GS Notes

1.5 - Geography Notes 03

12 min read

Elaborate Notes

Earth’s Movements and the Phenomenon of Seasons

The change in seasons is a fundamental geophysical phenomenon governed by the Earth’s relationship with the Sun. It is a result of a combination of factors, primarily the Earth’s revolution around the Sun and the fixed tilt of its rotational axis. To understand this complex interplay, it is useful to analyze hypothetical scenarios before examining the real-world conditions.

  • Hypothetical Scenarios of Earth’s Movement:

    • Case 1: No Rotation, No Tilt, No Revolution: In this static scenario, the Earth would be tidally locked with the Sun. One hemisphere would perpetually face the Sun, experiencing a constant, scorching day, while the other hemisphere would be locked in a permanent, frozen night. The angle of incidence of solar radiation would be highest (90°) at the subsolar point on the equator and decrease to zero at the terminator (the line separating day and night). There would be no concept of a “day” as we know it, nor any seasonal change.
    • Case 2: Rotation Present, No Tilt, No Revolution: If the Earth rotated on an axis perpendicular to its orbital plane (zero tilt), the Sun’s rays would always be directly overhead at the equator. The circle of illumination—the dividing line between day and night—would pass through both the North and South Poles. Consequently, every location on Earth, regardless of latitude, would experience exactly 12 hours of daylight and 12 hours of darkness every day of the year. While there would be a climatic gradient from the hot equator to the cold poles due to the varying angle of solar incidence, there would be no seasonal variation throughout the year.
    • Case 3: Rotation and Tilt Present, No Revolution: In this scenario, the Earth rotates on a tilted axis (23.5°), but does not revolve around the Sun. If the Northern Hemisphere were permanently tilted towards the Sun, it would experience perpetual summer, characterized by longer days and more direct sunlight. Conversely, the Southern Hemisphere would be locked in a permanent winter, with shorter days and oblique sunlight. The subsolar point would be fixed at the Tropic of Cancer (23.5° N). This demonstrates that tilt alone is insufficient to cause seasonal change; it merely creates permanent seasonal zones.
    • Case 4: Rotation, Tilt, and Revolution Present (Actual Condition): This is the reality. The Earth revolves around the Sun on its orbital plane in approximately 365.25 days. Crucially, its axis of rotation maintains a constant tilt of 23.5° from the perpendicular to its orbital plane, a phenomenon known as the parallelism of the axis. This constant orientation means that as the Earth orbits the Sun, the Northern Hemisphere is tilted towards the Sun for one part of the year and away from it for the other. This changing orientation relative to the Sun causes variations in the angle of incidence (intensity of sunlight) and the duration of daylight, leading to the cycle of seasons. This concept forms the basis of the Milankovitch cycles, a theory proposed by Serbian geophysicist Milutin Milankovitch in the 1920s, which explains long-term climate change based on variations in Earth’s orbital parameters (eccentricity, axial tilt, and precession).

  • Solstices and Equinoxes: These are four key points in Earth’s annual orbit that mark the transition of seasons.

    • Summer Solstice (Approx. June 21): On this day, the Earth’s North Pole is at its maximum tilt towards the Sun. The Sun’s rays fall vertically on the Tropic of Cancer (23.5° N). This results in the longest day and shortest night of the year in the Northern Hemisphere, marking the beginning of summer. Conversely, the Southern Hemisphere experiences its shortest day and longest night, marking the start of winter. North of the Arctic Circle (66.5° N), there is 24-hour daylight (Polar Day or “Midnight Sun”), while south of the Antarctic Circle (66.5° S), there is 24-hour darkness (Polar Night).
    • Winter Solstice (Approx. December 22): The Earth’s South Pole is now at its maximum tilt towards the Sun. The Sun’s rays are directly overhead at the Tropic of Capricorn (23.5° S). This is the shortest day and longest night in the Northern Hemisphere (start of winter) and the longest day and shortest night in the Southern Hemisphere (start of summer). The Antarctic Circle experiences 24-hour daylight, while the Arctic Circle is in 24-hour darkness.
    • Equinoxes (Approx. March 21 and September 23): The term ‘equinox’ is derived from Latin, meaning ‘equal night’. On these two days, the Earth’s axis is not tilted towards or away from the Sun. The Sun’s rays fall vertically on the Equator (0° latitude). The circle of illumination passes through both poles, resulting in approximately 12 hours of daylight and 12 hours of darkness for all latitudes across the globe.
      • Spring (Vernal) Equinox (March 21): Marks the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere.
      • Autumnal Equinox (September 23): Marks the beginning of autumn in the Northern Hemisphere and spring in the Southern Hemisphere.

