Movements of tides are mostly determined by

1. Ocean Dynamics 101: Waves, Tides, and Currents (basic)

Welcome to our journey into the heart of the ocean! To understand how the massive oceans circulate, we must first distinguish between the three primary ways ocean water moves: Waves, Tides, and Currents. While they might all look like "moving water" to the casual observer, they are driven by very different physical forces. Generally, we categorize these into horizontal motions (currents and waves) and vertical motions (tides) FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 13: Movements of Ocean Water, p.108.

Waves are essentially energy traveling across the ocean surface, usually triggered by the friction of wind blowing over the water. A crucial point to remember is that in a normal wave, the water molecules themselves do not travel across the ocean; they move in small circular orbits. It is the wave energy (the "wave train") that moves forward Physical Geography by PMF IAS, Chapter 32: Ocean Movements Ocean Currents And Tides, p.486. In contrast, Ocean Currents represent the actual physical movement of huge volumes of water in a definite direction, much like a river flowing within the sea. These are driven by wind (surface currents) or differences in water density caused by temperature and salinity (deep-sea currents).

Tides are the periodic rise and fall of sea levels, occurring once or twice a day. Unlike waves, tides are caused by the gravitational pull of the Moon and the Sun. However, gravity is only half the story. As the Earth and Moon rotate around their common center of mass, centrifugal force is generated. This force acts as a counter-balance to gravity, creating a second "tidal bulge" on the side of the Earth facing away from the moon. As the Earth rotates, coastal areas pass through these bulges, experiencing high and low tides FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 13: Movements of Ocean Water, p.109.

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 13: Movements of Ocean Water, p.108-109; Physical Geography by PMF IAS, Chapter 32: Ocean Movements Ocean Currents And Tides, p.486

2. Celestial Mechanics: Gravitational Pull and the Moon (basic)

Tides are the periodic rise and fall of the sea level, occurring once or twice a day. While many people think of this as a simple "pulling" of water, it is actually a complex dance of celestial mechanics involving three primary forces: the gravitational pull of the moon, the gravitational pull of the sun, and the centrifugal force generated by the Earth's rotation Physical Geography by PMF IAS, Ocean Movements Ocean Currents And Tides, p.500. Although the sun is massive, the moon is much closer to Earth, making its gravitational influence more than twice as powerful as the sun's in generating tides FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Movements of Ocean Water, p.110.

To understand how these forces create tides, we must look at the tidal bulges. On the side of the Earth facing the moon, the moon’s gravitational attraction is at its strongest, pulling the ocean water toward it and creating a high tide. However, there is a second high tide simultaneously occurring on the opposite side of the Earth. This happens because the centrifugal force (the outward force created by the Earth-Moon system's rotation) exceeds the moon's gravitational pull at that distance, pushing the water outward Physical Geography by PMF IAS, Ocean Movements Ocean Currents And Tides, p.501. Together, these two forces create two major bulges, which is why most coastal areas experience two high tides and two low tides every day as the Earth rotates through these bulges.

The intensity of these tides also fluctuates based on the moon's distance from Earth. When the moon is at its closest point to Earth, known as perigee, the gravitational pull is stronger, leading to unusually high and low tides. Conversely, when the moon is at its farthest point, called apogee, the forces are diminished, and the tidal range (the difference between high and low water) is much smaller FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Movements of Ocean Water, p.110. Interestingly, this constant "flexing" of the oceans and the Earth's solid body creates friction, which is actually slowing down the Earth's rotation by about 0.002 seconds per century Physical Geography by PMF IAS, Earths Interior, p.59.

Sources: Physical Geography by PMF IAS, Ocean Movements Ocean Currents And Tides, p.500-501; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Movements of Ocean Water, p.110; Physical Geography by PMF IAS, Earths Interior, p.59

3. Factors Influencing Ocean Currents: Coriolis and Salinity (intermediate)

To understand why the ocean moves the way it does, we must look beyond the wind. While surface winds act like a hand pushing a bowl of water, the Coriolis force and salinity act as the steering wheel and the engine's weight, respectively. These are categorized into primary forces (which initiate movement) and secondary forces (which influence how that water flows and sinks) Physical Geography by PMF IAS, Ocean Movements Ocean Currents And Tides, p.486.

