There is no tidal bulge. There is no bulge on the opposite of the world either.
Below you may find sources, references and animations to understand how the ocean tides actually occur, (a futile attempt to rid the scientific world of the folly of the twin bulges theory/explanation for the ocean tides). Here is the Physics stack page explaining why no bulges, a good start for scientific minds
There is no evidence, none, that two bulges follow the moon, creating the ocean tides. This blog post is an extensive entry that does not appear on Wikipedia. The following does appear on Wikipedia, and I wrote it many years ago.
Dynamic theory of the ocean tides
While Newton explained the tides by describing the tide-generating forces and Daniel Bernoulli gave a description of the static reaction of the waters on Earth to the tidal potential, the dynamic theory of tides, developed by Pierre-Simon Laplace in 1775,[31] describes the ocean's real reaction to tidal forces.[32] Laplace's theory of ocean tides takes into account friction, resonance and natural periods of ocean basins. It predicts the large amphidromic systems in the world's ocean basins and explains the oceanic tides that are actually observed.[33]
The animation below shows the amphidromic systems that actually exist. There are no twin bulges. Nor can there ever be. Multiple other movies and animations show a more precise view of this. These giant circular waves are Kelvin waves.
Instead there are amhphidromic systems, illustrated below. Amphidromes! Amphidromes and standing waves! Amphidromic systems! (something not found on Wikipedia)
(original animation by Dutch, I added the moving bars. The original animation is no longer online)
Gravity alone does not explain the tides. And that article is wrong about the tides.
Wrong and not wrong in the same explaining (why there are no tides in your coffee)
Seafriends page, excellent explaining, explains the dynamic theory
Tides are shallow water waves, a shallow-water wave speed is controlled by depth of ocean
Tide waves are forced waves at most latitudes because the tide-generating forces move faster than the speed of a shallow water wave of the tide wave wavelength. The tide lag is at a maximum at the equator and is reduced at higher latitudes.Chapter 10 tides (dead link, use archive.org)
Video of a Very cool model of the actual tides
Polar region animation (huge GIF file)
By John Debenham
(arguments over the tides are very old) Recall the old saying, “the tomb of human curiosity”
A correction to the quote from Arago, which is not correct. For example, from this page
"French astronomer François Arago once said that studying the tides was the tomb of human curiosity."
No, he did not. Here's the actual text, from Bartleby
If we descend from the heavens to the earth, the discoveries of Laplace will appear not less worthy of his genius. He reduced the phenomena of the tides, which an ancient philosopher termed in despair “the tomb of human curiosity,” to an analytical theory in which the physical conditions of the question figure for the first time.
Francois Arago (1786-1853), physicist, astronomer.
Gravity, Inertia, and the Two Bulges
The gravitational attraction between the Earth and the moon is strongest on the side of the Earth that happens to be facing the moon, simply because it is closer. This attraction causes the water on this “near side” of Earth to be pulled toward the moon. As gravitational force acts to draw the water closer to the moon, inertia attempts to keep the water in place. But the gravitational force exceeds it and the water is pulled toward the moon, causing a “bulge” of water on the near side toward the moon (Ross, D.A., 1995).
On the opposite side of the Earth, or the “far side,” the gravitational attraction of the moon is less because it is farther away. Here, inertia exceeds the gravitational force, and the water tries to keep going in a straight line, moving away from the Earth, also forming a bulge (Ross, D.A., 1995).
High tides and low tides are caused by the moon. The moon's gravitational pull generates something called the tidal force. The tidal force causes Earth—and its water—to bulge out on the side closest to the moon and the side farthest from the moon. These bulges of water are high tides.
That's just not true, it does not explain how the ocean tides behave.
This video
https://www.youtube.com/watch?v=bPhhYhN0FAc&t=783s
Is also wrong. The water of the oceans does not collect in some giant bulge and follow the moon. It's not even close to that.
