Earth as a Planet - Comprehensive Notes

Proof that Earth is Spherical in Shape

• Sighting of a Ship:
  • When a ship approaches the shore, one first sees the mast, then the deck, funnel, and finally the hull.
  • This is only possible if the Earth's surface is curved.
  • If the Earth were flat, the entire ship would be visible at once.
• The Bedford Level Experiment:
  • Three poles of equal length were fixed at intervals of 5 km in the Bedford Level canal area in England.
  • The middle pole appeared slightly higher when viewed from one end of the canal.
  • This could not happen on a flat surface, demonstrating the Earth's curved surface.
  • The experiment was repeated successfully in different places.
• Pole Star:
  • The Pole Star is seen at an angle of 90° at the North Pole and its angle decreases towards the Equator, where it is 0°.
  • This is possible only in an arc of a circle.
  • If the Earth were flat, the Pole Star would have the same height for all latitudes.
• Satellite Pictures:
  • Jean Baptiste Delambre (1749-1822), a French astronomer, discovered the flattening of the Earth at the poles.
  • Satellite pictures fed into computers confirm the Earth's spherical shape with flattened poles.
  • Conclusive evidence comes from photographs taken from space.
• Lunar Eclipse:
  • The shadow of the Earth on the moon during a lunar eclipse is clearly visible as an arc of a circle.
  • Since the Earth is bigger than the moon, the shadow is always circular.
• Circular Horizon:
  • The view of the Earth's surface from a height is circular.
  • As altitude increases, the circular horizon widens.
  • On a flat Earth, the visible horizon would be the same regardless of altitude.
• Sunrise and Sunset:
  • Sunrise and sunset occur at different times at different places.
  • The Earth rotates from west to east, so people in the east see the sun earlier.
  • On a flat Earth, sunrise and sunset would occur at the same time worldwide.
• Circumnavigation:
  • Magellan's ship Victoria completed a round-the-world voyage in 1522.
  • The ship returned to its starting point (Spain), proving the Earth's surface is round.
• The Earth as a Heavenly Body:
  • The Earth belongs to the Solar System, where all planets are spherical due to continuous rotation.
  • As a member of the Solar System, the Earth cannot be an exception.

Earth as a Planet

• The Earth is a planet, a spherical object in space that moves around the Sun (a star) and receives light from it.
• Planets do not have their own light; they move around their stars in fixed paths called orbits.

Earth an Oblate Spheroid

• The Earth is not a perfect sphere; it bulges slightly at the Equator and is flattened at the Poles.
• The Earth's diameter at the Equator is $12,756$ km, while its polar diameter is $12,714$ km.
• This difference is due to the centrifugal force of the Earth's rotation at a great speed, which forms a bulge at the Equator and compression at the Poles.
• The Earth is described as an Oblate Spheroid.
• The shape of the Earth is also described as Geoid, which means Earth-shaped.

Earth as the Home of Humankind

• The Earth is the only planet that supports life.
• It is covered with green vegetation, enormous blue-green oceans containing over a million islands, a large number of streams and rivers, huge land masses called continents with mountains, ice caps, and deserts.
• Life thrives virtually in every part of the Earth, from the coldest parts of the Poles to the warmest parts of the Equator.
• Earth has certain features that make it habitable and unique.

Distance from the Sun

• The Earth is at an optimum distance from the sun, making it neither too hot nor too cold.
• This distance makes life possible on Earth.
• If closer to the sun, life would burn; if farther, life would freeze.

Temperature

• The Earth has a suitable average temperature of $17°C$ for life to exist.
• Small changes in average temperature could cause species to perish due to extreme heat or cold.
• Mercury and Venus are very hot (more than $400°C$).
• Other planets in the Solar System are very cold (below $0°C$).
• Venus is hotter than Mercury because its atmosphere is mainly composed of carbon dioxide, producing the Greenhouse effect.

