Ocean tides are the predictable, periodic rise and fall of sea level, a phenomenon that governs coastal life and maritime activity. While the Sun exerts an immense gravitational force on Earth, the Moon is the primary driver of ocean tides. The most curious aspect of this cycle is the presence of two high tides and two low tides each day, known as semi-diurnal tides. Understanding this twice-daily rhythm requires examining the gravitational and inertial forces acting within the Earth-Moon system.
How the Moon Creates the Direct High Tide
The direct cause of the tides is the gravitational attraction between the Earth and the Moon. This force is strongest on the side of the Earth closest to the Moon, creating a distinct gradient of force across our planet. The water facing the Moon experiences the strongest gravitational tug. This direct pull draws the ocean water toward the Moon, effectively piling it up into a raised mound or bulge. This accumulation of water is the first high tide, often referred to as the direct high tide. The Moon’s proximity ensures that its gravitational gradient is steep enough to significantly deform the ocean’s surface, despite the Sun’s far greater overall mass.
The High Tide on the Opposite Side
The existence of a second, simultaneous high tide on the side of Earth facing away from the Moon is more complex. This requires understanding that the Earth and Moon revolve around a common point called the barycenter, which is the system’s center of mass. As the Earth revolves around this point, every part of the planet is subject to a constant outward force, often described as an inertial or “centrifugal” effect. This inertial force is uniform and acts in a direction opposite to the Moon. On the side of Earth farthest from the Moon, the Moon’s gravitational pull is at its weakest due to the increased distance. At this far-side location, the uniform outward inertial force is greater than the Moon’s weakened gravitational pull. This differential causes the ocean water to be pushed outward, away from the Moon, creating the second tidal bulge. This bulge is created by water being pushed away from the center of the Earth’s orbit, completing the pair of bulges necessary for the twice-daily tide cycle.
Why the Cycle Repeats Every Day
The pair of tidal bulges created by the Moon’s influence remains generally fixed relative to the Moon. The Earth rotates on its axis, completing one full rotation approximately every 24 hours. As the planet spins, any coastal location rotates underneath these two fixed bulges. When a point on Earth passes through a bulge, it experiences a high tide, and when it moves into the area between the bulges, it experiences a low tide. This rotation causes any location to pass through both tidal bulges over the course of a day, resulting in two high tides and two low tides. The cycle is not exactly 24 hours, but rather 24 hours and 50 minutes, defining what is known as a lunar day. This extra 50 minutes is required because the Moon is also orbiting the Earth, requiring the Earth to turn for an additional 50 minutes for a location to align beneath the Moon again.
How the Sun Changes Tide Height
While the Moon dictates the timing of the tides, the Sun, as the second major gravitational influence, modifies the height of the tides. The Sun’s tidal force amounts to about 46 percent of the Moon’s force. The interplay between the solar and lunar forces creates two distinct monthly tidal events. When the Sun, Earth, and Moon align in a straight line, their gravitational pulls combine, resulting in an amplified tidal effect. This alignment occurs during the full moon and the new moon phases, producing what are called Spring Tides. During a spring tide, the high tides are higher than average, and the low tides are lower than average, creating the maximum tidal range. When the Sun and Moon are positioned at a 90-degree angle relative to the Earth (during the first and third quarter moon phases), the Sun’s gravitational pull works against the Moon’s pull. These weaker tidal events are known as Neap Tides, characterized by a minimal tidal range where high tides are lower and low tides are higher than the average.