The Extent of Day and Night

  • Position of the Overhead Sun (Subsolar Point): This is the point on Earth’s surface where the Sun’s rays strike at a 90° angle. Due to the Earth’s axial tilt, this point appears to migrate north and south over the course of the year. This migration is confined between the Tropic of Cancer (23.5° N) and the Tropic of Capricorn (23.5° S).

    • Any location situated between these two tropics will experience the overhead sun twice a year.
    • Locations exactly on the Tropic of Cancer or the Tropic of Capricorn will experience the overhead sun only once a year (on their respective solstices).
    • Locations outside this tropical zone (the Torrid Zone) never experience the Sun directly overhead.

  • Polar Day and Polar Night: These phenomena occur within the polar circles (latitudes 66.5° to 90° N and S).

    • Polar Day: A period of more than 24 hours of continuous daylight. At the Arctic/Antarctic Circle, this occurs for one day during the respective summer solstice. The duration of continuous daylight increases towards the poles, lasting for approximately six months at the poles themselves. This phenomenon was a critical navigational and psychological factor for early polar explorers like Fridtjof Nansen during his Fram expedition (1893–1896) and Roald Amundsen’s journey to the South Pole (1911).
    • Polar Night: A period of more than 24 hours of continuous darkness, occurring during the respective winter solstice and lasting up to six months at the poles.

Time Zones

  • Concept and Standardization: A time zone is a region of the Earth that observes a uniform standard time for legal, commercial, and social purposes. The concept arose in the 19th century, driven by the need to synchronize railway schedules. Scottish-born Canadian engineer Sir Sanford Fleming was a key proponent of a worldwide system of standard time. The system was formally adopted at the International Meridian Conference in Washington, D.C., in 1884.

    • The Earth rotates 360° in 24 hours, which means it rotates 15° every hour (360/24). This forms the basis for the 24 standard time zones.
    • The Greenwich Meridian (0° longitude) was established as the Prime Meridian, the reference point for Coordinated Universal Time (UTC).
    • The rule for time calculation is EGA (East Gain Add) and WLS (West Lose Subtract). For every 15° of longitude eastward from Greenwich, the local time is one hour ahead of UTC. For every 15° westward, it is one hour behind. This translates to a 4-minute difference for every 1° of longitude.

  • Indian Standard Time (IST):

    • India’s longitudinal extent is from approximately 68°7’ E to 97°25’ E, a span of nearly 30°. This creates a natural time difference of about two hours between the westernmost point (Gujarat) and the easternmost point (Arunachal Pradesh).
    • Historically, British India had multiple time zones. The presidencies of Bombay, Calcutta, and Madras had their own local times. Additionally, tea plantations in Assam followed a local time known as ‘Chaibagan time’, which was one hour ahead of the eventual IST.
    • In 1906, India officially adopted the meridian of 82.5° E (or 82°30’ E) as the standard meridian for the whole country, establishing Indian Standard Time (IST). This meridian passes close to the city of Mirzapur in Uttar Pradesh. It was chosen because it is divisible by 7.5° and lies centrally within the country’s longitudinal expanse. Despite its official adoption, Bombay and Calcutta time zones were retained until 1955.

  • The Debate on Multiple Time Zones in India:

    • The vast east-west expanse leads to a significant mismatch between the solar cycle and the clock time, especially in the northeastern states. Sunrise and sunset occur much earlier there, leading to a loss of productive daylight hours in the evening and higher energy consumption.
    • Arguments for Multiple Time Zones: Proponents, including a panel at the National Physical Laboratory (NPL), have suggested creating a second time zone (e.g., IST-II at UTC+6:30) for the northeastern states. This could enhance productivity, save electricity, and better align work schedules with the natural diurnal cycle.
    • Issues in Adoption: The government has historically resisted this change, citing several potential challenges:
      • Economic Disruption: It could complicate operations in banking, stock markets, and other sectors that rely on synchronized national timing.
      • Administrative Complexity: Coordinating office and school timings across different zones could be challenging.
      • Transportation Chaos: A major concern is the risk of accidents and scheduling chaos in the railway network, which operates on a single national time.
      • Security and Communication: Different time zones could create communication gaps for security forces and agencies operating across the country. There is also a political apprehension that it might foster a sense of separation or regionalism.

