The daily rise and fall of sea levels, known as the tides, is a predictable planetary rhythm driven by gravity. This phenomenon is primarily the result of the gravitational forces exerted by the Moon and, to a lesser extent, the Sun, acting upon the fluid oceans of Earth. The seemingly simple pattern of high tide followed by low tide is actually more complex, typically featuring a cycle of two high tides and two low tides within a day. Understanding this twice-daily, or semi-diurnal, cycle requires looking at how these celestial forces create two distinct bulges of water on opposite sides of our planet.
The Gravitational Pull Creating the Nearest Bulge
The most direct cause of a high tide is the Moon’s gravitational attraction pulling water toward it. Because gravity’s strength is inversely proportional to the square of the distance between two objects, the side of Earth closest to the Moon experiences the strongest pull. This difference in force across the planet creates the tide-generating force.
The water on the side facing the Moon is pulled directly away from the Earth’s solid surface, accumulating into a bulge, which represents the first of the two daily high tides. The Moon is the dominant factor in this process, as its relative closeness to Earth gives it more than twice the tidal influence of the much more massive but distant Sun.
Explaining the Farthest Bulge
The formation of the second, opposite tidal bulge is less intuitive and is a consequence of the way the Earth and Moon orbit each other. The two bodies rotate around a common center of mass, called the barycenter. Because Earth is significantly more massive, this barycenter is located beneath our planet’s surface.
This continuous rotation around the barycenter creates an inertial force that acts equally and outwardly on every part of the Earth-Moon system. On the side of Earth facing the Moon, the Moon’s direct gravitational pull is strong enough to easily overcome this inertial force. However, on the side of Earth farthest from the Moon, the Moon’s gravitational pull is at its weakest point due to the increased distance.
At this far side, the outward-acting inertial force dominates the Moon’s diminished gravitational attraction. This causes the water to be flung outward, forming a second, distinct tidal bulge. The two bulges are created by a differential force: gravity dominates the near side, and the planetary system’s inertial rotation dominates the far side. The two bulges stretch the Earth’s oceans into an elongated oval shape that remains aligned with the Moon.
How Earth’s Spin Creates the Daily Cycle
These two bulges remain fixed, pointed toward and away from the Moon. The Earth is spinning, completing one full rotation every 24 hours. As a location on Earth rotates, it travels through both of these stationary bulges during a single spin.
Passing through the center of a bulge results in a high tide, while passing through the areas on the sides of the planet where the water is drawn away creates a low tide. This rotation beneath the two bulges generates the two high and two low tides observed daily. The full tidal cycle, known as a lunar day, is slightly longer than 24 hours, averaging 24 hours and 50 minutes.
This extra 50 minutes is necessary because the Moon moves forward in its orbit around the Earth. The Earth must rotate for an additional 50 minutes to “catch up” and bring the same location back beneath the Moon. Consequently, the timing of high and low tides shifts forward by nearly an hour each solar day.
External Factors That Modify Tidal Patterns
While the two-bulge model explains the semi-diurnal pattern, the Sun’s gravitational force introduces significant variations. When the Sun, Moon, and Earth align during the full and new moon phases, their gravitational pulls combine, resulting in the largest tidal range, known as spring tides. Conversely, when the Sun and Moon are at a 90-degree angle relative to Earth, their forces partially counteract each other, leading to smaller tidal ranges called neap tides.
Beyond these astronomical factors, local geography influences the observed tidal pattern and height. The shape of the coastline, the depth of the ocean basin, and the presence of narrow inlets can amplify or dampen the tidal wave. For example, a funnel-shaped bay can dramatically increase the height of the tide as the water is squeezed into a smaller area.
These factors can modify the standard semi-diurnal pattern into two other types. Some locations, like the Gulf of Mexico, experience diurnal tides, which feature only one high and one low tide per day. Other areas, such as the Pacific coast of the United States, see mixed semi-diurnal tides, characterized by two high and two low tides of significantly unequal heights.