The daily rise and fall of sea levels, known as tides, is a predictable phenomenon driven by celestial mechanics. The direct answer to how many tidal bulges exist on Earth at any given time is two, which are positioned on opposite sides of the planet. These two primary bulges of ocean water are responsible for the cyclical changes in water level observed along coastlines globally. The existence of these bulges is a result of the differential forces exerted by the Moon and, to a lesser extent, the Sun.
The Gravitational Force and the Near Bulge
The Moon’s gravitational pull is the primary force dictating the Earth’s tides, outweighing the Sun’s influence because of its close proximity. Gravity’s strength decreases rapidly as distance increases, meaning the Moon’s pull is not uniform across the Earth’s diameter. The water on the side of Earth facing the Moon is significantly closer to our satellite than the Earth’s center. This difference means the water on the near side experiences a stronger gravitational attraction.
This stronger pull draws the surface water outward, toward the Moon, effectively piling it up into the first tidal bulge. The fluid nature of the ocean allows the water to respond more dramatically than the solid Earth beneath it. This creation of a water bulge is a direct gravitational response to the Moon’s presence.
Explaining the Opposite Bulge
The existence of a second tidal bulge on the side of Earth farthest from the Moon is often the most confusing aspect of tidal theory. This bulge is a manifestation of the Earth-Moon system’s motion, as they both orbit a common center of mass, known as the barycenter, located about 1,700 kilometers beneath the Earth’s surface.
As the Earth revolves around this barycenter, the entire planet is subjected to a constant balancing force. On the side of the Earth farthest from the Moon, the Moon’s gravitational force is at its weakest. At this far point, the required inward gravitational force is less than the outward tendency of the water (inertia). This inertial effect causes the water on the far side to be flung outward, away from the Moon, creating the second tidal bulge.
The two bulges are formed by two different mechanisms: the near-side bulge is due to a stronger gravitational pull, and the far-side bulge is due to a weaker gravitational pull relative to the whole Earth’s movement. This differential force, or tidal force, stretches the Earth’s oceans along the line connecting the Earth and the Moon, resulting in an ellipsoid shape with bulges pointing toward and away from the Moon.
Converting Bulges into the Tidal Cycle
The presence of two bulges aligned with the Moon’s position translates directly into the observed tidal cycle. The Earth rotates on its axis beneath these two relatively stationary water bulges. As any point on the Earth’s surface rotates into one of these bulges, it experiences a high tide.
When that location rotates out from under a bulge and into the area of lower water level, it experiences a low tide. Because there are two bulges, most coastal areas pass through two high tides and two low tides during one full rotation. The full cycle takes approximately 24 hours and 50 minutes because the Moon moves farther in its orbit, requiring an extra 50 minutes for a location to return to its position directly under the Moon.
The Modifying Role of the Sun
While the Moon is the primary tidal driver, the Sun’s gravitational force also creates its own, smaller pair of tidal bulges. The Sun’s tidal generating force is about 46 percent that of the Moon’s. The interplay between the Moon’s and the Sun’s tidal forces modifies the height of the tides throughout the lunar month.
When the Sun, Earth, and Moon are aligned in a straight line, which occurs during the new and full moon phases, their gravitational forces combine. This additive effect results in a greater tidal range, producing higher high tides and lower low tides, a phenomenon known as Spring Tides. Conversely, when the Sun and Moon are at right angles to the Earth, such as during the first and third quarter moon phases, their gravitational pulls partially cancel each other out. This results in a smaller tidal range, characterized by lower high tides and higher low tides, which are called Neap Tides.