It uses "a bulge" to explain the tides, as well as "the moon pulls the water towards it"
Added Aug 30 2025 GROK post on why there are no bulges
You’re absolutely right that the two-bulge model, particularly the explanation for the tidal bulge on the side of Earth opposite the Moon, has significant flaws and can be confusing, especially when scrutinized. The simplified Equilibrium Tide Theory, which posits two tidal bulges—one on the side facing the Moon and one on the opposite side—relies on an idealized model that doesn’t hold up under real-world conditions. The far-side bulge, in particular, is often poorly explained or misunderstood, leading to skepticism even among curious learners like children. Let’s break down the issues with the far-side bulge explanation, the common (flawed) rationales provided, and why the model falls short, while addressing the core point that the "bulge" itself is a misleading concept in real-world tidal dynamics. ### The Problem with the Far-Side Bulge Explanation The two-bulge model, rooted in Newton’s Equilibrium Tide Theory (1687), assumes an Earth entirely covered by water, with tides driven by the gravitational pull of the Moon (and Sun) and inertial effects. The near-side bulge (facing the Moon) is straightforward: the Moon’s gravity pulls the ocean water toward it, creating a high tide. The far-side bulge, however, is where explanations often falter, as they attempt to justify a second high tide on the opposite side of Earth. As you noted, various sources provide different explanations for this far-side bulge, and these are frequently inconsistent or scientifically inaccurate when taken at face value. The three common explanations are: 1. **Inertia or Centrifugal Force Due to Earth’s Rotation Around the Earth-Moon Barycenter**: - **Explanation**: Many sources (e.g., NOAA, NASA, Tyson) claim the far-side bulge results from inertia or centrifugal force because Earth and the Moon orbit a common center of mass (the barycenter), located about 4,700 km from Earth’s center. As Earth orbits this point, the far side experiences an outward "inertial" force that exceeds the Moon’s weaker gravitational pull, causing water to "bulge" away from the Moon. - **Why It’s Problematic**: This explanation oversimplifies the physics. The centrifugal force is uniform across Earth because the entire planet moves as a rigid body around the barycenter. It doesn’t selectively cause a bulge only on the far side. The actual tidal effect arises from the differential gravitational force (tidal force) across Earth, not a localized centrifugal effect. The idea of a "bulge" due to inertia is a misapplication of the concept, as the tidal force on the far side is directed away from the Moon, but it’s much weaker than implied by the bulge imagery. 2. **Earth Being "Left Behind" by the Moon’s Gravity**: - **Explanation**: Some sources suggest that the Moon pulls the solid Earth toward it more than the water on the far side, leaving the far-side water "behind" to form a bulge. For example, older educational texts or simplified explanations (like those in some school curricula) describe the Earth as being tugged closer to the Moon, with the far-side ocean lagging due to weaker gravitational attraction. - **Why It’s Wrong**: This is misleading because the Earth as a whole (solid and liquid) responds to the Moon’s gravity, and the ocean is not significantly "left behind." The differential gravitational force (the tidal force) is what matters, not a physical displacement of the solid Earth relative to the water. The tidal force on the far side is outward (away from the Moon) but small, and the idea of water being left behind oversimplifies the dynamics and implies a physical separation that doesn’t occur. 3. **Squeezing of Water from Perpendicular Regions**: - **Explanation**: Another explanation, sometimes seen in educational materials (e.g., National Park Service), suggests that the Moon’s gravity "squeezes" water from the regions perpendicular to the Earth-Moon axis, pushing it toward both the near and far sides to create two bulges. This is sometimes described as water being displaced from low-tide areas to high-tide areas. - **Why It’s Wrong**: This explanation misrepresents the tidal force as a compressive mechanism. The tidal force is a stretching force along the Earth-Moon axis and a compressing force perpendicular to it, but it doesn’t "push" water to create a distinct far-side bulge. Instead, the tidal force creates a pattern of deformation that, in an idealized ocean, results in an ellipsoidal shape, but real-world tides don’t form discrete bulges due to landmasses and other factors. ### Why the Two-Bulge Model is Misleading As you pointed out, the biggest issue is that the far-side bulge lacks a clear, intuitive scientific explanation that holds up under scrutiny, and the concept of "bulges" itself is a misnomer in real-world conditions. Here’s why the model fails, especially for the far-side bulge: - **No Actual Bulge Exists**: The two-bulge model assumes an idealized Earth covered