Atmosphere

• The Earth's atmosphere contains a very small quantity of carbon dioxide.
• Atmosphere: The layer of air around the Earth.
  • It consists of gases like nitrogen (78%), oxygen (21%), carbon dioxide, helium, and argon (less than 1%).
  • The atmosphere receives heat from the sun by solar radiation and loses heat by Earth's radiation, maintaining balance.
  • The atmosphere contains life-supporting gases like nitrogen, oxygen, and carbon dioxide.
  • Ozone in the atmosphere absorbs harmful ultraviolet rays from the sun.
  • The atmosphere prevents heat loss from the Earth's surface and helps keep the Earth warm.

Water

• Earth is a watery planet with 70% of its surface covered by water.
• Distribution of water moderates the climate and surface conditions.
• Water can absorb a large amount of heat without much change in temperature.
• During the day, water bodies absorb a lot of heat, keeping the Earth cool.
• At night, water bodies release heat, preventing the surface from freezing.
• Water moves in a continuous cycle (hydrological cycle) with proper balance between evaporation, condensation, and precipitation.

Solid Crust (Lithosphere)

• Earth has a solid crust (Lithosphere).
• Weathering of rocks forms soil, essential for plant life.
• Soil provides nutrients for plant growth, supporting all forms of animal life directly or indirectly.

Biosphere

• The narrow realm of contact and interaction between the atmosphere, lithosphere, and hydrosphere.
• Provides all necessities for life: light, heat, water, food, and habitats.
• It is a thin layer approximately 15 km from the deepest ocean trench to the highest mountain peak.
• Extends to a depth of just 3 m below ground level and about 200 m deep in ocean waters.
• Life is found chiefly in two forms: the plant kingdom and the animal kingdom.

Ecosystem

• Ecosystem: Self-regulating and self-sustaining structural and functional unit of the biosphere.
  • Examples: pond, lake, desert, grassland, meadow, forests.
  • There is an exchange of materials and energy within the ecosystem and between adjoining ecosystems.
  • It consists of interconnected networks which constitutes the biosphere.

Life-Giving Cycles

Nutrition Cycle

• Represents a relationship between living and non-living things in the environment.
• Shows interdependence of all living things on Earth.
• Operates through processes like the food chain, photosynthesis, respiration, etc.
• Living things are divided into producers, consumers, and decomposers.
• Plants are producers of food through photosynthesis, from which energy originates.
• Consumers (herbivores and carnivores) live on producers.
• Man can modify his own environment and create changes in the food chain.
• All living beings undergo decomposition by saprotrophs or decomposers.

Carbon Cycle

• Carbon is the basic raw material of all life.
• All living things contain carbon.
• Atmospheric carbon dioxide is used by plants in the presence of sunlight and chlorophyll to make their own food by photosynthesis.
• During this process, $CO_2$ is fixed as starch by plants.
• Carbon moves along the food chain through consumers at different levels.
• After the death of plants and animals, carbon is decomposed and absorbed by saprophytic bacteria and fungi.
• Plants die and get buried, forming fossil fuels containing huge amounts of carbon.
• During respiration, plants, animals, and humans use atmospheric oxygen and release carbon dioxide back into the atmosphere.
• Burning carbon-containing fuels also releases $CO_2$ into the atmosphere.
• The cyclic movement of carbon is known as the carbon cycle.
• The carbon cycle helps maintain carbon balance in nature.
• Any imbalance of carbon in the atmosphere heats up the Earth, leading to Global Warming.
• Excessive use of gases that produce undue heat and global warming should be avoided.

Nitrogen Cycle

• The atmosphere is the largest reservoir (approximately 78%) of free nitrogen.
• Plants take nitrogen in the form of nitrates from the soil, which moves through the food chain to animals.
• Nitrogen is returned to the soil through manure, excreta, remains of plants and animals, and micro-organisms.

Oxygen Cycle

• Living things take in oxygen from the atmosphere to release energy from food.
• Oxygen is used with carbon, hydrogen, and nitrogen to build new molecules in their bodies.
• Oxygen is released back into the atmosphere by green plants during photosynthesis and by plants and animals as part of carbon dioxide.
• These cycles are nature's mechanism to maintain balance in the ecosystem.