By Chris Deziel, Updated March 24, 2022
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From the earliest days of human observation, people have linked the moon’s motion to the rhythmic rise and fall of the ocean. It was Isaac Newton who mathematically explained this relationship, revealing that the tides are primarily a product of gravity.
Gravity is the main driver of tides, but the Earth’s own rotation adds a crucial centrifugal component. As the planet spins, water is pushed outward, similar to how water arcs away from a spinning sprinkler. The Earth’s gravity keeps the water from escaping into space.
When the centrifugal force interacts with the gravitational pulls of the moon and the sun, high and low tides emerge. This interaction is why most coastal locations experience two high tides each day.
Newton’s Law of Gravitation states that the force between two masses is proportional to their masses and inversely proportional to the square of their distance:
F = Gm₁m₂/d²
Although the sun is roughly 27 million times more massive than the moon, it is about 400 times farther away. When both effects are considered, the moon’s gravitational pull on the Earth is about twice that of the sun.
During a new moon, the sun and moon line up on the same side of the Earth, amplifying their combined pull and producing the highest tides of the month, known as spring tides. In contrast, a full moon places the sun and moon on opposite sides, slightly reducing the tidal range.
The Earth and moon orbit a common center of mass, the barycenter, located roughly 1,068 miles (1,719 km) beneath the Earth's surface. This mutual orbit generates an additional centrifugal effect, much like a ball on a short string spinning around.
The combined forces create a permanent bulge in the oceans. At any point on the Earth, the tide pattern can be summarized as follows:
The moon’s average motion of 13.2° per day means the first high tide shifts roughly 50 minutes later each day.
Although the sun’s tidal effect is about half as strong as the moon’s, it is essential for accurate tide predictions. Visualizing the forces as overlapping “bubbles,” the moon’s bubble is twice the sun’s. These bubbles interfere, sometimes amplifying and sometimes canceling, shaping the final tidal pattern.
Real tides differ from the idealized bubble because the Earth is not a perfect water globe. Land masses confine water into basins, and factors such as wind, water depth, coastline shape, and the Coriolis effect further modify tidal behavior.
As a result, many Atlantic coastlines experience two high tides daily, while many Pacific locations have only one.
Regular tidal ebb and flow reshape coastlines, moving sediment and continually altering shorelines. Marine organisms have evolved to thrive in these predictable conditions, and human activities such as fishing have long adapted to the tidal cycle.
Tides also represent a powerful renewable energy source. Devices that harness tidal movement—either through turbines in tidal zones or dams that compress air with the water flow—can generate substantial electricity. Because water is far denser than air, tidal turbines can produce significantly more power than wind turbines of comparable size.
