Elaborate Notes
The shape of the earth
The modern understanding of Earth’s shape is that it is not a perfect sphere but an oblate spheroid, a form colloquially termed a geoid. This means it is slightly flattened at the poles and bulges at the equator.
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Cause of the Geoid Shape: The primary cause for this deviation from a perfect sphere is the centrifugal force generated by Earth’s continuous rotation on its axis. This force is greatest at the equator, where the rotational speed is highest (approximately 1,670 km/h), and diminishes to zero at the poles. This outward force has caused the equatorial regions to bulge over geological time, resulting in an equatorial diameter that is about 43 kilometers greater than the polar diameter. Sir Isaac Newton, in his Principia Mathematica (1687), was the first to theorize that a rotating, self-gravitating fluid body would take the form of an oblate spheroid.
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Evidence of the Geoid Shape:
- Circumnavigation of the Earth: The first practical demonstration of Earth’s sphericity was the expedition led by Ferdinand Magellan and completed by Juan Sebastián Elcano between 1519 and 1522. The ability to travel continuously in one direction and return to the starting point is only possible on a curved surface.
- Circular Horizon: As an observer’s altitude increases, the visible horizon expands, and it always appears as a circle. This phenomenon, observable from a tall building, mountain, or aircraft, is a direct consequence of the Earth’s curved surface. This was one of the earliest observations noted by ancient Greek philosophers like Aristotle (c. 384–322 BC).
- Ship’s Visibility: When a ship sails away from an observer on the shore, its hull disappears first, followed by the mast. Conversely, an approaching ship’s mast is visible before its hull. This gradual appearance and disappearance over the horizon would not occur on a flat surface. The Bedford Level experiment, conducted by Samuel Rowbotham in the 19th century to ‘prove’ a flat Earth, was later scientifically debunked by experiments like the one conducted by Alfred Russel Wallace in 1870, which correctly accounted for atmospheric refraction and confirmed the curvature.
- Varying Times of Sunrise and Sunset: The rotation of a spherical Earth from west to east causes the sun to rise in the east and set in the west. Different locations experience sunrise and sunset at different times. If the Earth were flat, all locations would experience sunrise and sunset simultaneously.
- Lunar Eclipse: During a lunar eclipse, the Earth passes between the Sun and the Moon, casting its shadow on the Moon. This shadow is consistently circular, regardless of the Earth’s orientation. Aristotle noted that only a spherical object can produce a circular shadow in all orientations.
- Other Planetary Bodies: Observations through telescopes, since the time of Galileo Galilei in the early 17th century, have shown that other celestial bodies like the Moon, Sun, and other planets are spherical. The principle of mediocrity suggests that Earth, being a planet formed under similar physical laws, would also be spherical.
Latitude and longitude
Latitude and Longitude form a coordinate grid system, known as the graticule, used to specify any location on Earth’s surface. The concept was first systematically developed by the Greek astronomer and geographer Hipparchus (c. 190–c. 120 BC).
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Latitude:
- Definition: Latitude is the angular distance of a point on the Earth’s surface, measured in degrees north or south of the Equator. The angle is measured from the center of the Earth.
- Parallels of Latitude: These are imaginary circles running east-west, parallel to the Equator. All points on a given parallel have the same latitude.
- Key Parallels:
- The Equator (0°) is the largest parallel and a Great Circle, dividing the Earth into the Northern and Southern Hemispheres.
- Other important parallels include the Tropic of Cancer (23.5° N), Tropic of Capricorn (23.5° S), Arctic Circle (66.5° N), and Antarctic Circle (66.5° S).
- Properties:
- The length of the parallels decreases as one moves from the Equator towards the poles. At the poles (90° N and 90° S), they are merely points.
- The distance between any two consecutive parallels, one degree apart, is relatively constant, approximately 111 kilometers. This consistency arises because the lines are parallel.
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Longitude:
- Definition: Longitude is the angular distance of a point on the Earth’s surface, measured in degrees east or west of the Prime Meridian.
- Meridians of Longitude: These are imaginary semi-circles that run from the North Pole to the South Pole. All points on a given meridian have the same longitude.
- Key Meridian: The Prime Meridian (0°), by international agreement in 1884, passes through the Royal Observatory at Greenwich, London. The 180° meridian, also known as the International Date Line (with some deviations), is directly opposite the Prime Meridian.
- Properties:
- All meridians are of equal length.
- Meridians are not parallel; they converge at the North and South Poles.
