Assertion (A) : The weight of a body decreases with the increase of latitude on earth Reason (R) : The earth is not perfect sphere

1. Earth's True Shape: The Geoid and Oblate Spheroid (basic)

While we often imagine Earth as a perfect blue marble, it is actually an oblate spheroid — a sphere that is slightly 'squashed' at the top and bottom and 'bulged' in the middle. This unique shape is technically called a Geoid, which literally means 'Earth-shaped' Physical Geography by PMF IAS, Chapter 18, p. 241. This deformation isn't accidental; it is the result of Earth’s constant rotation. Because the Earth rotates on its axis, it generates a centrifugal force that acts outward from the center. This force is strongest at the equator, where the rotational speed is highest, causing the planet’s mass to bulge outward, making the equatorial radius roughly 21 kilometers longer than the polar radius Fundamentals of Physical Geography, Geography Class XI (NCERT 2025 ed.), Movements of Ocean Water, p. 109.

This 'bulging' has a direct impact on how much you weigh depending on where you stand. Gravity is not uniform across the globe for two main reasons. First, because of the equatorial bulge, a person standing at the equator is actually farther away from the Earth's center of mass than someone standing at the North or South Pole. Since gravitational pull weakens with distance, the pull is slightly weaker at the equator. Second, the same centrifugal force that caused the bulge also works against gravity at the equator, effectively 'lifting' you up very slightly Physical Geography by PMF IAS, Chapter 18, p. 241.

Consequently, the gravitational force is greater at the poles and less at the equator. If you were to take a highly sensitive spring balance and travel from the equator to the North Pole, you would find that your measured weight increases as you move toward higher latitudes. Understanding this shape is fundamental to geography because it affects everything from how we map the world to how we calculate the paths of satellites and the behavior of ocean tides.

Sources: Physical Geography by PMF IAS, Latitudes and Longitudes, p.241; Fundamentals of Physical Geography, Geography Class XI (NCERT 2025 ed.), Movements of Ocean Water, p.109

2. Earth's Rotation and Its Physical Consequences (basic)

The Earth is constantly in motion, spinning on an imaginary line called its axis that connects the North and South Poles. This spinning motion is known as rotation, and the Earth completes one full turn every 23 hours, 56 minutes, and 4 seconds (roughly 24 hours) Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.251. Crucially, this rotation occurs from West to East, which is why we observe the Sun, Moon, and stars appearing to rise in the East and set in the West Science-Class VII . NCERT, Earth, Moon, and the Sun, p.171. This movement creates the fundamental cycle of day and night, separated by a boundary called the circle of illumination. Beyond just day and night, rotation has a profound impact on the Earth's physical shape. Because the Earth is spinning, it generates an outward-pushing centrifugal force. Since the speed of rotation is highest at the Equator and zero at the poles, this force has "stretched" the Earth over billions of years into an oblate spheroid (or Geoid) shape—meaning it bulges at the Equator and is flattened at the poles Physical Geography by PMF IAS, Latitudes and Longitudes, p.241. This equatorial bulge means the Earth's radius is significantly larger at the Equator than at the poles. This change in shape and rotation speed creates a measurable difference in gravity and weight across the globe. You would actually weigh slightly less at the Equator and more at the poles for two reasons:

• Distance from Center: At the poles, you are closer to the Earth's center of mass due to the flattening, making the gravitational pull stronger.
• Centrifugal Force: At the Equator, the centrifugal force is at its maximum and acts in the opposite direction to gravity, effectively "canceling out" a tiny fraction of your weight. At the poles, this force is non-existent Physical Geography by PMF IAS, Latitudes and Longitudes, p.241.

Feature	Equatorial Region	Polar Region
Rotational Speed	Maximum	Minimum (Zero at the axis)
Centrifugal Force	Strongest (opposes gravity)	Negligible/Zero
Distance to Center	Greater (due to bulge)	Shorter (due to flattening)
Effective Weight	Lowest	Highest

Sources: Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.251; Science-Class VII . NCERT, Earth, Moon, and the Sun, p.171; Physical Geography by PMF IAS, Latitudes and Longitudes, p.241

3. Fundamental Physics: Gravity vs. Weight (basic)

To understand how Earth's rotation affects our daily lives, we must first master the distinction between mass and weight. Mass is the measure of the actual quantity of matter in an object; it is constant and does not change regardless of where you are in the universe Science, Class VIII, NCERT (Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p. 142. Weight, however, is a force—specifically, the force with which Earth's gravity pulls on that mass. Because weight depends on the strength of gravity, it can actually change as you move across the Earth's surface Science, Class VIII, NCERT (Revised ed 2025), Exploring Forces, p. 75.

