Buzz
Robin Hood's Bay in North Yorkshire, a picturesque spot on England's Northeast coast, is currently experiencing low tide. Captured beautifully in photos by R A Kearton/Getty Images.Tides have posed significant challenges in the early part of this millennium. With rising sea levels, coastal regions now face more frequent flooding during high tides. From 2000 to 2017, the U.S. saw a 50 percent rise in high tide flooding events. These floods disrupt transportation, damage critical infrastructure, and overwhelm stormwater systems.
Approximately 37 percent of the world's population resides within 62 miles (100 kilometers) of coastal areas. The increasing frequency of flooding has sparked curiosity about tidal mechanisms. What drives tides? Why do certain regions experience more extreme tidal variations? And why isn't the ocean's surface uniform globally? This exploration delves into the science and unique characteristics of Earth's tides.
The Moon's Gravitational Pull
Take a look at the illustration provided. You’ll see Earth enveloped by an ocean layer resembling the shape of a rugby ball, with two distinct bulges. One bulge appears on the side of the planet facing the moon, while the other is positioned on the exact opposite side.
What’s the reason behind these bulges? Simply put, they are largely the result of the gravitational force exerted by the moon on Earth.
This gravitational force operates in two distinct ways. It can exert a "vertical" pull, meaning it acts perpendicular to Earth’s surface. Additionally, it can create a "horizontal" pull, which acts parallel to the planet’s surface.
The point on Earth directly below the moon at any moment is referred to as the sublunar point. Conversely, the point on the opposite side of the planet, directly facing away from the moon, is known as the antipodal point.
It’s not by chance that the ocean bulges are most pronounced directly above these two locations. At both the sublunar and antipodal points, the moon’s gravitational force has no horizontal effect — a characteristic also absent at the two points on Earth positioned 90 degrees from these locations.
These four regions stand out because, unlike anywhere else on Earth, they don’t experience a horizontal force that drives ocean water toward either the sublunar point (where the moon’s gravity is strongest) or the antipodal point (where it’s weakest). This explains why the ocean rises prominently over these two areas.
The Sun and the Tides
Earth rotates fully on its axis every 24 hours. During this rotation, any specific location on the planet’s surface, such as Long Island or Australia, will move through both of these ocean bulges.
In most regions, when your location is directly beneath a bulge, you’ll experience high tide. However, as the area moves into the gap between the bulges, the tide recedes. While this is generally true, there are exceptions, as we’ll soon explore.
Now, let’s explore another key player in tidal behavior. The sun also exerts a gravitational force on the oceans, but since it’s much farther away, its impact on tides is less significant compared to the moon’s. However, this massive sphere of gas and plasma still plays a noticeable role in amplifying tidal bulges regularly.
"Tides reach their peak ... when the sun and moon align," explains Duncan Agnew, a geophysicist at the University of California, San Diego, via email.
He points out that this alignment occurs during two distinct lunar phases: Full moons and new moons, creating exceptionally large tides known as spring tides. (It’s worth noting that the term has no connection to the spring season; these tides happen year-round.)
During spring tides, the contrast between high and low tides becomes more pronounced: high tides reach exceptionally high levels, while low tides drop to unusually low levels.
Tides are less extreme when the sun and moon are positioned at right angles to each other (relative to Earth). This configuration results in what’s known as neap tides, where the difference between high and low tides is minimal.
When Continents Interfere
Prepare yourself: The situation is about to become even more intricate. While Earth is often called a "blue planet," 29 percent of its surface is covered by land. Geographic features like coves, cliffs, and other landforms can disrupt tidal patterns, amplifying them in some areas and diminishing them in others.
Most coastal regions experience two high tides daily, occurring roughly every 12 hours and 25 minutes. However, deviations from this pattern are common. "Ocean tides are a complex phenomenon driven by tidal forces acting on water that is free to move within ocean basins," explains Agnew.
Many beaches along the Gulf of Mexico experience only one high tide each day, a result of restricted water movement. In Nova Scotia, water entering the V-shaped Bay of Fundy is forced upward as it travels inland, creating significant differences between low and high tides, known as bore tides.
Tides can serve as a renewable energy source. The Rance Tidal Power Station in Brittany, France, has harnessed tides from the English Channel to produce electricity since 1966. A larger facility of this kind was later constructed in South Korea.