- The distance between two meridians is maximum at the Equator (approximately 111 km for a 1° separation) and decreases to zero at the poles where they all meet.
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Great Circle:
- Definition: A Great Circle is any circle drawn on the surface of a sphere whose center and radius are the same as the sphere’s center and radius. It represents the largest possible circumference on the sphere’s surface.
- Properties:
- It divides the sphere into two equal halves or hemispheres. The Equator is a unique parallel of latitude that is also a Great Circle. All meridians of longitude are semi-circles of a Great Circle.
- The shortest distance between any two points on the surface of the Earth lies along the arc of the Great Circle connecting them. This path is known as a Great Circle Route and is crucial for intercontinental aviation and shipping to save fuel and time.
The rotation of the earth
Rotation refers to the spinning of the Earth on its own axis. This motion is fundamental to many of Earth’s physical and biological systems.
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Axis of Rotation and Axial Tilt: The Earth rotates on an imaginary line, the axis, which passes through the North and South Poles.
- The plane on which the Earth revolves around the Sun is called the orbital plane or the ecliptic plane.
- Earth’s axis is not perpendicular to its orbital plane. It is tilted at an angle of 23.5 degrees from the perpendicular line (normal).
- Consequently, the angle between the Earth’s axis and its orbital plane is 66.5 degrees. This axial tilt is the primary reason for the seasons.
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Direction of Rotation: The Earth rotates from west to east, which is counter-clockwise when viewed from above the North Pole. This is why the Sun, Moon, and stars appear to rise in the east and set in the west.
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Period of Rotation:
- Solar Day: This is the time it takes for the Earth to rotate once relative to the Sun, i.e., the time between two successive noons when the Sun is at its highest point in the sky. It is defined as 24 hours. It is slightly longer than a sidereal day because as the Earth rotates, it also moves along its orbit around the Sun, requiring a little extra rotation to “catch up” to the same position relative to the Sun.
- Sidereal Day: This is the time it takes for the Earth to complete one full 360-degree rotation relative to distant, fixed stars. It is 23 hours, 56 minutes, and 4.09 seconds. This is the true rotational period of the Earth.
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Speed of Earth’s Rotation:
- The speed of rotation is not uniform across the planet’s surface. It is an angular velocity, but the linear speed varies with latitude.
- The speed is maximum at the Equator (approx. 1,670 km/h) and decreases with increasing latitude, becoming zero at the poles.
- This variation has practical implications; for example, space agencies launch rockets from sites near the equator (e.g., Kourou in French Guiana, Sriharikota in India) to take advantage of the Earth’s rotational speed, which provides an initial velocity boost to the launch vehicle, saving fuel.
Revolution of the earth
Revolution is the motion of the Earth in its orbit around the Sun.
- Orbit and Speed: The Earth revolves around the Sun in an elliptical orbit, not a perfect circle, a discovery credited to Johannes Kepler’s laws of planetary motion (early 17th century).
- The average speed of revolution is approximately 29.8 km/s, or about 107,000 km/h.
- Period of Revolution: The time taken for one complete revolution is approximately 365.25 days (365 days, 5 hours, 48 minutes, and 46 seconds). The extra quarter of a day is accounted for by adding a leap day (February 29th) every four years.
- Direction of Revolution: Similar to its rotation, the Earth revolves around the Sun in a counter-clockwise direction when viewed from above the North Pole.
- Perihelion and Aphelion: Due to the elliptical orbit, the distance between the Earth and the Sun varies throughout the year.
- Perihelion: The point in the orbit where the Earth is closest to the Sun (about 147.1 million km). This occurs around January 3rd.
- Aphelion: The point in the orbit where the Earth is farthest from the Sun (about 152.1 million km). This occurs around July 4th.
- It is a common misconception that this varying distance causes seasons. The Northern Hemisphere experiences winter during Perihelion and summer during Aphelion.
Seasons
Geographically, there are four distinct seasons: Summer, Winter, Autumn (Fall), and Spring. These are not caused by the Earth’s varying distance from the Sun but by the 23.5-degree axial tilt combined with the Earth’s revolution. As the Earth orbits the Sun, the tilt remains constant and pointed in the same direction (towards Polaris, the North Star). This causes different parts of the Earth to receive more direct or indirect solar radiation at different times of the year, leading to the cycle of seasons.
Prelims Pointers
- The specific shape of the Earth is called a Geoid or Oblate Spheroid.
- The Earth is flattened at the poles and bulges at the equator due to centrifugal force from its rotation.
- First circumnavigation of the Earth was completed by the Magellan-Elcano expedition (1519-1522).
