiurnal tidal period.

Modern tidal prediction 🔗

Today’s tidal predictions combine elements from all these theories with harmonic analysis-a mathematical technique pioneered by Lord Kelvin in the 1860s. Scientists identify and track nearly 400 separate tidal constituents, each representing different astronomical influences on the tides. This approach, refined by A.T. Doodson in the 1920s, forms the basis of modern tidal prediction systems used worldwide for navigation, coastal management, and disaster preparedness.

Understanding tides remains crucial for coastal communities. Accurate tidal predictions help prevent ships from running aground, inform construction projects in coastal zones, and provide essential data for managing fisheries. In the context of disaster management, tidal information becomes critical when combined with storm surge predictions during tropical cyclones, helping authorities plan evacuations and emergency responses.

What do you think? How might rising sea levels due to climate change interact with existing tidal patterns to affect coastal communities? What role should tidal prediction play in planning coastal infrastructure for the future?

References

1. https://oceanservice.noaa.gov/education/tutorial_tides/tides02_cause.html
2. https://science.nasa.gov/moon/tides/
3. https://oceanservice.noaa.gov/education/tutorial_tides/tides07_cycles.html
4. https://www.timeanddate.com/astronomy/moon/tides.html
5. https://oceanservice.noaa.gov/facts/springtide.html
6. https://oceanservice.noaa.gov/education/tutorial_tides/tides06_variations.html
7. https://www.vims.edu/research/units/labgroups/tc_tutorial/static.php
8. https://en.wikipedia.org/wiki/Theory_of_tides
9. https://rwu.pressbooks.pub/webboceanography/chapter/11-2-dynamic-theory-of-tides/