Which of the following statements regarding the duration of day and night is correct?

1. Earth's Motions: Rotation and Revolution (basic)

To understand how time and seasons work, we must first look at the two primary ways our planet moves in space. The first is rotation, which is the Earth spinning on its own axis—an imaginary line connecting the North and South Poles through the center. This movement happens from West to East, taking approximately 24 hours to complete one full turn Science-Class VII . NCERT(Revised ed 2025), Earth, Moon, and the Sun, p.184. This spinning is the reason we experience the cycle of day and night. As the Earth rotates, only one half faces the Sun at any given time; the boundary that separates the lighted half from the dark half is known as the Circle of Illumination Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.251.

The second motion is revolution, where the Earth travels in an elliptical orbit around the Sun. This journey takes about 365.25 days (one year) to complete Science-Class VII . NCERT(Revised ed 2025), Earth, Moon, and the Sun, p.175. While rotation gives us our daily rhythm, revolution—combined with the Earth's tilt—dictates our yearly cycle of seasons. It is important to distinguish these: rotation is like a top spinning in place, while revolution is like a runner circling a track.

Feature	Rotation	Revolution
Path	On its own axis	Around the Sun (Orbit)
Direction	West to East	Counter-clockwise (as seen from North)
Time Taken	~24 hours (1 day)	~365 days (1 year)
Primary Effect	Day and Night	Seasons and Year length

What makes Earth's movement truly interesting for geography is that its axis is not perfectly vertical. It is tilted at an angle of 23.5° from the vertical (the normal to the orbital plane). This means the axis makes an angle of 66.5° with the orbital plane itself Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.251. This tilt is "fixed" in space, meaning as the Earth revolves around the Sun, different parts of the planet lean toward or away from the Sun at different times of the year, leading to variations in day length and the change of seasons Certificate Physical and Human Geography, GC Leong, The Earth's Crust, p.15.

Sources: Science-Class VII . NCERT(Revised ed 2025), Earth, Moon, and the Sun, p.175, 184; Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.251; Certificate Physical and Human Geography, GC Leong, The Earth's Crust, p.15

2. Axial Tilt and the Circle of Illumination (basic)

To understand why the length of our days changes throughout the year, we must first look at the Earth's Axial Tilt. Imagine a line passing through the North and South Poles—this is the Earth's axis. Instead of being perfectly vertical relative to its path around the Sun, this axis is tilted at an angle of 23.5° from the perpendicular Science-Class VII . NCERT(Revised ed 2025), Earth, Moon, and the Sun, p.177. This tilt is not a temporary wobble; the Earth maintains this specific orientation as it orbits the Sun, a phenomenon often called parallelism of the axis.

Now, let's introduce the Circle of Illumination. Since the Earth is a sphere, the Sun can only light up one half of it at any given moment. The imaginary line that separates the portion of the Earth experiencing daylight from the portion in darkness is called the Circle of Illumination Certificate Physical and Human Geography , GC Leong, The Earth's Crust, p.14. If the Earth were not tilted, this circle would always pass directly through both poles, and every place on Earth would have exactly 12 hours of day and 12 hours of night every single day.

However, because of the 23.5° tilt, the Circle of Illumination rarely passes through the poles. Instead, it "cuts" across the latitudes at an angle. This leads to several critical geographical effects:

• At the Equator: The Circle of Illumination always bisects (cuts exactly in half) the Equator, regardless of the tilt. Therefore, the Equator consistently experiences roughly 12 hours of daylight and 12 hours of darkness throughout the year.
• Increasing Latitudes: As you move away from the Equator toward the poles, the tilt causes the Circle of Illumination to divide the latitudes into unequal parts. This is why summer days are longer and winter days are shorter Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.253.
• The Poles: At the extremes, the tilt can leave a pole entirely on the sunlit side or entirely on the dark side for months at a time, leading to the Midnight Sun or Polar Night.

Latitude	Day/Night Variation	Reason
0° (Equator)	Minimal to None	Circle of Illumination always bisects the Equator.
Mid-Latitudes	Moderate	Tilt creates unequal portions of day and night.
90° (Poles)	Extreme (6 months)	The pole remains entirely inside or outside the Circle of Illumination.

Sources: Science-Class VII . NCERT(Revised ed 2025), Earth, Moon, and the Sun, p.177; Certificate Physical and Human Geography , GC Leong, The Earth's Crust, p.14; Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.253

3. The Equinoxes: Global Equal Day and Night (intermediate)

Imagine the Earth as a spinning top slightly tilted at an angle of 23.5°. As it travels around the Sun, this tilt usually means one hemisphere is leaning toward the light while the other leans away. However, twice a year, the Earth reaches a specific point in its orbit where its axis is tilted neither toward nor away from the Sun. At these moments, the Sun’s direct rays fall vertically on the Equator. These instances are known as Equinoxes (derived from Latin, meaning 'equal night'). On March 21st and September 23rd, the circle of illumination (the line dividing day and night) passes exactly through the North and South Poles, resulting in approximately 12 hours of daylight and 12 hours of darkness for every location on the planet Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.254. While we say 'equal day and night,' there is a small scientific catch: Atmospheric Refraction. Because our atmosphere is dense, it bends sunlight. This allows us to see the Sun slightly before it actually rises and for a few minutes after it has set. Consequently, even on the equinox, the day is technically a few minutes longer than the night at the Equator Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.255. Additionally, the equinox marks a dramatic shift at the poles. On the March Equinox, the Sun rises at the North Pole for the first time in six months, while it simultaneously sets at the South Pole, ushering in the long polar night Environment and Ecology by Majid Hussain, Major Crops and Cropping Patterns in India, p.126.

