UPSC CSE Prelims
Motions of the Earth Previous Year Questions (PYQs)

Detailed Explanation:

Scientific studies confirm that the Earth's rotational axis is gradually shifting, a phenomenon known as polar drift. Melting polar ice redistributes mass from the poles toward the equator, changing the Earth's moment of inertia and affecting its rotation. Solar flares and CMEs do impact the upper atmosphere, but they do not significantly alter the Earth's rotational axis.

✅ Statement I is Correct: Satellite observations show that Earth's rotational axis is drifting and its rotation rate is changing slightly.

✅ Statement II is Correct: Solar flares and CMEs transfer large amounts of energy to the thermosphere and magnetosphere.

✅ Statement III is Correct: Melted polar ice redistributes water toward lower latitudes, especially near the equator.

❌ Statement II does NOT explain Statement I: Solar activity affects the atmosphere and satellites but not the Earth's rotational axis significantly.

✅ Statement III explains Statement I: Redistribution of mass from poles to equator changes Earth's moment of inertia, causing polar drift and slight rotational changes.

Concept Note:

Moment of Inertia is the resistance of a rotating object to changes in its rotation. When mass moves away from the rotational axis (toward the equator), the moment of inertia increases, leading to changes in rotational speed and axis orientation.

Detailed Explanation:

Answer: Option 3 — Both 1 and 2

The Coriolis force is an apparent deflection force caused by Earth's rotation, affecting moving objects including wind and ocean currents. Both statements about the Coriolis force are correct based on fundamental principles of atmospheric and oceanographic dynamics.

✅ Statement 1 – Correct: The Coriolis force is directly proportional to the velocity of the moving object (F = 2mΩv sinφ), so higher wind velocity results in greater Coriolis force and deflection.

✅ Statement 2 – Correct: The Coriolis force varies with the sine of latitude (sinφ), being maximum at the poles (sin 90° = 1) and zero at the equator (sin 0° = 0), where no deflection occurs.

📝 Short Notes: Coriolis Force

• Definition: An apparent deflection force acting on moving objects due to Earth's rotation, named after French scientist Gaspard-Gustave de Coriolis.
• Mathematical Expression: F = 2mΩv sinφ, where m = mass, Ω = Earth's angular velocity, v = object velocity, φ = latitude.
• Direction of Deflection: Objects are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
• Latitudinal Variation: Zero at the equator, increases with latitude, and maximum at the poles; this explains why tropical cyclones do not form within 5° of the equator.
• Dependency on Velocity: Greater the speed of the moving object (wind, ocean current, missile), greater the Coriolis deflection.
• Impact on Wind Patterns: Responsible for the rotation of cyclones, anticyclones, trade winds deflection, and the geostrophic wind balance.
• Ocean Currents: Causes major ocean currents to flow in circular patterns (gyres) - clockwise in Northern Hemisphere and counterclockwise in Southern Hemisphere.
• Practical Applications: Critical for long-range artillery, missile trajectories, aviation routes, and understanding global circulation patterns.

Detailed Explanation:

Answer: Option 4 — 2 and 4

On June 21, the Summer Solstice in the Northern Hemisphere, the sun's rays are directly overhead at the Tropic of Cancer, and all latitudes north of the Equator experience daylight longer than 12 hours. The Arctic Circle experiences 24 hours of continuous daylight (Midnight Sun), while the Equator always has exactly 12 hours of daylight throughout the year.

✅ Statement 1 (Equator) – Incorrect: The Equator experiences exactly 12 hours of daylight on all days of the year, not more than 12 hours.
✅ Statement 2 (Tropic of Cancer) – Correct: On June 21, the Tropic of Cancer receives direct sunlight and experiences more than 12 hours of daylight.
❌ Statement 3 (Tropic of Capricorn) – Incorrect: The Tropic of Capricorn lies in the Southern Hemisphere, which is tilted away from the Sun on June 21, resulting in less than 12 hours of daylight.
✅ Statement 4 (Arctic Circle) – Correct: The Arctic Circle experiences 24 hours of continuous daylight on June 21, known as the Midnight Sun phenomenon.

📝 Short Notes: Summer Solstice and Day Length

• Summer Solstice (June 21): The Northern Hemisphere is tilted at its maximum (23.5°) toward the Sun, resulting in the longest day of the year in the Northern Hemisphere.
• Direct Rays at Tropic of Cancer: On this day, the sun's rays fall vertically at the Tropic of Cancer (23.5° N), which receives maximum solar energy.
• Day Length Pattern: All latitudes north of the Equator experience more than 12 hours of daylight on June 21, with day length increasing toward the North Pole.
• Equator: The Equator experiences exactly 12 hours of daylight and 12 hours of night on all days of the year due to its position midway between the poles.
• Midnight Sun: At the Arctic Circle (66.5° N) and all latitudes beyond it toward the North Pole, the sun does not set on June 21, resulting in 24 hours of continuous daylight.
• Southern Hemisphere: On June 21 (Winter Solstice in Southern Hemisphere), all latitudes south of the Equator experience less than 12 hours of daylight, with the Antarctic Circle experiencing 24 hours of darkness.
• Tropic of Capricorn: Located at 23.5° S, it experiences its shortest day on June 21 with significantly less than 12 hours of daylight.

Detailed Explanation:

Answer: Option 1 — does not set below the horizon at the Arctic Circle

On 21st June, the Northern Hemisphere experiences the Summer Solstice, when the North Pole is tilted at its maximum angle (23.5°) towards the Sun. This results in the Arctic Circle (66.5°N) experiencing 24 hours of continuous daylight, meaning the Sun does not set below the horizon. Conversely, the Antarctic Circle experiences polar night, the Equator has equal day and night, and the Sun is directly overhead at the Tropic of Cancer (not Capricorn).

📝 Short Notes: Solstices and Sun's Position

• Arctic Circle: Located at 66.5°N latitude; experiences midnight sun during summer solstice
• Antarctic Circle: Located at 66.5°S latitude; experiences midnight sun during winter solstice (December)
• Tropic of Cancer: 23.5°N - Sun directly overhead on 21st June
• Tropic of Capricorn: 23.5°S - Sun directly overhead on 22nd December
• Earth's axial tilt: 23.5° from perpendicular to orbital plane causes seasonal variations

Detailed Explanation:

Seasonal variation in day and night length occurs due to the Earth's revolution around the Sun combined with its axial tilt of 23.5°.

The tilted axis causes different hemispheres to receive varying amounts of direct sunlight throughout the year – when the Northern Hemisphere is tilted toward the Sun (summer solstice), it experiences longer days; when tilted away (winter solstice), it experiences shorter days.

❌ Option 1 – Incorrect: Earth's rotation causes the daily cycle of day and night (24 hours), not seasonal variation in their length.

❌ Option 3 – Incorrect: Latitudinal position determines the magnitude of variation (polar regions experience extreme variation, equatorial regions minimal), but not the cause of seasonal changes.

❌ Option 4 – Incorrect: The elliptical orbit affects Earth-Sun distance but does not cause seasonal day-length variation; the tilt is the primary factor.