There are two primary reasons why your weight varies depending on your latitude (your distance north or south of the equator):

• The Shape of the Earth: Earth is not a perfect sphere; it is an oblate spheroid. Due to its rotation, it bulges at the equator and flattens at the poles. This means if you are standing at the North or South Pole, you are physically closer to the Earth's center than if you were standing at the equator. Since gravitational attraction is stronger when you are closer to the center of mass, the pull is naturally stronger at the poles Physical Geography by PMF IAS, Latitudes and Longitudes, p. 241.
• Centrifugal Force: As the Earth rotates, it generates an outward-pushing centrifugal force. This force is at its maximum at the equator (where the rotational speed is highest) and is non-existent at the poles. This outward force acts in the opposite direction to gravity, slightly "canceling out" some of the downward pull. Therefore, the net force (your weight) is slightly less at the equator Physical Geography by PMF IAS, Latitudes and Longitudes, p. 241.

In summary, as you move from the equator toward the poles, your weight increases because the gravitational pull gets stronger and the counteracting centrifugal force disappears. This is also why we see "gravity anomalies" in different regions—local variations in mass distribution, such as deep oceanic trenches, can further cause slight fluctuations in the value of 'g' Physical Geography by PMF IAS, Tectonics, p. 108.

Feature	At the Equator	At the Poles
Distance from Earth's Center	Greater (due to bulge)	Smaller (flattened)
Centrifugal Force	Maximum	Zero
Measured Weight	Lower	Higher

Sources: Science, Class VIII, NCERT (Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.142; Science, Class VIII, NCERT (Revised ed 2025), Exploring Forces, p.75; Physical Geography by PMF IAS, Latitudes and Longitudes, p.241; Physical Geography by PMF IAS, Tectonics, p.108

4. Latitudinal Heat Zones and Solar Insulation (intermediate)

To understand why the Earth is divided into distinct heat zones, we must first master the concept of Insolation (Incoming Solar Radiation). Insolation is the solar energy received at the Earth's surface, typically measured in Watts per square meter (W/m²). While the Sun radiates energy uniformly, the Earth does not receive it uniformly. The primary reason is the Earth's spherical shape and the 66½° angle of inclination its axis makes with the plane of its orbit NCERT Class XI, Solar Radiation, Heat Balance and Temperature, p.67.

The intensity of insolation depends heavily on the angle of incidence. Near the Equator, solar rays are nearly vertical. Vertical rays are concentrated over a small surface area and travel a shorter distance through the atmosphere, losing less energy to scattering and absorption. As we move toward the poles, the angle of the rays becomes more oblique (slanting). Slanting rays spread over a much larger area and must penetrate a thicker layer of the atmosphere, significantly reducing their heating power NCERT Class XI, Solar Radiation, Heat Balance and Temperature, p.67. This fundamental difference creates three primary Latitudinal Heat Zones:

Heat Zone	Latitudinal Range	Characteristics
Torrid Zone	Between Tropic of Cancer (23.5°N) and Capricorn (23.5°S)	Receives maximum heat as the Sun is overhead at least once a year.
Temperate Zone	23.5° to 66.5° in both hemispheres	The angle of the sun's rays goes on decreasing; moderate temperatures.
Frigid Zone	66.5° to the Poles	The sun never rises far above the horizon; extremely cold due to very slanting rays.

An interesting nuance to note is that the Equator does not receive the maximum insolation on Earth. Instead, the maximum insolation is recorded over subtropical deserts. This is because the Equator experiences heavy cloud cover which reflects some solar radiation, whereas deserts have clear skies, allowing more radiation to reach the surface NCERT Class XI, Solar Radiation, Heat Balance and Temperature, p.68. Furthermore, Earth maintains its temperature by absorbing short-wave radiation during the day and radiating it back as long-wave infrared radiation at night PMF IAS, Horizontal Distribution of Temperature, p.282.

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Solar Radiation, Heat Balance and Temperature, p.67; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Solar Radiation, Heat Balance and Temperature, p.68; Physical Geography by PMF IAS, Manjunath Thamminidi, Horizontal Distribution of Temperature, p.282

5. The Coriolis Effect and Atmospheric Circulation (intermediate)

To understand the Coriolis Effect, imagine you are standing on a giant merry-go-round (the Earth). Because the Earth is a sphere, points at the equator have to travel a much larger distance in 24 hours than points near the poles. This means the rotational speed of the Earth’s surface is highest at the equator (~1,670 km/h) and effectively zero at the poles Physical Geography by PMF IAS, Pressure Systems and Wind System, p.308. When air moves from the equator toward the poles, it retains its high eastward momentum, 'outrunning' the slower-moving ground beneath it. To an observer on the ground, the air appears to curve. This apparent force that deflects moving objects (like wind or ocean currents) is what we call the Coriolis force.

The strength of this deflection is not uniform across the globe. It is governed by the formula 2νω sin ϕ, where 'ϕ' represents the latitude. Because the sine of 0° is zero, the Coriolis force is absent at the equator. As you move toward higher latitudes, the value of sin ϕ increases, meaning the force reaches its maximum intensity at the poles Physical Geography by PMF IAS, Pressure Systems and Wind System, p.309. This is a critical reason why tropical cyclones (which require a 'spin' or cyclonic vortex) do not form between 0° and 5° latitude; the Coriolis force there is simply too weak to initiate the rotation Physical Geography by PMF IAS, Tropical Cyclones, p.356.