The seasons associated with these dates are mirror images in the two hemispheres:

Date	Northern Hemisphere	Southern Hemisphere
March 21st	Vernal (Spring) Equinox	Autumnal Equinox
September 23rd	Autumnal Equinox	Vernal (Spring) Equinox

Interestingly, the duration of these seasons is not perfectly identical. Because the Earth's orbit is elliptical, its speed varies. Earth moves slower when it is farther from the Sun (during the Northern Hemisphere summer). This causes the journey from the March equinox to the September equinox to take slightly longer than the return leg. As a result, in the Northern Hemisphere, summer is roughly 92 days long, while winter is only about 89 days Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.256.

Sources: Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.254; Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.255; Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.256; Environment and Ecology by Majid Hussain, Major Crops and Cropping Patterns in India, p.126

4. The Solstices: Seasonal Extremes (intermediate)

To understand the solstices, we must first appreciate the axial tilt of the Earth. Our planet doesn't sit upright as it orbits the Sun; instead, it is tilted at an angle of 23.5°. This tilt, combined with Earth's revolution, means that at two specific points in the year, one of the hemispheres reaches its maximum inclination toward the Sun. These moments are the Solstices (from the Latin sol for sun and sistere to stand still), marking the peak of seasonal extremes Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.252.

On June 21st (the Summer Solstice for the Northern Hemisphere), the North Pole is tilted most directly toward the Sun. Consequently, the Sun's rays fall vertically over the Tropic of Cancer (23.5° N). This causes the Northern Hemisphere to experience its longest day and shortest night of the year. During this time, the entire region north of the Arctic Circle experiences 24 hours of daylight, often called the 'Midnight Sun' Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.252. Conversely, in the Southern Hemisphere, it is the Winter Solstice, characterized by the shortest day and longest night Science-Class VII . NCERT(Revised ed 2025), Earth, Moon, and the Sun, p.179.

On December 22nd, the situation reverses. The Southern Hemisphere is now tilted toward the Sun, which shines directly over the Tropic of Capricorn (23.5° S). This is the Summer Solstice for the Southern Hemisphere and the Winter Solstice for the Northern Hemisphere Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.253. While people in Australia enjoy their longest day of the year, those in the Northern Hemisphere experience their shortest day and longest night. Interestingly, at the Equator, the length of day and night remains nearly equal (~12 hours) regardless of the solstice, because the circle of illumination always bisects the Equator Certificate Physical and Human Geography , GC Leong, The Earth's Crust, p.16.

Feature	June Solstice (June 21)	December Solstice (Dec 22)
Sun Overhead at...	Tropic of Cancer (23.5° N)	Tropic of Capricorn (23.5° S)
Northern Hemisphere	Summer (Longest Day)	Winter (Shortest Day)
Southern Hemisphere	Winter (Shortest Day)	Summer (Longest Day)
Polar Phenomenon	24-hr light at North Pole	24-hr light at South Pole

Sources: Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.252; Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.253; Science-Class VII . NCERT(Revised ed 2025), Earth, Moon, and the Sun, p.179; Certificate Physical and Human Geography , GC Leong, The Earth's Crust, p.16

5. Connected Concept: Latitudinal Heat Zones (intermediate)

To understand why Earth’s temperature varies so drastically from the Equator to the poles, we must first look at the angle of incidence—the angle at which the Sun’s rays strike the Earth's surface. At the Equator (low latitudes), the Sun’s rays are nearly vertical or perpendicular. This means the solar energy is concentrated over a smaller surface area, leading to intense heating. As we move toward the poles (high latitudes), the curvature of the Earth causes the rays to become increasingly slanting. These slanting rays must travel through a thicker layer of the atmosphere and spread their energy over a much larger surface area, which significantly reduces the intensity of heat received Exploring Society: India and Beyond, Social Science-Class VII, NCERT, Climates of India, p.49. Based on this variation in solar intensity, we divide the Earth into three primary Latitudinal Heat Zones. These zones help us understand the broad climatic patterns of our planet:

Heat Zone	Latitudinal Range	Characteristics
Torrid Zone	Between the Tropic of Cancer (23.5°N) and Tropic of Capricorn (23.5°S)	Receives maximum heat; the Sun is exactly overhead at least once a year.
Temperate Zone	Between the Tropics and the Arctic/Antarctic Circles (66.5° N/S)	Moderate temperatures; the Sun is never overhead, and the angle of rays is moderate Exploring Society: India and Beyond, Social Science-Class VI, NCERT, Locating Places on the Earth, p.14.
Frigid Zone	Between the Arctic/Antarctic Circles and the Poles	Extremely cold; the Sun remains near or below the horizon, providing very little heat.

