To a perpendicular to the plane of ecliptic, Earth’s axis of rotation makes an angle of 23~ degrees. Had this angle been zero degree, which one among the following would result ?

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

To understand how time and seasons function, we must first master the two fundamental movements of our planet: Rotation and Revolution. Think of Earth as a spinning top that is simultaneously racing along a giant circular track around the Sun. These two motions happen at the same time but govern very different aspects of our lives. Rotation is the spinning of the Earth on its axis — an imaginary line that passes through the North and South Poles. Earth rotates from West to East, which is why the Sun appears to rise in the east and set in the west. When viewed from above the North Pole, this motion is anti-clockwise Science-Class VII . NCERT, Chapter 12, p.171. It takes approximately 24 hours to complete one full turn. The most immediate result of rotation is the cycle of day and night. At any given moment, only half of the Earth faces the Sun; the boundary that separates the lighted half from the dark half is known as the Circle of Illumination Physical Geography by PMF IAS, Chapter 19, p.251. While spinning, Earth also orbits the Sun in a motion called Revolution. The path it follows is called an orbit, and the plane it travels on is the orbital plane or ecliptic Physical Geography by PMF IAS, Chapter 19, p.252. One full revolution takes about 365 ¼ days. To keep our calendars simple, we count a year as 365 days and "save up" the extra six hours. Every four years, these 24 hours (6 hours × 4 years) form one full day, which is added to February to create a Leap Year of 366 days Science-Class VII . NCERT, Chapter 12, p.176.

Feature	Rotation	Revolution
Definition	Spinning on its own axis.	Movement around the Sun.
Time Taken	~24 hours (one day).	~365 ¼ days (one year).
Primary Effect	Day and Night cycle.	Seasons and the Solar Year.

Sources: Science-Class VII . NCERT(Revised ed 2025), Earth, Moon, and the Sun, p.171; Science-Class VII . NCERT(Revised ed 2025), Earth, Moon, and the Sun, p.176; Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.251; Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.252

2. Geometry of the Axial Tilt and Orbital Plane (basic)

To understand why we have seasons or why days grow shorter in winter, we must first look at the Earth’s specific orientation in space. Imagine the Earth traveling around the Sun on a giant, flat sheet of glass. This flat path is known as the Orbital Plane or the Ecliptic Physical Geography by PMF IAS, Chapter 19, p.252. If the Earth sat "straight up" on this plane, its axis of rotation would be perfectly vertical (at a 90° angle to the plane).

However, the Earth is not upright. Its axis is tilted. When we describe this tilt, we use two different reference points, and it is vital for your exams to distinguish between them:

Reference Point	Angle	Description
The Normal	23.5°	The angle measured from a line perpendicular (vertical) to the orbital plane.
The Orbital Plane	66.5°	The angle measured from the flat plane of the orbit itself.

As the Earth revolves around the Sun, it maintains this tilt in a fixed direction—a phenomenon often called the parallelism of the axis. This means the North Pole always points toward the same spot in space (near the North Star, Polaris), regardless of where the Earth is in its 365¼-day journey Science-Class VII . NCERT(Revised ed 2025), Chapter 12, p.177. This constant lean is what causes different parts of the Earth to receive varying intensities of sunlight throughout the year, laying the foundation for our seasons and the changing altitude of the midday sun GC Leong, Chapter 2, p.15.

Sources: Physical Geography by PMF IAS, Chapter 19: The Motions of The Earth and Their Effects, p.251-252; Science-Class VII . NCERT(Revised ed 2025), Chapter 12: Earth, Moon, and the Sun, p.177; Certificate Physical and Human Geography, GC Leong, Chapter 2: The Earth's Crust, p.15

3. Solstices, Equinoxes, and the Apparent Path of the Sun (intermediate)

To understand why we have seasons and why the length of our days changes, we must first look at the Earth’s axial tilt. The Earth does not sit upright; it is tilted at an angle of 23.5° relative to its orbital plane. As the Earth orbits the Sun, this tilt remains fixed in space, meaning that for half the year, the Northern Hemisphere leans toward the Sun, and for the other half, it leans away. This geometry creates the apparent path of the Sun—the illusion that the Sun migrates north and south between the Tropics throughout the year Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.252.

The Solstices represent the extremes of this migration. On June 21st (Summer Solstice), the Northern Hemisphere is at its maximum tilt toward the Sun, which shines directly over the Tropic of Cancer (23.5°N). Consequently, the Northern Hemisphere experiences its longest day and the Arctic Circle enjoys 24 hours of daylight. Conversely, on December 22nd (Winter Solstice), the Sun is vertically overhead at the Tropic of Capricorn (23.5°S). This brings the longest day to the Southern Hemisphere, while the Northern Hemisphere faces its shortest day and longest night Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.253.

Between these extremes are the Equinoxes (meaning "equal nights"). Occurring around March 21st (Vernal) and September 23rd (Autumnal), these are the moments when the Sun is directly over the Equator. At this point, neither pole is tilted toward the Sun, and the "circle of illumination" passes exactly through the North and South Poles, resulting in exactly 12 hours of day and 12 hours of night everywhere on Earth Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.254.