In terms of direction, we follow Ferrel’s Law: in the Northern Hemisphere, moving objects are deflected to their right, and in the Southern Hemisphere, they are deflected to their left Certificate Physical and Human Geography, GC Leong, Climate, p.139. This force always acts perpendicular to the direction of motion and the Pressure Gradient Force (PGF). While the PGF pushes air directly from high to low pressure, the Coriolis force pulls it sideways, eventually forcing the wind to blow parallel to isobars in the upper atmosphere or in a circular motion around pressure centers Fundamentals of Physical Geography, NCERT Class XI, Atmospheric Circulation and Weather Systems, p.79.

Feature	Northern Hemisphere	Southern Hemisphere
Deflection Direction	To the Right	To the Left
Impact on Trade Winds	Become North-East Trades	Become South-East Trades
Coriolis at Equator	Zero (No deflection)
Coriolis at Poles	Maximum Strength

Sources: Physical Geography by PMF IAS, Pressure Systems and Wind System, p.308-309; Physical Geography by PMF IAS, Tropical Cyclones, p.356; Fundamentals of Physical Geography, NCERT Class XI, Atmospheric Circulation and Weather Systems, p.79; Certificate Physical and Human Geography, GC Leong, Climate, p.139

6. Variation of 'g' (Gravity) Across the Globe (exam-level)

Have you ever wondered if your weight would change if you traveled from the bustling streets of Mumbai to the frozen plains of the North Pole? It actually does! Even though your mass (the amount of matter in you) remains constant, your weight (the force with which gravity pulls you) varies across the globe. This variation in the acceleration due to gravity (g) is driven by three main factors: Earth's shape, its rotation, and its internal composition.

First, we must understand that the Earth is not a perfect sphere; it is an oblate spheroid. Because of the equatorial bulge, the surface at the equator is further away from the Earth's center of mass than the surface at the poles Physical Geography by PMF IAS, Latitudes and Longitudes, p.241. Since the force of gravity weakens as distance increases, the gravitational pull is naturally greater at the poles and less at the equator FUNDAMENTALS OF PHYSICAL GEOGRAPHY (NCERT), The Origin and Evolution of the Earth, p.19.

Second, the Earth's rotation introduces a centrifugal force. Think of this as a "pole-fleeing force" that pushes objects away from the axis of rotation Physical Geography by PMF IAS, Tectonics, p.95. This force is strongest at the equator, where the speed of rotation is highest, and it becomes zero at the poles. Because centrifugal force acts in the opposite direction to gravity, it effectively "cancels out" a tiny portion of the gravitational pull at the equator. The combination of being further from the center and fighting the maximum centrifugal force makes the equator the place where you would weigh the least.

Finally, scientists observe gravity anomalies. These are differences between the expected value of g and the actual measured value at a specific location. These anomalies occur because the mass of material within the Earth’s crust is not distributed evenly. For example, a region with very dense ore deposits will exert a slightly stronger gravitational pull than a region with low-density rocks FUNDAMENTALS OF PHYSICAL GEOGRAPHY (NCERT), The Origin and Evolution of the Earth, p.19.

Feature	At the Equator	At the Poles
Distance from Earth's Center	Greater (due to bulge)	Lesser (flattened)
Centrifugal Force	Maximum (acts against gravity)	Zero
Effective Gravity (g)	Minimum	Maximum

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY (NCERT), The Origin and Evolution of the Earth, p.19; Physical Geography by PMF IAS, Latitudes and Longitudes, p.241; Physical Geography by PMF IAS, Tectonics, p.95

7. Solving the Original PYQ (exam-level)

In this question, we see the practical application of two fundamental concepts you have just mastered: Earth's geometry and gravitational mechanics. To solve this, you must synthesize the fact that the Earth is an oblate spheroid—bulging at the equator and flattened at the poles—with the understanding of how centrifugal force and distance from the Earth's center influence effective gravity (g). As explained in Physical Geography by PMF IAS, the Earth’s shape is a direct result of its rotation, which creates a variation in the radius from the center to the surface.

Let's walk through the logic: the Reason (R) is true because the equatorial radius is significantly larger than the polar radius. Because gravity is inversely proportional to the square of the distance from the center, gravitational pull is actually stronger at the poles (where you are closer to the center) than at the equator. Additionally, centrifugal acceleration from Earth's rotation is maximum at the equator and zero at the poles, further reducing your apparent weight at lower latitudes. Therefore, as you move toward higher latitudes, your weight actually increases, which makes Assertion (A) false. This leads us directly to the Correct Answer: (D).

A common UPSC trap is to present a Reason (R) that is a scientifically accurate statement in isolation, tempting students to select option (A) or (B) without critically verifying the Assertion (A). Many students assume that any change in latitude must result in a "decrease" simply because they misremember the direction of the trend. Always pause to determine if the direction of change described in the assertion matches the physical reality established by the reason. Here, while the Earth's non-spherical shape is the cause of weight variation, it causes an increase, not a decrease, as one moves from the equator to the poles.