The rhythmic rise and fall of sea level, known as the tide, is a fundamental phenomenon of our planet’s oceans. Most coastal locations around the world experience a pattern of two high tides and two low tides within a period slightly longer than a solar day. This semi-diurnal cycle is a direct result of the gravitational interaction between the Earth, the Moon, and to a lesser extent, the Sun. Understanding why there are two high tides each day requires understanding the complex interplay of forces that distort the Earth’s oceans into two simultaneous bulges.
The Primary High Tide: Gravitational Pull
The most intuitive part of the tidal mechanism is the gravitational attraction between the Earth and the Moon. Isaac Newton’s law of universal gravitation explains that the strength of this pull is inversely proportional to the square of the distance between them. Because the Moon is relatively close to Earth, its gravitational influence is the dominant force in generating tides. The water on the side of Earth facing the Moon is significantly closer to the Moon than the Earth’s center. Consequently, the water is pulled toward the Moon more strongly than the solid Earth beneath, creating the first, or primary, tidal bulge.
The Mechanism of the Second High Tide
The existence of a second tidal bulge on the side of Earth facing away from the Moon is equally important. This phenomenon is explained by considering the entire Earth-Moon system as two bodies orbiting a common center of mass, known as the barycenter. Both the Earth and the Moon revolve around this point, which is located beneath the Earth’s surface.
As the Earth-Moon system rotates around the barycenter, every part of the Earth experiences an outward-directed inertial force due to this shared orbital motion. This force acts equally on all parts of the Earth. On the side farthest from the Moon, the Moon’s gravitational pull is at its weakest due to the greater distance.
On this far side, the outward inertial force overcomes the Moon’s comparatively weaker inward gravitational pull. This effectively pulls the solid Earth away from the water, resulting in the formation of the second tidal bulge. This ensures that the Earth’s oceans are distorted into a football-like shape with two bulges aligned roughly with the Moon.
As the Earth rotates on its axis approximately every 24 hours, a coastal location passes through both of these tidal bulges once. When a location is directly under one of the bulges, it experiences a high tide. The two high tides and two low tides per day are a direct consequence of the Earth rotating beneath these two stationary bulges.
The Timing Cycle: Why Not Exactly 12 Hours
The high tides are not precisely 12 hours apart because the Moon is constantly moving in its orbit around the Earth in the same direction that the Earth rotates. The Earth completes one full rotation in a solar day, which is 24 hours. However, during that time, the Moon has advanced in its orbit. For a specific point on Earth to return to its position directly under the tidal bulge, it must rotate for an additional amount of time to “catch up” to the Moon’s new orbital position. This extra rotation takes approximately 50 minutes. Therefore, a full tidal cycle, known as a lunar day, lasts 24 hours and 50 minutes. Since the Earth rotates through two high-tide bulges in that lunar day, the time between one high tide and the next is 12 hours and 25 minutes.
Solar Influence on Tidal Magnitude
While the Moon is the primary engine driving the timing and occurrence of the two daily high tides, the Sun’s gravity also creates its own, smaller set of tidal bulges on Earth. Although the Sun is vastly more massive than the Moon, its immense distance means its tide-generating force is only about half as effective as the Moon’s. The Sun’s influence becomes noticeable not in the timing of the tides, but in their magnitude, or height.
Spring Tides
The highest and lowest tides occur during a phenomenon known as Spring Tides, which happens twice a month. Spring Tides occur when the Sun, Earth, and Moon are aligned in a straight line, which is during the full and new moon phases. In this alignment, the gravitational forces of the Sun and Moon combine, reinforcing each other to produce higher high tides and lower low tides than average.
Neap Tides
Conversely, moderate tides known as Neap Tides occur when the Sun and Moon are at right angles to the Earth, which is during the first and third quarter moon phases. In this arrangement, the Sun’s gravitational pull partially cancels out the Moon’s pull, resulting in a smaller difference between high and low tide levels. These variations in magnitude demonstrate the combined, yet distinct, roles of both the Moon and the Sun in the ocean’s rhythmic movements.