The rhythmic rise and fall of sea level, known as the tides, is primarily driven by the gravitational interaction between Earth and the Moon. Most coastal locations experience two high tides and two low tides within roughly a 24-hour cycle. This pattern results from the existence of two simultaneous bulges of ocean water on opposite sides of the planet. Understanding why there are two of these oceanic mounds requires a look into the differential forces governing the Earth-Moon system.
The Gravitational Pull
The first tidal bulge, known as the near-side bulge, is caused by the Moon’s gravitational attraction on the side of Earth closest to it. Because gravitational pull weakens with distance, the water facing the Moon is pulled with the greatest force. This stronger attraction pulls the ocean water away from the solid Earth, creating the bulge.
This phenomenon is driven by the differential gravitational force, or tidal force. Tides are caused not by the absolute strength of the Moon’s gravity, but by the difference in its pull across the Earth’s diameter. The water closest to the Moon is pulled strongly, the Earth’s center is pulled less, and the water on the far side is pulled the least. This stretching force generates the tidal phenomenon. The Moon’s proximity creates a steep gravitational gradient, resulting in a much larger tidal effect than the Sun, even though the Sun’s overall gravity is stronger.
The Inertial Counterbalance
The second tidal bulge, or far-side bulge, faces away from the Moon and arises from the mechanics of the Earth-Moon orbit. The Earth and Moon orbit a common center of mass called the barycenter. Since Earth is much more massive, the barycenter is located beneath the Earth’s surface.
As the Earth orbits this barycenter, the motion generates an inertial force that is uniform across the entire planet. This force acts like a push, always directed away from the Moon. On the far side of the Earth, the Moon’s gravitational pull is weakest, allowing the uniform inertial force to dominate. This dominance causes the ocean water on the far side to bulge outward, creating the second tidal bulge.
The Complete Tidal Model
The differential gravitational pull and the inertial counterbalance operate simultaneously to create the two tidal bulges. The net effect is an elongated, football-shaped envelope of water surrounding the globe. One bulge points toward the Moon, pulled by gravity, and the opposite bulge points away, pushed by the inertial effect.
The two bulges remain aligned with the Moon as it orbits Earth. The daily cycle of high and low tides is caused by the Earth’s rotation on its axis. As the Earth spins, coastal areas pass through these two stationary bulges. Rotating into a bulge results in high tide, and rotating through the lower water level between them results in low tide. Because of the Moon’s orbit, the cycle of two high and two low tides occurs about 50 minutes later each day.
Solar Influence on Tidal Strength
While the Moon is the primary driver of Earth’s tides, the Sun also exerts a gravitational pull that modifies the height of the lunar bulges. The Sun’s tidal force is about 46% as strong as the Moon’s. The alignment of the Sun, Earth, and Moon determines the magnitude of the tides.
When the Sun, Earth, and Moon are aligned in a straight line (during New Moon and Full Moon), their gravitational forces combine. This creates an amplified tidal bulge, resulting in the highest high tides and lowest low tides, known as Spring Tides. Conversely, when the Sun and Moon are at a right angle relative to Earth (during the first and third quarter Moon phases), their forces partially cancel. This perpendicular alignment produces Neap Tides, characterized by a smaller tidal range.