The ocean’s surface is characterized by the predictable, periodic rise and fall of sea level known as the tides. This rhythm is observable along nearly all coastlines, where the water level climbs to a peak (high tide) before receding to a minimum (low tide). The most common pattern observed globally is the semidiurnal tide, meaning coastal locations experience two high tides and two low tides of roughly equal height each day. This consistent, twice-daily cycle results directly from the Earth’s relationship with the Moon.
The Moon’s Gravitational Influence
The primary force dictating the ocean’s rhythm is the gravitational pull of the Moon. Despite the Sun’s size, the Moon’s proximity grants it more than twice the tide-generating influence on Earth’s oceans. This gravitational force is not uniform; it acts as a differential force, meaning its strength changes based on distance across the Earth’s surface.
The side of Earth directly facing the Moon experiences the strongest gravitational attraction. Here, the ocean water is pulled outward and upward toward the Moon, forming the first large water bulge. This direct pull draws the seawater away from the solid Earth beneath it, gathering the water into an elevated mound. This bulge accounts for one of the two daily high tides a coastal observer experiences.
Why Two Water Bulges Form
The semidiurnal cycle requires a second tidal bulge on the side of the Earth facing away from the Moon. This second bulge forms not due to gravity pulling the water outward, but because of inertia related to the orbital motion of the Earth-Moon system. The Earth and Moon revolve around a common center of mass, known as the barycenter, located approximately 4,670 kilometers from the Earth’s center.
As the Earth and Moon orbit this shared barycenter, the Earth is constantly being pulled toward the Moon. This orbital motion generates an inertial effect—often simplified as “centrifugal force”—that is uniform across the entire Earth. On the side furthest from the Moon, the Moon’s gravitational pull on the water is at its weakest.
The solid body of the Earth is pulled toward the Moon more strongly than the water on the far side, effectively pulling the Earth away from that distant water. This leaves the water behind, creating the second tidal bulge opposite the Moon. Thus, the Moon’s differential gravitational force and the resulting inertial force work in tandem to create two elevated regions of water on opposite sides of the planet.
The two bulges are separated by regions where the gravitational and inertial forces are balanced, causing the water level to be at its lowest. These regions are located 90 degrees away from the Moon and correspond to the two low tides that occur daily. The entire system of two high-tide bulges and two low-tide troughs is aligned along the line connecting the centers of the Earth and the Moon.
How Earth’s Rotation Determines Timing
The twice-daily cycle of the tides results from the Earth rotating on its axis beneath these oceanic bulges. As the planet spins, any coastal location passes through both the near-side and far-side bulges during one full rotation. When a point passes directly under a bulge, it experiences a high tide; when it passes through the trough between the bulges, it experiences a low tide.
If the Moon were stationary, this cycle would align perfectly with the 24-hour solar day. However, the Moon constantly moves in its orbit around the Earth, in the same direction that the Earth rotates. By the time Earth completes a 24-hour rotation, the Moon has moved slightly ahead. The Earth must therefore rotate for an extra 50 minutes to bring a coastal location back directly beneath the Moon.
The total time required for a specific point on Earth to return to the same position relative to the Moon is known as a lunar day, lasting approximately 24 hours and 50 minutes. Since a coastal area passes through two high-tide bulges during this period, the interval between consecutive high tides is about 12 hours and 25 minutes. This slight daily shift explains why the timing of high and low tides changes by nearly an hour from one solar day to the next.
Geographical Constraints and Tidal Variations
While the two-bulge mechanism establishes the semidiurnal pattern as the global norm, the actual manifestation of tides varies significantly due to local geography. Continents act as obstructions, blocking the free movement of the tidal bulges and forcing the tidal wave to navigate complex ocean basins. The shape of the coastline, the depth of the continental shelves, and the presence of narrow bays can dramatically modify the tidal wave.
Diurnal Tides
In certain locations, such as the Gulf of Mexico, the interplay of basin size and the tidal period can lead to resonance effects. This results in a diurnal pattern, where only one high and one low tide occur per lunar day.
Mixed Semidiurnal Tides
Other regions, particularly along the Pacific coast of North America, experience mixed semidiurnal tides. These still feature two high and two low tides, but with significant differences in height between the two highs and the two lows.
The Sun also plays a secondary role in modifying the height of the two daily tides. When the Sun, Moon, and Earth align during a New or Full Moon, their gravitational forces combine to produce larger-than-average tidal ranges known as Spring Tides. Conversely, when the Sun and Moon are at a 90-degree angle to the Earth, their forces partially cancel out, resulting in smaller-than-average Neap Tides.