Event	Date (Approx)	Sun's Vertical Position	Key Characteristic
Summer Solstice	June 21	Tropic of Cancer (23.5°N)	Longest day in Northern Hemisphere
Autumnal Equinox	Sept 23	Equator (0°)	Equal day/night globally
Winter Solstice	Dec 22	Tropic of Capricorn (23.5°S)	Shortest day in Northern Hemisphere
Vernal Equinox	March 21	Equator (0°)	Equal day/night globally

An interesting detail is that our seasons are not perfectly equal in length. Because Earth's orbit is elliptical, it moves slower when it is farther from the Sun (Kepler’s Second Law). In the Northern Hemisphere, we are actually farther from the Sun during our summer, meaning the Earth moves slower in its orbit. This results in the Northern summer being about 92 days long, while the winter is slightly shorter at 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.252; Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.253; 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.256

4. The Circle of Illumination and Global Light Distribution (intermediate)

To understand how light is distributed across our planet, we must first visualize the Circle of Illumination. This is the imaginary line that separates the portion of the Earth experiencing daylight from the portion in darkness. Because the Earth is a sphere, the Sun can only light up one half at a time. This boundary is actually a Great Circle, meaning it bisects the Earth into two equal halves GC Leong, Certificate Physical and Human Geography, Chapter 1, p.14. You can visualize this by holding a ball near a lamp; the line separating the light and shadow will always appear as a curve that wraps around the sphere NCERT Science Class VIII, Keeping Time with the Skies, p.174.

The distribution of light is not static because of the Earth's axial tilt of 23.5°. As the Earth rotates, points on the surface move into and out of this circle. Because of the tilt, the Circle of Illumination usually cuts across the parallels of latitude at an angle. This is why day and night lengths vary as you move toward the poles. For example, during the Northern Hemisphere's summer, the North Pole is tilted toward the sun, placing it entirely within the illuminated half even as the Earth rotates NCERT Science Class VII, Earth, Moon, and the Sun, p.186.

What if the Earth had zero tilt? This is a favorite conceptual thought experiment in geography. If the Earth's axis were perfectly perpendicular to its orbital plane, the Circle of Illumination would always pass precisely through the North and South Poles. This would create a permanent state of Equinox. At every single latitude, from the Equator to the poles, the day would be exactly 12 hours long, and the night would be exactly 12 hours long, every single day of the year Physical Geography by PMF IAS, Chapter 19, p.267.

Feature	Current (23.5° Tilt)	Hypothetical (0° Tilt)
Circle of Illumination	Shifts relative to poles	Always passes through both poles
Day Length	Varies by latitude and season	Equal (12 hours) everywhere
Seasonal Change	Distinct seasons exist	No seasons; constant climate

Sources: Certificate Physical and Human Geography, GC Leong, The Earth's Crust, p.14; NCERT Science Class VIII, Keeping Time with the Skies, p.174; NCERT Science Class VII, Earth, Moon, and the Sun, p.186; Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.267

5. Latitudinal Heat Zones and Solar Intensity (intermediate)

To understand why the Earth has different climates, we must first look at how the Sun’s energy, or Insolation (Incoming Solar Radiation), is distributed across a curved surface. Because the Earth is a geoid, the Sun's rays do not hit every part of the planet at the same angle. At the Equator, the rays are vertical or nearly perpendicular, meaning the solar energy is concentrated over a small surface area, leading to intense heating Exploring Society: India and Beyond (Class VII), Climates of India, p.49. As you move toward the poles, the Earth curves away, causing the rays to hit at an increasingly slanting angle. These slanting rays must travel through a thicker layer of the atmosphere and spread their energy over a much larger area, which significantly reduces the heat intensity per unit of surface Physical Geography by PMF IAS, Horizontal Distribution of Temperature, p.282.

This variation in solar intensity allows us to divide the Earth into three primary Latitudinal Heat Zones. The Torrid Zone (between the Tropics of Cancer and Capricorn) receives the most direct sunlight and is the hottest. The Temperate Zones (between the Tropics and the Arctic/Antarctic Circles) experience moderate temperatures because the Sun is never directly overhead but never too low. Finally, the Frigid Zones (beyond the Polar Circles) receive very slanting rays and remain cold year-round Exploring Society: India and Beyond (Class VI), Locating Places on the Earth, p.14.

The boundaries of these zones are dictated by the Earth's axial tilt of 23.5° (which makes an angle of 66.5° with its orbital plane). This tilt ensures that the 'overhead' Sun shifts throughout the year, but the fundamental decrease in intensity from the Equator to the Poles remains a constant rule of physical geography Fundamentals of Physical Geography (Class XI), Solar Radiation, Heat Balance and Temperature, p.67.

Heat Zone	Latitudinal Range	Solar Characteristic
Torrid Zone	23.5° N to 23.5° S	Sun is directly overhead at least once a year; maximum heat.
Temperate Zone	23.5° to 66.5° (N & S)	Sun's angle is moderate; never overhead; moderate heat.
Frigid Zone	66.5° to 90° (N & S)	Sun's rays are extremely slanting; least solar intensity.

