A tidal bulge is a temporary deformation of the ocean’s surface, described as a mound of water rising higher than the surrounding average sea level. This phenomenon is a direct consequence of gravitational interactions between Earth and celestial bodies, particularly the Moon. It represents a significant redistribution of the planet’s ocean water across its spherical surface. The mechanics behind the formation of these bulges are what ultimately govern the rise and fall of the ocean, creating the familiar pattern of tides.
The Primary Bulge: How Gravity Pulls Water
The formation of the first tidal bulge is the most straightforward consequence of the Moon’s presence near Earth. Gravity is the force of attraction between any two masses, and while the Moon is much smaller than the Sun, its relative closeness gives its gravitational pull a dominating influence on our oceans. Ocean water is fluid and mobile, making it highly susceptible to this external gravitational force.
The Moon’s gravitational attraction is strongest on the side of Earth directly facing it because that water is closest to the Moon. This pull draws the nearest ocean water toward the Moon, effectively lifting it up and away from the solid Earth beneath it. The result is a large, elevated mass of water positioned squarely beneath the Moon.
This mound of water is the primary tidal bulge, and its existence is intuitive: the Moon’s gravity is simply dragging the water closest to it. This explains one area of high water, but since a full tidal cycle involves two high tides per day, a separate mechanism must account for the second bulge.
Explaining the Second Bulge on the Opposite Side
The existence of a second tidal bulge on the side of Earth farthest from the Moon is a counter-intuitive effect that requires understanding the concept of differential gravity. The Moon’s gravity does not just pull on the oceans; it pulls on the entire Earth system, including the solid planet itself. The force of gravity weakens rapidly with distance, meaning the Moon’s pull is not uniform across Earth’s diameter.
The pull is strongest on the near side (where the primary bulge forms) and weakest on the far side. The solid mass of the Earth, particularly its center, is pulled toward the Moon more strongly than the water on the planet’s distant side. This difference in force across the planet is what is known as the tidal force.
This differential pull causes the solid Earth to be pulled slightly away from the distant ocean water. The water on the far side essentially lags behind as the Earth’s mass is drawn toward the Moon. This action stretches the Earth slightly into an ovoid shape, and the water left behind forms the second tidal bulge, pointing directly away from the Moon.
The overall effect is that the Moon’s gravity acts to stretch the planet, creating simultaneous mounds of water on both the side facing it and the side opposite it. These two bulges are sustained by the consistent difference in gravitational pull, rather than a single direct attraction.
Connecting Tidal Bulges to High and Low Tides
The two tidal bulges—one facing the Moon and one opposite—remain relatively fixed in their alignment with the Moon. The Earth, however, is spinning on its axis, completing one rotation approximately every 24 hours. It is this rotation of the Earth beneath the two stable bulges that causes the regular cycle of tides.
As any location on Earth rotates, it passes through the two elevated mounds of water. When a coastline enters a bulge, the local sea level rises, and that location experiences a high tide. When it rotates out of a bulge and into the area of lower water level between the two mounds, it experiences a low tide.
Since there are two bulges, the rotation causes a given point on the globe to pass through two high-water points and two low-water points during a single rotation. This explains why most coastal areas experience two high tides and two low tides each day. Because the Moon is also orbiting Earth, the tidal cycle is slightly longer than 24 hours, typically taking about 24 hours and 50 minutes to complete.