If the Earth’s axis were perpendicular to the plane of its orbit, which one among the following would not have happened ?

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

Welcome to your first step in mastering Earth’s dynamics! To understand time, seasons, and life on our planet, we must first look at how the Earth moves. Think of the Earth as a grand performer executing two distinct types of motion simultaneously: it rotates like a spinning top and revolves like a runner on a track.

Rotation is the Earth spinning on its own axis—an imaginary line connecting the North and South Poles Science-Class VII . NCERT(Revised ed 2025), Chapter 12: Earth, Moon, and the Sun, p.171. The Earth turns from West to East, which is why we see the Sun, Moon, and stars appear to rise in the East and set in the West Science-Class VII . NCERT(Revised ed 2025), Chapter 12: Earth, Moon, and the Sun, p.184. This 24-hour cycle is what gives us the fundamental rhythm of day and night.

While spinning, the Earth also travels around the Sun in a movement called Revolution. It takes about 365.25 days to complete one full journey along its orbital path Science-Class VII . NCERT(Revised ed 2025), Chapter 12: Earth, Moon, and the Sun, p.175. However, the most critical detail for a UPSC aspirant is the tilt of the Earth’s axis. It isn't upright; instead, it makes an angle of 66.5° with its orbital plane (the ecliptic) and 23.5° with the vertical (normal) Physical Geography by PMF IAS, Chapter 19: The Motions of The Earth and Their Effects, p.251. This tilt, combined with revolution, is the primary reason we experience seasons.

Feature	Rotation	Revolution
Definition	Spinning on its own axis	Movement around the Sun
Time Taken	~24 Hours (1 Day)	~365.25 Days (1 Year)
Primary Effect	Day and Night cycle	Changing of Seasons

Sources: Science-Class VII . NCERT(Revised ed 2025), Chapter 12: Earth, Moon, and the Sun, p.171, 175, 184; Physical Geography by PMF IAS, Chapter 19: The Motions of The Earth and Their Effects, p.251

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

To understand why we have seasons and why the length of our days changes, we must first look at the Earth’s Axial Tilt, also known as Obliquity. Imagine the Earth’s path around the Sun as a flat tabletop called the orbital plane (or ecliptic). Instead of spinning perfectly upright like a top on this table, the Earth leans over at an angle. Specifically, the Earth’s axis makes an angle of 23.5° with the "normal" (a vertical line perpendicular to the plane) and 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 not a temporary wobble; the Earth maintains this fixed orientation as it journeys around the Sun.

While the Earth rotates, exactly one-half of it is always lit by the Sun. The imaginary line that separates the lighted half (day) from the dark half (night) is called the Circle of Illumination. Because of the axial tilt, this circle usually does not coincide with the Earth’s axis. Think of it this way: the axis is the stick the Earth spins on, but the circle of illumination is the spotlight from the Sun. Because the "stick" is tilted, the spotlight hits the Earth unevenly. This is the root cause of why days are longer in summer than in winter and why we experience a cycle of seasons Science-Class VII . NCERT(Revised ed 2025), Earth, Moon, and the Sun, p.177.

Consider the hypothetical scenario where the Earth had zero tilt (a vertical axis). In such a world:

• The Sun would always be directly overhead at the Equator.
• The Circle of Illumination would pass exactly through the North and South Poles every single day.
• Every location on Earth would experience equal days and nights (12 hours each) throughout the entire year Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.254.
• Most importantly, the seasons would disappear because no hemisphere would ever "lean" toward or away from the Sun.

Sources: Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.251; Science-Class VII . NCERT(Revised ed 2025), Earth, Moon, and the Sun, p.177; Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.254

3. The Mechanism of Seasons: Solstices and Equinoxes (intermediate)

To understand why we have seasons, we must look at the Axial Tilt (obliquity) of the Earth. Imagine the Earth spinning like a slightly leaning top at an angle of 23.5° relative to its orbital plane. If the Earth stood perfectly upright (zero tilt), the Sun would always be directly over the Equator, and every place on Earth would experience 12 hours of day and 12 hours of night forever, with no seasonal change Science-Class VII NCERT, Chapter 12, p.177. However, because of this tilt, different parts of the Earth receive varying amounts of direct sunlight as we journey around the Sun.

The Solstices represent the extremes of this journey. Around June 21st, the Northern Hemisphere is tilted most toward the Sun, which shines directly over the Tropic of Cancer. This is the Summer Solstice for the North, featuring the longest day of the year. Conversely, around December 22nd, the Sun shines directly over the Tropic of Capricorn. This marks the Winter Solstice for the Northern Hemisphere—the shortest day and longest night—while the Southern Hemisphere celebrates its summer Physical Geography by PMF IAS, Chapter 19, p.253.