Sources: Exploring Society: India and Beyond (Class VII), Climates of India, p.49; Physical Geography by PMF IAS, Horizontal Distribution of Temperature, p.282; Exploring Society: India and Beyond (Class VI), Locating Places on the Earth, p.14; Fundamentals of Physical Geography (Class XI), Solar Radiation, Heat Balance and Temperature, p.67

6. Variations in Day and Night Length across Latitudes (exam-level)

To understand why days are long in summer and short in winter, we must look at the Earth’s axial tilt. The Earth’s axis is not vertical; it is tilted at an angle of 23.5° from the perpendicular to its orbital plane (or 66.5° with the orbital plane itself). As the Earth revolves around the Sun, this tilt remains fixed in space, pointing toward the North Star. This creates a geometric setup where, at any given time of the year, one hemisphere is tilted toward the Sun while the other is tilted away. Physical Geography by PMF IAS, Chapter 19, p.267

The boundary between day and night is called the Circle of Illumination. If the Earth had no tilt, this circle would always pass exactly through the North and South Poles, cutting every latitude in half. In such a scenario, every place on Earth would experience exactly 12 hours of day and 12 hours of night throughout the year. However, because of the tilt, the Circle of Illumination usually cuts across latitudes at an angle. At the Equator, the impact is minimal because the circle always bisects it into two equal halves, leading to nearly constant 12-hour days year-round. Science-Class VII NCERT, Chapter 12, p.177

As we move toward the poles, the variation becomes dramatic. During the Northern Hemisphere's summer, the North Pole is tilted toward the Sun. Consequently, the Circle of Illumination swings "behind" the pole, leaving the entire region within the Arctic Circle (66.5° N) in continuous sunlight for 24 hours. Conversely, the Antarctic Circle (66.5° S) is shrouded in 24-hour darkness. As the Earth moves to the other side of its orbit six months later, these conditions reverse. GC Leong, Chapter 2, p.7

Latitude	Day/Night Variation	Reason
Equator (0°)	Negligible variation (approx. 12h day/night).	The Circle of Illumination always bisects the Equator.
Mid-Latitudes	Distinct seasonal changes in day length.	The tilt causes unequal portions of the latitude to be in light vs. dark.
Poles (90°)	Extreme (6 months day, 6 months night).	The pole remains entirely on one side of the Circle of Illumination for half the year.

Sources: Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.267; Science-Class VII NCERT, Earth, Moon, and the Sun, p.177; Certificate Physical and Human Geography, GC Leong, The Earth's Crust, p.7

7. Hypothetical Scenario: Impacts of Zero Axial Tilt (exam-level)

To understand what happens if Earth had zero axial tilt, we first need to appreciate our current reality. Earth’s axis is currently tilted at 23.5° relative to the line perpendicular to its orbital plane Physical Geography by PMF IAS, Chapter 19, p. 251. This tilt is the fundamental reason we have seasons. It causes the Sun’s direct rays to migrate between the Tropics of Cancer and Capricorn throughout the year. If we were to "straighten" the Earth so its axis was perfectly vertical (0° tilt), the Sun would remain permanently overhead at the Equator. The Sun would never move north toward Delhi or south toward Canberra; it would simply stay fixed over the center of the Earth every single day of the year.

The most immediate and dramatic consequence would be the total disappearance of seasons. Seasons exist because the tilt causes different hemispheres to lean toward the Sun at different times, changing the intensity and duration of sunlight Physical Geography by PMF IAS, Chapter 19, p. 267. Without this tilt, the amount of solar energy reaching any specific latitude would remain constant every day. You would have a "permanent spring" or "permanent autumn" depending on your distance from the equator. There would be no summer heatwaves driven by high solar altitude and no winter chills driven by low solar angles.

Furthermore, this scenario creates a state of permanent equinox. Currently, an equinox occurs only twice a year when the Sun is directly over the equator, resulting in equal day and night lengths globally GC Leong, Chapter 2, p. 7. With zero tilt, the circle of illumination (the boundary between day and night) would pass directly through the North and South Poles every day. Consequently, every location on Earth would experience 12 hours of daylight and 12 hours of darkness, 365 days a year. The "Midnight Sun" at the poles would vanish; instead, the Sun would simply skim the horizon indefinitely at the poles, never fully rising and never fully setting.

Feature	Current Earth (23.5° Tilt)	Hypothetical Earth (0° Tilt)
Seasons	Distinct (Spring, Summer, Autumn, Winter)	None (Permanent climate per latitude)
Day Length	Varies by season and latitude	Exactly 12 hours everywhere, always
Overhead Sun	Migrates between 23.5°N and 23.5°S	Permanently fixed at the Equator (0°)
Polar Regions	6 months day / 6 months night	Sun stays on the horizon 24/7

Sources: Physical Geography by PMF IAS, Chapter 19: The Motions of The Earth and Their Effects, p.251; Physical Geography by PMF IAS, Chapter 19: The Motions of The Earth and Their Effects, p.267; Certificate Physical and Human Geography, GC Leong, Chapter 2: The Earth's Crust, p.7

8. Solving the Original PYQ (exam-level)

Review the concepts above and try solving the question.