The Coriolis force is an apparent force caused by the Earth's rotation. Imagine trying to draw a straight line on a spinning record; the line would curve. Similarly, as the Earth rotates from West to East, it deflects the path of ocean currents. In the Northern Hemisphere, currents are deflected to the right, while in the Southern Hemisphere, they move to the left. This is why major ocean gyres rotate clockwise in the North Atlantic but counter-clockwise in the South Pacific Physical Geography by PMF IAS, Ocean Movements Ocean Currents And Tides, p.487. It is crucial to note that while Coriolis shapes the path of these currents, it is not responsible for the formation of counter-currents, which are driven by other pressure gradients Physical Geography by PMF IAS, Ocean Movements Ocean Currents And Tides, p.489.

While Coriolis dictates horizontal direction, Salinity (along with temperature) dictates vertical movement. This is known as Thermohaline Circulation. Water with high salinity is denser and heavier; therefore, it sinks. This sinking creates a vacuum at the surface, pulling in surrounding water to replace it. This process forms the "Great Ocean Conveyor Belt," a global system that circulates water between the surface and the deep ocean Physical Geography by PMF IAS, Ocean temperature and salinity, p.516. While salinity has a massive impact on these deep-sea vertical currents, its influence on horizontal surface movement is relatively less significant compared to wind or gravity.

Sources: Physical Geography by PMF IAS, Ocean Movements Ocean Currents And Tides, p.486-489; Physical Geography by PMF IAS, Ocean temperature and salinity, p.516

4. Coastal Geomorphology: Impact of Water Movement (intermediate)

Coastal geomorphology is the study of how the land meets the sea, a dynamic zone where the energy of waves, tides, and currents constantly reshapes the Earth's surface. Unlike rivers that follow a linear path, marine agents act along a front, making coastal evolution a complex interplay of erosion and deposition. The primary driver is wave energy, which is generated by wind. When waves approach the shore, they don't just carry water; they carry immense energy that performs the work of a 'geomorphic agent'—weathering rocks, transporting sediment, and building new landforms Fundamentals of Physical Geography, Chapter 6, p.47.

The erosive power of the sea is most visible through hydraulic action. As waves strike a cliff, they force water and air into tiny cracks (fissures) and joints. This trapped air is suddenly compressed and then released as the wave retreats, acting like a miniature explosion that shatters the rock over time Physical Geography by PMF IAS, Chapter 16, p.216. Additionally, waves use rock fragments as 'tools' to grind down the shore, a process known as abrasion or corrasion. While waves do the heavy lifting, tides determine the vertical range over which this erosion occurs, shifting the 'attack zone' twice a day between high and low tide marks.

Interestingly, nature provides its own defense against this relentless movement. In tropical regions like India, mangrove ecosystems act as biological buffers. In the Sunderbans, dense tidal mangroves stabilize the shoreline against tidal surges, while on the West Coast, they occupy the intertidal regions of estuaries Environment by Shankar IAS Academy, Chapter 4, p.49. Without these 'bio-shields,' many coastal areas would succumb much faster to the 'high eroding' status often mapped by satellite imagery Environment by Shankar IAS Academy, Chapter 4, p.55.

Sources: Fundamentals of Physical Geography, Geomorphic Processes, p.47; Physical Geography by PMF IAS, Coastal Landforms, p.216; Fundamentals of Physical Geography, Landforms and their Evolution, p.57; Environment by Shankar IAS Academy, Aquatic Ecosystem, p.49, 55

5. Tidal Varieties: Spring Tides, Neap Tides, and Cycles (intermediate)

While the basic rhythm of tides is daily, the height of the tide (the vertical distance between high and low water) is not constant. It varies significantly throughout the month based on the changing relative positions of the Sun and the Moon with respect to the Earth. These variations are categorized into two main types: Spring Tides and Neap Tides. Understanding these is crucial for navigators, as it determines how much "clearance" a ship has over a harbor floor NCERT Class XI Fundamentals of Physical Geography, Chapter 13, p.110.