Between these extremes lie the Equinoxes (meaning "equal nights"). Occurring around March 21st (Vernal) and September 23rd (Autumnal), these are the moments when the Earth’s axis is tilted neither toward nor away from the Sun. The Sun sits directly over the Equator, resulting in roughly 12 hours of daylight and 12 hours of darkness for the entire planet Physical Geography by PMF IAS, Chapter 19, p.254. At the exact moment of the Vernal Equinox, the Sun rises precisely at the North Pole and sets at the South Pole Environment and Ecology, Majid Hussain, Major Crops and Cropping Patterns in India, p.126.

Interestingly, the length of our seasons isn't perfectly equal. Because Earth's orbit is elliptical, it moves slower when it is farther from the Sun (aphelion) during the Northern Hemisphere summer. Consequently, summer in the Northern Hemisphere actually lasts about 92 days, which is slightly longer than the 89 days of winter Physical Geography by PMF IAS, Chapter 19, p.256.

Event	Approx. Date	Sun's Direct Rays	Northern Hemisphere Season
Summer Solstice	June 21	Tropic of Cancer	Summer (Longest Day)
Autumnal Equinox	Sept 23	Equator	Autumn (Equal Day/Night)
Winter Solstice	Dec 22	Tropic of Capricorn	Winter (Shortest Day)
Vernal Equinox	March 21	Equator	Spring (Equal Day/Night)

Sources: Science-Class VII NCERT, Chapter 12: Earth, Moon, and the Sun, p.177; Physical Geography by PMF IAS, Chapter 19: The Motions of The Earth and Their Effects, p.253; Physical Geography by PMF IAS, Chapter 19: The Motions of The Earth and Their Effects, p.254; Physical Geography by PMF IAS, Chapter 19: The Motions of The Earth and Their Effects, p.256; Environment and Ecology, Majid Hussain, Major Crops and Cropping Patterns in India, p.126

4. Heat Zones and Latitudinal Distribution of Light (intermediate)

To understand why the Earth isn't heated uniformly, we must start with its shape and orientation. Because the Earth is a geoid (spherical), the Sun’s rays do not strike all parts of the surface at the same angle. Near the Equator, the rays fall vertically (at a 90° angle), concentrating heat over a small, focused area. As you move toward the Poles, the curvature causes the rays to strike at a more slanting angle. These slanting rays must travel through a thicker layer of the atmosphere and spread their energy over a much larger surface area, leading to lower temperatures FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI, Solar Radiation, Heat Balance and Temperature, p.67.

This variation in insolation (incoming solar radiation) allows us to divide the Earth into three primary Heat Zones. The Torrid Zone, located between the Tropics of Cancer and Capricorn, receives the most direct sunlight year-round. Beyond the Tropics, we enter the Temperate Zones, where the Sun is never directly overhead, leading to moderate climates Exploring Society: India and Beyond, Social Science-Class VI, Locating Places on the Earth, p.14. Finally, the Frigid Zones near the North and South Poles receive extremely slanting rays, resulting in very low temperatures and distinct phenomena like the 'midnight sun' or permanent winter darkness.

Heat Zone	Latitudinal Range	Characteristics
Torrid Zone	23.5° N to 23.5° S	Maximum heat; Sun passes directly overhead twice a year.
Temperate Zone	23.5° to 66.5° (N & S)	Moderate temperatures; Sun is never overhead; distinct seasons.
Frigid Zone	66.5° to 90° (N & S)	Extremely cold; Sun remains near or below the horizon.

The intensity of this distribution is further dictated by the Earth's axial tilt. The axis is inclined at an angle of 66.5° to its orbital plane (or 23.5° from the perpendicular) FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI, Solar Radiation, Heat Balance and Temperature, p.67. If the Earth’s axis were perfectly vertical (perpendicular to its orbit), every place on Earth would have exactly 12 hours of day and 12 hours of night every single day, and the concept of changing seasons would virtually disappear because the Sun would always remain directly over the Equator Science-Class VII, Earth, Moon, and the Sun, p.177.

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI, Solar Radiation, Heat Balance and Temperature, p.67; Exploring Society: India and Beyond, Social Science-Class VI, Locating Places on the Earth, p.14; Science-Class VII, Earth, Moon, and the Sun, p.177

5. Phenomena at the Poles: 6-Month Days and Nights (intermediate)

One of the most awe-inspiring phenomena on Earth is the six-month day and six-month night experienced at the geographic poles. To understand why this happens, we must look at the axial tilt of the Earth (approximately 23.5°) in relation to its orbit around the Sun. Because the Earth’s axis remains pointed in the same direction in space (towards the North Star), different parts of the Earth are tilted toward or away from the Sun at different times of the year.

At the North Pole, the "day" begins at the Vernal Equinox (around March 21). On this day, the Sun rises above the horizon and does not set again for six full months. Instead of rising and setting daily, the Sun appears to move in a horizontal circle around the sky, gradually climbing higher until the Summer Solstice, and then descending back toward the horizon Science-Class VII . NCERT(Revised ed 2025), Chapter 12, p. 179. The reverse occurs at the South Pole; when one pole is bathed in constant Polar Day (Midnight Sun), the other is shrouded in Polar Night Physical Geography by PMF IAS, Chapter 19, p. 254.

