The cyclical rise and fall of sea level, known as the tide, is a visible demonstration of Earth’s connection to the cosmos. While a standard pattern exists, the actual number of high and low tides a coastline experiences daily is not always simple. Tides are vast, slow-moving waves traveling across ocean basins, driven by gravitational forces.
The Standard Tidal Cycle
Most coastal locations follow a predictable rhythm, experiencing two high tides and two low tides. This cycle does not align with the 24-hour solar day, but instead follows the timing of the Moon. The complete cycle occurs over a tidal day, which lasts approximately 24 hours and 50 minutes. This extra 50 minutes accounts for the time needed for a point on Earth to rotate back to the same position relative to the Moon, which has moved slightly in its orbit.
The Gravitational Mechanism
The primary forces behind the tides are the gravitational pulls exerted by the Moon and, to a lesser extent, the Sun. Due to its close proximity to Earth, the Moon is the dominant driver of tidal forces, having about twice the effect of the Sun. Tides are generated not by the strength of gravity itself, but by the difference in gravitational pull across the diameter of the Earth, known as the differential or tidal force.
The Sun’s gravity acts as a secondary, modulating force, amplifying or reducing the Moon’s effect depending on their alignment. When the Sun, Earth, and Moon align in a straight line (during new and full moons), their forces combine to create the largest tidal range, known as spring tides. Conversely, when the Sun and Moon are positioned at a 90-degree angle relative to Earth (during the first and third quarter moons), their gravitational pulls partially counteract each other, resulting in smaller tidal ranges called neap tides.
Why Two Tides Occur Simultaneously
The presence of two high tides and two low tides results directly from the differential gravitational force creating two simultaneous bulges of water on opposite sides of the planet. The Moon’s gravity pulls the ocean water toward it, creating the first tidal bulge on the side of Earth facing the Moon. This is the intuitive effect, where the water is directly pulled outward.
The second, less intuitive bulge forms on the side of the Earth farthest from the Moon. This bulge is caused by the inertia of the water, which is attempting to move in a straight line, combined with the fact that the Moon’s gravity pulls the solid Earth more strongly than it pulls the distant water. Essentially, the Moon’s gravity pulls the entire mass of the Earth away from the water on the far side. This leaves the water behind, bulging outward in the opposite direction from the Moon.
As the Earth rotates beneath these two fixed water bulges, any point on the planet’s surface passes through a high-water area, a low-water area, the second high-water area, and finally the second low-water area. This continuous rotation causes the rhythmic sequence of two high and two low tides over the course of the 24-hour and 50-minute tidal day.
Geographic Influence on Tidal Patterns
While the gravitational forces of the Moon and Sun drive the global tidal mechanism, local geography dictates the actual observed pattern at any specific coastline. If Earth were perfectly smooth and covered entirely by a deep ocean, all areas would experience the standard semi-diurnal pattern of two equal high and two equal low tides. However, continental landmasses, varying ocean depths, and the shape of coastlines significantly modify the movement of the tidal bulges.
These geographic constraints lead to three main types of tidal regimes observed worldwide.
Semi-Diurnal Pattern
This pattern, common along the U.S. Atlantic coast, involves two high and two low tides of approximately equal height per tidal day.
Mixed Semi-Diurnal Pattern
This pattern, seen along the U.S. Pacific coast, also has two high and two low tides daily, but the heights of the two high tides and the two low tides are significantly unequal.
Diurnal Pattern
This pattern is characterized by only one high tide and one low tide within the 24-hour and 50-minute cycle. This is often observed in partially enclosed basins, such as certain areas of the Gulf of Mexico, where the geometry of the coastline and ocean floor restricts the natural oscillation of the water. While the astronomical answer to the number of low tides is usually two, the local answer depends entirely on the unique hydrological characteristics of the coastline.