Prelims Pointers

  • Axial Tilt: Earth’s axis is tilted at an angle of 23.5° from the perpendicular to its orbital plane (or 66.5° to the orbital plane).
  • Parallelism of Axis: The Earth’s axis always points in the same direction in space (towards Polaris, the North Star) as it revolves around the Sun.
  • Summer Solstice: June 21; Sun overhead at Tropic of Cancer (23.5° N). Longest day in Northern Hemisphere.
  • Winter Solstice: December 22; Sun overhead at Tropic of Capricorn (23.5° S). Shortest day in Northern Hemisphere.
  • Spring (Vernal) Equinox: March 21; Sun overhead at the Equator (0°).
  • Autumnal Equinox: September 23; Sun overhead at the Equator (0°).
  • Equal Day/Night: Occurs on Equinoxes (March 21 & Sept 23) across all latitudes. The equator experiences approx. 12 hours of day and night throughout the year.
  • Subsolar Point: The location where the Sun is directly overhead (90°). It migrates between 23.5° N and 23.5° S.
  • Polar Circles: Arctic Circle (66.5° N) and Antarctic Circle (66.5° S). These are the latitudes where 24-hour day/night begins on solstices.
  • Time Calculation: Earth rotates 360° in 24 hours.
    • 15° of longitude corresponds to a 1-hour time difference.
    • 1° of longitude corresponds to a 4-minute time difference.
  • Prime Meridian: 0° longitude, passes through Greenwich, London. It is the basis for Coordinated Universal Time (UTC).
  • Indian Standard Time (IST): Based on the 82.5° E (82°30’ E) longitude.
  • IST Meridian Location: Passes near Mirzapur, Uttar Pradesh.
  • IST and UTC: IST is 5 hours and 30 minutes ahead of UTC (UTC+5:30).
  • International Meridian Conference: Held in 1884 in Washington, D.C., established the Greenwich Meridian as the prime meridian.
  • Chaibagan Time: A local time used in Assam’s tea gardens, which was one hour ahead of IST.

Mains Insights

GS Paper I (Geography & Society)

  1. Interplay of Geographical Phenomena: The phenomenon of seasons is not caused by a single factor but is a complex interplay of Earth’s revolution, its constant axial tilt (parallelism), and the resulting variation in insolation (angle of incidence and duration of daylight). This demonstrates how interconnected Earth’s systems are.
  2. Latitudinal Heat Balance and its Consequences: The differential heating of the Earth due to the Sun’s varying angle is the primary driver of global atmospheric circulation, pressure belts, and wind systems. The seasonal migration of the overhead sun causes the seasonal shifting of these pressure and wind belts, which in turn governs global climate patterns, including monsoons in regions like India.
  3. The Debate on Multiple Time Zones in India - A Socio-Economic Analysis:
    • Cause: India’s large longitudinal extent (~30°) causes a nearly two-hour difference in sunrise/sunset times between its easternmost and westernmost points.
    • Effect: A single time zone (IST) leads to a significant mismatch between the biological clock (driven by the Sun) and the administrative clock in the Northeast. This results in:
      • Loss of Productivity: Daylight hours are lost in the morning as offices open late relative to sunrise.
      • Increased Energy Consumption: More electricity is needed for lighting in the evening.
      • Social and Health Impacts: The misalignment can affect sleep patterns and overall well-being.
    • Historiographical View: The adoption of a single IST in 1906 was a decision driven by the need for administrative and railway uniformity in British India. However, the context has changed, and the current debate reflects a shift towards optimizing regional productivity and energy efficiency.

GS Paper II (Governance & Administration)

  1. Federalism and Regional Aspirations: The demand for a separate time zone from northeastern states can be viewed as an expression of regional aspiration for administrative arrangements that are better suited to their geographical reality. The central government’s reluctance highlights the classic tension between national uniformity and regional specificity.
  2. Policy Paralysis vs. Pragmatism: The government’s cautious stance against multiple time zones stems from legitimate concerns about administrative complexity, transport synchronization (especially railways), and national security. However, critics argue this reflects policy inertia. A pragmatic solution could involve advancing IST by 30 minutes for the entire country or introducing changes without creating a formal second time zone, such as adjusting administrative timings in certain regions.

GS Paper III (Economy & Science)

  1. Energy Efficiency and Economic Productivity: Scientific studies, including those by the National Physical Laboratory (NPL), have estimated that advancing IST or creating a second time zone could save billions of units of electricity annually. Furthermore, aligning work hours with daylight hours is projected to increase worker productivity and reduce accidents.
  2. The Role of Scientific Institutions: The debate highlights the crucial role of scientific bodies like the NPL (which maintains IST) and the Council of Scientific and Industrial Research (CSIR) in providing data-driven policy recommendations. The government’s decision-making process must balance these scientific inputs with political, social, and administrative considerations.
  3. Technological Solutions to Administrative Hurdles: While challenges like railway synchronization are significant, modern technology (automated signaling, digital scheduling) can mitigate many of the risks associated with multiple time zones that were formidable in the 20th century. The debate should consider these technological advancements.

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