Origin and Evolution of Earth

Geography as a Discipline

• Definition of Geography: Geography is the study of the Earth's surface, its physical features, and the relationships between humans and their environment. The term was coined by the Greek scholar Eratosthenes (276-194 BC).

• Branches of Geography:

  • Physical Geography: Studies natural features like landforms, climate, and ecosystems.

  • Human Geography: Focuses on human activities, cultures, and their spatial distribution.

• Interdisciplinary Nature: Geography overlaps with disciplines like geology, meteorology, sociology, and economics, making it a holistic science.

Theories of Origin of Earth and Universe

• Nebular Hypothesis (Kant-Laplace Hypothesis) – Proposed by Immanuel Kant (1755) & Pierre-Simon Laplace (1796), suggesting that the solar system evolved from a spinning nebula.

• Big Bang Theory (George Lemaitre, 1931) – Universe originated 13.8 billion years ago from a singular explosion.

• Modern Nebular Hypothesis (Otto Schmidt & Carl Weizsäcker, 1950s) – Dust-gas clouds condensed into the Sun and planets.

Some Basic terms about Space(Prelims Specific)

• Einstein's Theory of Relativity (1915) – The General Theory of Relativity states that gravity is not a force but the curvature of spacetime caused by massive objects. It Helps in understanding black holes, neutron stars, and cosmic events, revolutionizing astrophysics.

• Gravitational Waves (Predicted in 1916) – Ripples in spacetime caused by accelerating massive objects (e.g., merging black holes).

• Gravitational Lensing - Light around a massive object, such as a black hole, is bent, causing it to act as a lens for the things that lie behind it.

• Singularity - A singularity (gravitational singularity or (spacetime singularity) is a condition in which gravity is so intense that spacetime ceases to exist and our laws of physics become invalid. Singularities were first predicted as a result of Einstein’s Theory of General Relativity, which resulted in the theoretical existence of black holes. 

• Galaxy-Galaxy is a system of millions or billions of stars, together with gas and dust, held together by gravitational attraction. They are the major building blocks of the universe.

• Life Cycle of a Star-  It is formed by a nebula, to its death as a white dwarf(in case of small star) or a neutron star(in case of large star). 

• Nebula: A cloud of gas (mostly hydrogen and helium) and dust in space. Nebulae are the birthplaces of stars.

• Supernova - A supernova is the explosive death of a star and often results in the star obtaining the brightness of 100 million suns for a short time

• Neutron star - Neutron stars are composed mainly of neutrons and are produced after a supernova, forcing the protons and electrons to combine to produce a neutron star

• Black Holes - Black holes are believed to form from massive stars at the end of their lifetimes. The density of matter in a black hole cannot be measured (infinite!). The gravitational pull is so great that nothing can escape from it, not even light.

• Constellations - The stars forming a group that has a recognisable shape is called a constellation. A few famous constellations are Great Bear (the Big Dipper or Saptarshi or Ursa Major), Orion (hunter).

  
  

Solar System

• The Solar System formed 4.6 billion years ago from a rotating nebular cloud.

• Sun – Making up 99.86% of the Solar System’s mass, primarily hydrogen and helium.

• Planets – Inner (rocky) planets: Mercury, Venus, Earth, Mars; Outer (gas/ice giants): Jupiter, Saturn, Uranus, Neptune.

• Dwarf Planets & Moons – Pluto, Ceres, and others; Jupiter and Saturn have the most moons.

  Mnemonics to remember order of planets: "My Very Educated Mother Just Served Us Noodles" (Mercury → Neptune).

  
  

Geological Time Scale (For Prelims)

• Precambrian (4.6 billion – 541 million years ago) – Earth’s early history, first life forms.

• Paleozoic Era (541-252 million years ago) – Explosion of marine life.

• Mesozoic Era (252-66 million years ago) – Dinosaurs dominated; ended with asteroid impact.

• Cenozoic Era (66 million years ago – Present) – Mammals evolved, modern continents formed.