The regular rise and fall of the ocean’s surface, known as the tide, is a predictable effect of gravity. Tides are giant waves primarily driven by the gravitational pull of the Moon and, to a lesser extent, the Sun. The cycle of water level changes, from high tide to low tide and back again, is referred to as the tidal cycle. Understanding the length of this cycle requires considering the Moon’s orbital movement rather than the standard 24-hour solar day.
The Length of the Full Tidal Day
The full period for a tidal cycle to repeat itself is 24 hours and 50 minutes, a duration known as the lunar day. This timing is determined by the Earth’s rotation relative to the Moon, not the Sun. A solar day is approximately 24 hours, defined by the time it takes for a point on Earth to return to the same alignment with the Sun.
While the Earth is spinning, the Moon is also orbiting the Earth in the same direction. Because the Moon moves slightly in its orbit, the Earth must rotate an extra 50 minutes for a point on the planet to directly face the Moon again. This longer period dictates the timing of the tides and is the measure of a complete tidal cycle.
The Common High-to-High Interval
Most coastal locations experience two high tides and two low tides during one 24-hour 50-minute lunar day. This pattern is known as a semidiurnal tide, meaning “half-daily,” and results in a high-to-high tide interval of about 12 hours and 25 minutes. This interval is exactly half of the full lunar day and is caused by the Moon’s gravitational influence on the Earth’s oceans.
The Moon’s gravity creates a bulge of water on the side of the Earth closest to it. Simultaneously, a second bulge forms on the opposite side due to the centrifugal force generated by the Earth-Moon system’s revolution. As the Earth rotates through these two bulges during the lunar day, a location passes through two high waters and two low waters. The interval between a high tide and the subsequent low tide is approximately 6 hours and 12.5 minutes.
Global Variations in Tidal Patterns
While the semidiurnal pattern is the most common globally, the standard 12-hour 25-minute interval is not universal. The shape of ocean basins, continental landmasses, and seafloor topography modify the tidal wave, leading to three distinct patterns. The semidiurnal pattern features two high tides and two low tides of approximately equal height within a lunar day, typical along the East Coast of North America and much of Europe.
A diurnal tide pattern is an exception, having only one high tide and one low tide per lunar day. This results in a full 24-hour 50-minute interval between successive high tides, commonly found in areas such as the Gulf of Mexico.
The third pattern is the mixed semidiurnal tide, which also has two high and two low tides, but with significant differences in height between them. This pattern is characteristic of the U.S. West Coast and parts of Southeast Asia.