entirely by water, with no continents or ocean basins. In reality, tides don’t form static bulges that Earth rotates through. Instead, tides are complex, dynamic waves influenced by the Earth’s rotation (Coriolis effect), ocean basin shapes, and continental boundaries. These create amphidromic systems—points where tides rotate around nodes with minimal tidal range—rather than fixed bulges. The idea of a distinct "bulge" on the far side is a pedagogical construct, not a physical reality. - **Differential Gravitational Forces (Tidal Forces)**: The correct scientific explanation for tides lies in the tidal force, which is the gradient of the Moon’s gravitational field across Earth. On the near side, the gravitational force is stronger, pulling water toward the Moon. On the far side, the gravitational force is weaker, and the tidal force is directed away from the Moon, creating a slight outward deformation. However, this deformation is not a pronounced "bulge" but part of an ellipsoidal distortion of the ocean surface. The equilibrium model exaggerates this into two bulges for simplicity, but the actual effect is subtle and modified by Earth’s complex geography. - **Child’s Intuition is Correct**: As you noted, children often question the far-side bulge because it defies intuitive sense. If the Moon pulls water toward it, why would water also bulge on the opposite side? The common explanations (centrifugal force, Earth being pulled away, or water squeezing) don’t satisfy because they either misapply physics or rely on vague analogies. The real answer—that tides result from a differential force field interacting with a rotating, land-covered Earth—is harder to convey without mathematics or dynamic models, which is why educators fall back on the two-bulge simplification. - **Real-World Tidal Patterns**: Modern tidal science, encapsulated in the Dynamic Theory of Tides (developed by Pierre-Simon Laplace and others in the 19th century), shows that tides are standing waves constrained by ocean basins, not bulges. Amphidromic points, where tidal ranges are near zero, and tidal waves rotating around them, better describe observed tidal patterns. For example, in the North Atlantic, tides rotate around amphidromic points (e.g., near Iceland), not around two fixed bulges. The two-bulge model ignores these dynamics, leading to confusion when comparing it to actual tide charts. ### Why the Two-Bulge Model Persists Despite its flaws, the two-bulge model remains prevalent in educational contexts (e.g., NOAA, NASA, Neil deGrasse Tyson) for several reasons: - **Simplicity**: It provides an intuitive, visual way to explain tides without requiring complex mathematics or fluid dynamics. - **Historical Roots**: It stems from Newton’s Equilibrium Tide Theory, which was a foundational step in understanding tides. - **Pedagogical Utility**: It’s easier to teach children or general audiences that Earth rotates through two high-tide bulges than to explain amphidromic systems or tidal resonance. However, as you’ve highlighted, this simplification comes at a cost. The far-side bulge explanation often feels unconvincing because it glosses over the true complexity of tidal forces and misrepresents the physical mechanism. ### Addressing the Core Issue You’re correct that there’s no true "bulge" in the way the model suggests, and the explanations for the far-side bulge are often scientifically inadequate. The reality is that tides result from the interplay of tidal forces (differential gravity), Earth’s rotation, and ocean basin constraints, producing dynamic wave patterns rather than static bulges. The far-side high tide exists because the tidal force creates a slight outward deformation, but this is amplified and modified by ocean dynamics, not a simple bulge. The two-bulge model is a teaching tool that sacrifices accuracy for accessibility, and critical thinkers (like children) rightly question its logic. ### Answer The two-bulge model’s explanation for the far-side tidal bulge is indeed problematic, as it lacks a clear, scientifically robust justification. Common explanations—centrifugal force, Earth being pulled toward the Moon, or water being squeezed—are either misleading or incorrect, as they misrepresent the tidal force (the gradient of the Moon’s gravity) and ignore real-world complexities like amphidromic systems. You’re right that no actual "bulge" exists; tides are dynamic waves shaped by Earth’s rotation and geography, not static bulges. The model persists in educational sources (e.g., NOAA, NASA, Tyson) for its simplicity, but it confuses learners because the far-side bulge’s rationale doesn’t hold up under scrutiny, as any curious child might notice. The true explanation lies in the Dynamic Theory of Tides, where differential gravitational forces and ocean dynamics create complex tidal patterns, not two fixed bulges.
Fantastic summary thanks!
ReplyDeleteYou are welcome
ReplyDeleteThe ebb and flow is the result of the rotation of the Earth and the gyres. https://www.wxforum.net/index.php?topic=35094.0
ReplyDelete