Tides are the predictable rise and fall of sea levels, driven by the gravitational interaction between the Earth, the Moon, and the Sun. A full tidal cycle, known as a tidal day or lunar day, lasts approximately 24 hours and 50 minutes. This duration means that in most coastal regions, the interval between one high tide and the next is about 12 hours and 25 minutes.
The Fundamental Duration of the Tidal Day
The basic rhythm of the ocean’s tide is governed by the length of time it takes for a specific point on Earth to rotate back to the same position relative to the Moon. This lunar day is the true time frame for the tidal forces acting on the Earth. The gravitational pull of the Moon creates a bulge of water on the side of the Earth facing it, and a corresponding bulge on the opposite side due to inertia.
Locations that experience two high tides and two low tides each cycle follow a semidiurnal pattern. In this pattern, the water level moves from high tide to low tide in about 6 hours and 12.5 minutes. This predictable pattern of two highs and two lows within the 24 hour and 50 minute lunar day establishes the baseline for tidal timing across the globe.
The Influence of Lunar Movement on Tidal Timing
The tidal day is 50 minutes longer than the solar day because the Moon is constantly moving in its orbit around the Earth. While the Earth completes one full rotation on its axis in 24 hours, the Moon shifts its position eastward in the same direction that the Earth spins.
After the Earth has rotated for 24 hours, a location on the surface is no longer directly under the Moon. The Earth must continue to rotate for an additional 50 minutes to “catch up” to the Moon’s new position in its orbit. This extra rotation time is necessary to bring that specific point back under the direct influence of the Moon’s gravitational pull. The 50-minute delay is a direct consequence of the Moon’s orbital motion, ensuring that the tides consistently arrive later each solar day.
Variations in Local Tidal Patterns
Although the Moon dictates the underlying 24-hour, 50-minute cycle, the actual pattern observed at the coast is modified by geographic factors. Large landmasses, ocean depth, and the shape of the seafloor impede the free movement of the tidal bulges, establishing three main categories of tidal patterns. The semidiurnal pattern, featuring two high tides and two low tides of approximately equal height every lunar day, is the most common. This pattern is typical along the eastern coasts of North America and much of Europe.
Diurnal Pattern
Some regions, such as the Gulf of Mexico, experience a diurnal pattern, characterized by only one high tide and one low tide within the lunar day. This single-cycle pattern is less common globally than the semidiurnal type.
Mixed Tide Pattern
A third category is the mixed tide, which still has two high and two low tides, but they are of unequal height. One high tide may be substantially higher than the other. This pattern is common along the Pacific coast of North America.