Location	March 21 to Sept 22	Sept 23 to March 20
North Pole	Continuous Daylight (6 Months)	Continuous Darkness (6 Months)
South Pole	Continuous Darkness (6 Months)	Continuous Daylight (6 Months)

It is important to note that this extreme condition only occurs at the poles themselves. As you move away from the poles toward the Arctic or Antarctic Circles (66.5° N/S), the period of continuous daylight shortens from six months to just a few weeks, and eventually to a single 24-hour day on the Summer Solstice Certificate Physical and Human Geography, GC Leong, Chapter 1, p. 7. If the Earth’s axis were perpendicular (no tilt) to its orbit, this phenomenon would vanish entirely; every place on Earth would have exactly 12 hours of day and night, and the Sun would sit perpetually on the horizon at the poles without ever fully rising or setting.

Sources: Science-Class VII . NCERT(Revised ed 2025), Chapter 12: Earth, Moon, and the Sun, p.179; Physical Geography by PMF IAS, Chapter 19: The Motions of The Earth and Their Effects, p.253-254; Certificate Physical and Human Geography, GC Leong, Chapter 1: The Earth and the Universe, p.7

6. Hypothetical Earth: What if the Tilt Changes? (exam-level)

To truly grasp the mechanics of our planet, we must imagine what happens if we change its fundamental geometry. Currently, Earth sits at a tilt of 23.5° relative to its orbital plane. If Earth’s axis were perpendicular (a 0° tilt) to its orbit, the Sun’s most direct, vertical rays would strike the Equator every single day of the year. This would effectively put the planet in a state of permanent Equinox Science-Class VII, Earth, Moon, and the Sun, p.177. In this hypothetical scenario, the primary driver of seasonal variation—the shifting of the Sun's direct rays between the Tropics—would disappear. Without the tilt, there would be no summer or winter; the weather patterns at any given latitude would remain largely stagnant throughout the year. Furthermore, the variation in the length of day and night would cease. Currently, we enjoy longer days in summer because of the tilt Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.267. On a non-tilted Earth, every location from the Arctic Circle to the Antarctic Circle would experience exactly 12 hours of daylight and 12 hours of night every day. The most unique change would occur at the Poles. Currently, the poles experience six months of continuous day followed by six months of continuous night Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.253. If the tilt were removed, the Sun would neither rise high into the sky nor sink far below the horizon. Instead, the Sun would sit permanently on the horizon, creating a perpetual state of twilight.

Feature	Current (23.5° Tilt)	Hypothetical (0° Tilt)
Seasons	Distinct Spring, Summer, Autumn, Winter	No seasonal changes
Day/Night Length	Varies by season and latitude	Exactly 12 hours each, everywhere
Polar Light	6 months day / 6 months night	Permanent Sun on the horizon

Sources: Science-Class VII, Earth, Moon, and the Sun, p.177; Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.267; Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.253

7. Solving the Original PYQ (exam-level)

This question perfectly bridges the gap between the theoretical motions of the Earth and their practical consequences. Having just mastered the concepts of rotation, revolution, and axial tilt, you can now see how the 23.5° inclination is the single most important variable in our planetary geography. When you encounter a "what if" scenario like this, start by visualizing the Circle of Illumination. If the axis were perpendicular (zero obliquity), this circle would perfectly bisect the Earth from pole to pole at every point in its orbit. As noted in Science-Class VII . NCERT (Revised ed 2025), this means the Sun's rays would consistently strike the Equator at a 90-degree angle, effectively locking the Earth into a permanent Equinox state.

To arrive at the correct answer, we must identify which outcome is geometrically impossible in this perpendicular setup. Since the Sun would always be directly overhead at the Equator, the Circle of Illumination would always touch both the North and South Poles. Consequently, the Sun would sit exactly on the horizon for both poles—never fully rising and never fully setting. This creates a state of permanent twilight rather than total darkness. Therefore, the statement that (A) The North Pole will always lie in dark is the correct choice because it is the only event that would not occur. As explained in Physical Geography by PMF IAS, darkness at the poles only happens when a pole is tilted away from the Sun, which cannot happen if the axis is perpendicular.

UPSC frequently uses "not" in the question stem to test if you can distinguish between actual effects and common misconceptions. Options B, C, and D are the actual results of a perpendicular axis: equal days and nights would prevail because every latitude is halved by the light; seasonal changes would vanish because the varying intensity of solar radiation across the year is eliminated; and the Sun would indeed remain perpendicular to the Equator. The trap here is misinterpreting "perpendicular" to mean a fixed side facing the Sun; remember, the Earth is still rotating on its axis, ensuring every part of the planet (except the exact poles on the horizon) sees the Sun every 24 hours.