The regular, rhythmic rise and fall of sea level, known as the tide, is a fundamental process shaping coastal environments. This movement is primarily a consequence of the gravitational interaction between the Earth, the Moon, and to a lesser extent, the Sun. For the vast majority of coastlines worldwide, the tidal pattern is remarkably consistent, resulting in two high tides and two low tides occurring over the course of approximately one day.
The Standard Semidiurnal Cycle
The most common tidal pattern observed globally is the semidiurnal cycle, which means “half-daily.” This cycle is characterized by a shore experiencing two high tides and two low tides of roughly equal height. The full cycle takes about 24 hours and 50 minutes to complete, meaning the period between one high tide and the next is approximately 12 hours and 25 minutes. Consequently, the water level takes about 6 hours and 12.5 minutes to transition from high tide to low tide, or vice versa.
The Gravitational Mechanics of Tides
Tides are generated by the gravitational pull of the Moon and the Sun, which creates two large bulges in the Earth’s oceans. The Moon’s gravitational force is the primary driver because, although it is much smaller than the Sun, it is significantly closer to Earth. This proximity means its gravitational influence on Earth’s oceans is more than twice that of the Sun.
One tidal bulge forms on the side of the Earth directly facing the Moon, where the Moon’s gravity pulls the water toward it. A second, equally important bulge forms on the side of the Earth opposite the Moon because the Moon’s gravitational pull is weaker there, allowing the ocean water to be left behind as the solid Earth is pulled toward the Moon. The planet rotates once on its axis approximately every 24 hours beneath these two stationary bulges, causing a coastal location to experience a high tide as it passes through the center of each bulge.
Regional Differences in Tidal Patterns
While the semidiurnal pattern is the most common, local geography and the angle of the Moon’s orbit lead to significant regional variations. The shape and depth of an ocean basin, as well as the presence of continental landmasses, interfere with the free movement of the tidal bulges, modifying the local tidal signature. These factors result in two other primary tidal classifications.
Some regions experience a diurnal tidal cycle, which involves only one high tide and one low tide per lunar day, such as portions of the Gulf of Mexico. Other coasts have a mixed semidiurnal cycle, where there are still two high tides and two low tides, but the heights of the successive high tides are markedly unequal. The Pacific Coast of the United States is a well-known example of a mixed tidal area.
The Daily Delay of High Tide
The high tide does not occur at the exact same time every day, but is instead delayed by about 50 minutes due to the Moon’s orbital motion. A solar day is defined by the Earth completing one full rotation relative to the Sun, taking 24 hours. During that time, the Moon has continued its orbit around the Earth in the same direction as the Earth’s rotation.
The Earth must rotate for an extra 50 minutes to “catch up” to the Moon and bring a specific coastal location back under the point of maximum lunar gravitational influence. This extended period is known as the lunar day, which lasts 24 hours and 50 minutes. Since the two high tides are spaced half of a lunar day apart, the time of high tide shifts later by 50 minutes each successive solar day.