- The Bedford Level experiment is historically associated with debates about the Earth’s curvature.
- A circular shadow of the Earth on the Moon during a lunar eclipse is evidence of its spherical shape.
- Latitude is the angular distance north or south of the Equator (0°).
- Lines of latitude are called parallels and are parallel to each other.
- The distance between one degree of latitude is approximately 111 km.
- Longitude is the angular distance east or west of the Prime Meridian (0°).
- Lines of longitude are called meridians and converge at the poles.
- The distance between one degree of longitude is 111 km at the Equator and zero at the poles.
- A Great Circle is the largest circle that can be drawn on a sphere and represents the shortest distance between two points on the surface.
- The Earth’s axis of rotation is tilted at an angle of 23.5° from the perpendicular to its orbital plane.
- The angle between the Earth’s axis and its orbital plane is 66.5°.
- Direction of rotation is West to East (counter-clockwise).
- A Solar Day is 24 hours (rotation relative to the Sun).
- A Sidereal Day is 23 hours, 56 minutes (true rotation relative to stars).
- Rockets are preferably launched from locations near the equator to gain an initial velocity boost.
- The path of Earth’s revolution around the Sun is elliptical.
- Period of revolution is 365.25 days (365 days and 6 hours approx.).
- Perihelion: Earth is nearest to the Sun, around January 3rd.
- Aphelion: Earth is farthest from the Sun, around July 4th.
Mains Insights
Geopolitical and Scientific Significance of Earth’s Shape and Grid System
- Evolution of Scientific Thought: The understanding of Earth’s shape from a flat disc (in ancient mythologies) to a perfect sphere (Pythagoras, Aristotle) and finally to an oblate spheroid (Newton) marks a significant progression in scientific methodology—from philosophical reasoning and simple observation to mathematical modeling and empirical verification. This journey reflects humanity’s developing capacity for abstract thought and precision measurement.
- The Longitude Problem: While latitude could be determined relatively easily using celestial navigation (e.g., measuring the angle of Polaris), determining longitude was a major scientific and navigational challenge until the 18th century. The inability to accurately determine longitude led to numerous shipwrecks and economic losses. The solution, pioneered by John Harrison’s marine chronometer, was a landmark achievement that revolutionized maritime trade and naval power, underscoring the critical link between scientific innovation, economic imperatives, and national security.
- Standardization and International Cooperation: The establishment of the Prime Meridian at Greenwich in 1884 at the International Meridian Conference was a political and economic decision as much as a scientific one. It reflected the dominance of the British Empire and the global reach of its maritime charts. It also represented a crucial step in global standardization, essential for international travel, communication, and timekeeping (Time Zones).
Causal Relationships of Earth’s Motions and Their Consequences
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Rotation and its Manifold Effects:
- Cause: Earth’s spin on its axis.
- Effects:
- Day and Night Cycle: The most direct consequence, governing the diurnal rhythms of all life forms.
- Coriolis Effect: The deflection of moving objects (like wind and ocean currents) to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This is a fundamental concept in climatology and oceanography, explaining the direction of trade winds, cyclones, and ocean gyres.
- Tides: While primarily caused by the Moon’s gravity, the Earth’s rotation plays a role in the timing and pattern of the twice-daily (semidiurnal) tides.
- Geoid Shape: As discussed, the centrifugal force from rotation creates the equatorial bulge.
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Revolution, Axial Tilt, and Seasonality:
- Cause: The combination of Earth’s revolution around the Sun and the constant 23.5° tilt of its axis.
- Effect: The occurrence of seasons. It is crucial to understand this is not due to the varying Earth-Sun distance (Perihelion/Aphelion). When the Northern Hemisphere is tilted towards the Sun, it receives more direct solar radiation, leading to summer. Six months later, when it is tilted away, it receives less direct, more oblique rays, resulting in winter. The opposite occurs simultaneously in the Southern Hemisphere.
- Significance: This predictable cycle of seasons is the foundation for global climate patterns, agricultural cycles, ecosystems, and human cultures. The stability of Earth’s axial tilt over long periods has been a key factor in providing a stable climate conducive to the evolution of complex life.
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Interplay of Concepts: The speed of rotation provides an initial thrust for rocket launches, a practical application of a fundamental geographical concept. Great Circle routes, the shortest distance on a sphere, are a direct application of understanding Earth’s shape for economic benefit (saving fuel and time in aviation). These examples highlight how abstract geographical principles have tangible, real-world applications in technology and economics.