Tides represent the rhythmic rise and fall of sea levels across the globe. The common pattern observed in most coastal regions is the semidiurnal tide, meaning the sea experiences two high tides and two low tides within a specific timeframe. Understanding this twice-daily cycle requires looking beyond a simple gravitational pull to the complex interaction of celestial mechanics involving the Earth, the Moon, and the Sun. The presence of two high tides, separated by about 12 hours and 25 minutes, is the direct result of two distinct bulges of water forming on opposite sides of the planet.
The Moon’s Primary Gravitational Pull
The Moon is the primary driver of Earth’s tides, despite its relatively small size, because of its close proximity to our planet. The gravitational force between two bodies is inversely proportional to the square of the distance between them. This differential force, known as the tidal force, means the Moon’s pull is significantly stronger on the side of Earth facing it than on the far side, stretching the Earth and its oceans.
The ocean water on the side of the Earth directly facing the Moon is pulled toward it, creating the first tidal bulge. This bulge represents one of the two daily high tides. This direct gravitational attraction alone only accounts for a single high tide per day, leaving the second one unexplained.
The High Tide on the Opposite Side
The creation of the second tidal bulge occurs on the side of the Earth facing away from the Moon. The Earth and Moon orbit a common center of mass, called the barycenter, located about 4,700 kilometers from Earth’s center. As the Earth orbits this point, all parts of the planet experience a uniform, outward-directed force, often described as inertia or a centrifugal effect.
On the side of the Earth facing the Moon, the Moon’s gravitational pull is strong enough to overcome this uniform outward force, leading to the direct bulge. Conversely, on the side facing away from the Moon, the Moon’s gravitational pull is weakest. Here, the uniform outward force is unopposed by the Moon’s gravity, causing the ocean water to bulge outward and form the second high tide. Thus, the two high tides result from the Moon’s direct pull on the near side and the planet’s orbital inertia creating an effective outward force on the far side. The two low tides occur in the areas between these bulges, where the water has been drawn away.
Earth’s Rotation and the Timing of Tides
The existence of the two tidal bulges explains why a location experiences two high tides and two low tides, but the Earth’s rotation dictates the timing. As the Earth spins on its axis, a specific point on the planet rotates through both of the stationary high-tide bulges and the two low-tide troughs in a cycle. This rotation causes the water level to rise and fall twice during one complete pass.
The full cycle of two high and two low tides averages about 24 hours and 50 minutes, not exactly 24 hours. This difference occurs because the Moon moves forward in its orbit while the Earth completes one rotation. The Earth must rotate for an extra 50 minutes to “catch up” to the Moon’s new position, ensuring alignment under the Moon’s gravitational influence again. Consequently, the time between successive high tides is approximately 12 hours and 25 minutes.
How the Sun Affects Tidal Height
While the Moon dictates the semidiurnal pattern of two tides per day, the Sun plays a modifying role by influencing the height of those tides. The Sun is significantly more massive than the Moon, but its immense distance means its tidal force is only about 46% as strong as the Moon’s. The Sun’s gravity creates its own set of tidal bulges that are superimposed onto the Moon’s.
When the Sun, Moon, and Earth align in a straight line (during the new and full moon phases), their gravitational forces combine. This additive effect produces Spring Tides, characterized by a greater-than-average tidal range, resulting in extra-high high tides and extra-low low tides. Conversely, when the Sun and Moon are positioned at a 90-degree angle relative to the Earth, their tidal forces partially counteract each other. This alignment (during the first and third quarter moon phases) results in Neap Tides, which have a smaller-than-average tidal range.