Spring Tides occur when the Sun, the Moon, and the Earth are aligned in a straight line (a configuration known as syzygy). This happens twice a month: during the New Moon (when the Sun and Moon are on the same side of Earth) and the Full Moon (when they are on opposite sides). In this position, the gravitational pulls of the Sun and Moon reinforce each other, creating a massive net pull. This results in the maximum tidal range: the high tides are exceptionally high, and the low tides are exceptionally low PMF IAS Physical Geography, Chapter 32, p.503.

Conversely, Neap Tides occur roughly seven days after the spring tides, when the Sun and Moon are at right angles to each other with respect to the Earth (known as quadrature). This happens during the first and third quarter moon phases. Because the Sun's gravity now acts in a direction perpendicular to the Moon's, it partially counteracts or "diminishes" the Moon's pull. Consequently, the tidal range is at its minimum: the high tide is lower than usual, and the low tide is higher than usual PMF IAS Physical Geography, Chapter 32, p.504-505.

Sources: NCERT Class XI Fundamentals of Physical Geography, Chapter 13: Movements of Ocean Water, p.110; PMF IAS Physical Geography, Chapter 32: Ocean Movements Ocean Currents And Tides, p.503-506

6. The Role of Centrifugal Force and Earth's Rotation in Tides (exam-level)

When we think of tides, we often credit the Moon’s gravity entirely. However, to truly master oceanography, you must understand that tides are the result of a precise tug-of-war between two opposing forces: the gravitational pull of celestial bodies (the Moon and Sun) and the centrifugal force generated by the Earth-Moon orbital system. While gravity pulls ocean water toward the Moon, centrifugal force acts in the opposite direction, attempting to push the water away from the center of rotation.

The interaction of these forces creates what we call the tide-generating force, which is essentially the net difference between gravitational attraction and centrifugal force Fundamentals of Physical Geography, Class XI (NCERT 2025 ed.), Chapter 13, p.109. Because the Moon’s gravity is stronger on the side of the Earth facing it, it overcomes the centrifugal force to create the primary tidal bulge. On the opposite side of the Earth, the Moon’s gravitational pull is weaker due to the increased distance; here, the centrifugal force becomes the dominant factor, pushing the water outward to create a secondary tidal bulge Physical Geography by PMF IAS, Chapter 32, p.501. This explains why we see two high tides on opposite sides of the planet simultaneously.

While these forces create the bulges, it is the Earth's rotation that acts as the "timer." As the Earth rotates on its axis, coastal locations are carried into and out of these two bulges. This is why most places experience two high tides and two low tides every day. Interestingly, this movement isn't perfectly fluid; the friction between the moving water and the rotating Earth actually acts as a brake, causing the Earth’s rotation to slow down by a tiny fraction of a second every century Physical Geography by PMF IAS, Chapter 3, p.59.

Sources: Fundamentals of Physical Geography, Class XI (NCERT 2025 ed.), Chapter 13: Movements of Ocean Water, p.109; Physical Geography by PMF IAS, Chapter 32: Ocean Movements Ocean Currents And Tides, p.501; Physical Geography by PMF IAS, Chapter 3: Earths Interior, p.59

7. Solving the Original PYQ (exam-level)

Now that you have mastered the foundational building blocks of oceanography, this question tests your ability to identify the active mechanism behind tidal cycles. You have learned that the gravitational pull of the Moon and Sun creates "bulges" in the ocean. However, the movement of these tides across the globe is primarily a result of the rotation of the Earth. As the Earth spins on its axis, specific coastal locations are carried through these stationary tidal bulges, which results in the periodic rise and fall of sea levels. As highlighted in FUNDAMENTALS OF PHYSICAL GEOGRAPHY (NCERT), the Earth's rotation also generates the centrifugal force necessary to create the second tidal bulge on the side opposite the Moon.

When evaluating the choices, look for the factor that explains the regularity of the tides. Wind velocity (Option B) is a common trap; while wind can cause water to pile up against a coast, it creates irregular "storm surges" or waves, not the predictable periodic movement of tides. Revolution of the Earth (Option D) refers to our orbit around the Sun, which influences seasonal variations rather than the daily tidal rhythm. The albedo effect (Option A) is completely unrelated, as it deals with the reflectivity of the Earth's surface regarding solar radiation. Therefore, the rotation of the Earth is the correct answer because it is the fundamental motion that dictates the daily timing and occurrence of tides, a concept meticulously explained in Physical Geography by PMF IAS.