Ocean tides are the rhythmic rise and fall of sea levels globally. Most areas experience two high tides and two low tides daily, with approximately 12 hours and 25 minutes separating successive high tides. This regular pattern is a fundamental aspect of Earth’s interaction with celestial bodies.
The Moon’s Gravitational Pull
The Moon’s gravitational pull is the primary driver of ocean tides. As the Moon orbits Earth, its gravity attracts ocean water. The side of Earth directly facing the Moon experiences the strongest pull, causing water to bulge outwards. This bulge forms one of the two daily high tides.
Gravitational force weakens with increasing distance. The Moon’s pull is strongest on the side of Earth closest to it and progressively weaker on the opposite side. This differential gravitational force creates the tidal effect.
Understanding the Far-Side Bulge
A second high tide occurs on the side of Earth opposite the Moon. While the Moon’s gravity directly pulls water on the near side, it also affects the entire Earth. The Moon’s gravitational pull is stronger on the solid body of Earth than on the water on the far side. This difference causes the solid Earth to be pulled away from the water on its far side.
As the solid Earth is drawn towards the Moon, water on the opposite side, experiencing a weaker pull, lags behind. This relative displacement creates a second bulge of water on the side farthest from the Moon. Earth rotates through these two distinct tidal bulges daily, leading to two high tides.
Why Tides Occur Every 12 Hours and 25 Minutes
Earth’s rotation brings locations through these two tidal bulges daily. As a point on Earth passes through one bulge, it experiences a high tide, and approximately 12 hours later, another high tide as it passes through the second bulge.
The period between high tides is not exactly 12 hours because the Moon moves in its orbit around Earth. The Moon orbits in the same direction Earth spins. By the time Earth completes one rotation (24 hours), the Moon has advanced slightly. Earth needs to rotate an additional 50 minutes, on average, for a location to realign with the Moon. This extended period, a lunar day (approximately 24 hours and 50 minutes), causes high tides to be separated by about 12 hours and 25 minutes.
Additional Factors Influencing Tides
While the Moon is the primary influence on Earth’s tides, the Sun also plays a role due to its gravitational pull. Although the Sun is far more massive than the Moon, its greater distance from Earth means its tidal influence is about half that of the Moon. When the Sun, Moon, and Earth align, typically during new and full moons, their combined gravitational forces create enhanced high tides and lower low tides, known as spring tides.
Conversely, when the Sun and Moon are at right angles to each other relative to Earth, usually during the first and third quarter moon phases, their gravitational pulls partially counteract. This results in weaker high tides and higher low tides, referred to as neap tides. Local geography, such as coastline shape, ocean basin depths, and narrow inlets or bays, can also significantly modify tidal patterns and the range between high and low tides. Wind and atmospheric pressure can cause minor variations in predicted tidal heights.