Finally, it is crucial to understand the Global Heat Balance. There is a surplus of net radiation between roughly 40°N and 40°S, meaning these areas receive more heat than they lose. Conversely, the polar regions have a deficit. To prevent the tropics from getting progressively hotter and the poles from freezing completely, the Earth's system redistributes this surplus heat poleward through winds (like the jet stream) and ocean currents Fundamentals of Physical Geography, Geography Class XI, NCERT, Solar Radiation, Heat Balance and Temperature, p.70. Most of this intense heat transfer occurs across the mid-latitudes (30° to 50°), which is why these regions often experience stormy weather and temperate cyclones Physical Geography by PMF IAS, Horizontal Distribution of Temperature, p.293.

Sources: Exploring Society: India and Beyond, Social Science-Class VII, NCERT, Climates of India, p.49; Exploring Society: India and Beyond, Social Science-Class VI, NCERT, Locating Places on the Earth, p.14; Fundamentals of Physical Geography, Geography Class XI, NCERT, Solar Radiation, Heat Balance and Temperature, p.70; Physical Geography by PMF IAS, Horizontal Distribution of Temperature, p.293

6. Latitudinal Variation in Day Length (exam-level)

To understand why the length of a day varies, we must look at the Earth's axial tilt of 23.5°. As the Earth orbits the Sun, this tilt ensures that different latitudes receive different amounts of sunlight at different times of the year. The boundary between day and night on Earth is called the circle of illumination. Because the Earth is a sphere, this circle always cuts the Equator (0°) exactly in half, regardless of the tilt. Consequently, at the Equator, the duration of day and night remains nearly equal (approximately 12 hours each) all year round Science-Class VII . NCERT, Earth, Moon, and the Sun, p.179.

As you move away from the Equator toward the poles, the circle of illumination no longer bisects the latitudes equally. In the summer hemisphere, a larger portion of the latitude is tilted toward the Sun, leading to longer days. In the winter hemisphere, the opposite occurs. This variation becomes more dramatic as latitude increases. For instance, in southern India (closer to the Equator), the difference between the longest and shortest day is barely an hour, whereas in northern regions like Leh or Kashmir, the seasonal difference in daylight is much more pronounced INDIA PHYSICAL ENVIRONMENT, Geography Class XI, India — Location, p.2.

At the most extreme latitudes—beyond the Arctic Circle (66.5° N) and Antarctic Circle (66.5° S)—the variation reaches its peak. During the summer solstice, the Sun may not set at all, a phenomenon known as the Midnight Sun or Polar Day. At the North and South Poles themselves, this effect is so extreme that the Sun rises and sets only once a year, resulting in six months of continuous daylight followed by six months of continuous darkness (Polar Night) Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.253-254.

Comparison of Day Length Variation

Latitude Region	Day/Night Duration	Seasonal Variation
Equator (0°)	Always ~12 hours each	Negligible / None
Mid-Latitudes (e.g., 45°)	Varies (e.g., 8h to 16h)	Moderate
Poles (90°)	6 months Day / 6 months Night	Extreme

Sources: Science-Class VII . NCERT, Earth, Moon, and the Sun, p.179; INDIA PHYSICAL ENVIRONMENT, Geography Class XI, India — Location, p.2; Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.253-254

7. Solving the Original PYQ (exam-level)

Now that you have mastered the mechanics of Earth's axial tilt and its orbit, this question brings those building blocks together. The key lies in the Circle of Illumination—the boundary between day and night. Because of the Earth's geometry, this circle always bisects the Equator exactly in half, regardless of the season. This means that at 0° latitude, the duration of day and night remains nearly constant at 12 hours each throughout the year. As you move toward higher latitudes, the tilt causes the Circle of Illumination to cut the latitude lines more unevenly, leading to longer days in summer and shorter days in winter. This variance reaches its extreme at the Poles, where the difference is a full six months. Therefore, the difference is least near the Equator and progressively increases away from it, making (A) the correct answer.

UPSC often uses the Tropics as a distractor in options (C) and (D) because students frequently associate the Tropics with the sun's overhead movement. However, duration of daylight is a function of latitudinal distance from the center, not just where the sun is vertical. Option (B) is a classic inverse trap; it flips the logic to catch students who might confuse the intensity of sunlight (which is highest at the Equator) with the variation in day length. As explained in Physical Geography by PMF IAS, the geometric bisection at the Equator ensures stability, while the 23.5-degree tilt drives the seasonal extremes as one travels poleward.