Geographical Terms & Concepts

The Solar System consists of the Sun and all celestial bodies bound to it by gravitational pull — 8 planets, their moons, dwarf planets, asteroids, comets, and other small bodies.

• Stars — Huge, luminous balls of hot gases that produce their own light and heat (e.g., the Sun).
• Planets — Non-luminous bodies that revolve around stars in fixed orbits (e.g., Earth). Per the International Astronomical Union (IAU) 26th General Assembly (2006, Prague), a planet must: (1) be a circular/spherical body, (2) orbit in a fixed path around the Sun, and (3) possess considerably more mass than any satellite — otherwise it is a Natural Satellite or a Dwarf Planet.
• Moons (Satellites) — Natural bodies that revolve around planets (e.g., Moon).
• Asteroids — Small, rocky bodies revolving around the Sun, mostly found in the Asteroid Belt.
• Comets — Icy bodies revolving around the Sun in elongated orbits; develop a tail when near the Sun. Comets originate from the Oort Cloud at the periphery of the Solar System.
• Meteoroids — Small particles of rocks and metals that burn on entering Earth's atmosphere.
• White Hole — A hypothetical concept; exactly opposite to a Black Hole — it emits stars and energy.

1 Astronomical Unit (AU) — The unit of distance in space; 1 AU = 149,597,870.7 km (approx. 150 million km), equal to the distance between the Earth and the Sun. Distances of all other heavenly bodies are measured with respect to the AU.

• Oort Cloud — A vast region at the periphery of the Solar System from which comets are produced and scattered.
• Milky Way Galaxy — According to the Big Bang Theory, cosmic substance formed a single dense mass that scattered to form galaxies. One such galaxy is the Milky Way — a spiral-shaped galaxy discovered by Galileo in 1670.
• Perihelion & Aphelion — As Earth follows an elliptical orbit, its distance from the Sun is never constant. Earth is nearest to the Sun on 3rd January (called Perihelion) and farthest on 4th July (called Aphelion).
• Supernova — Stars larger than the Sun end their life with a supernova explosion; they shrink so intensely that all gases collapse and gravity becomes so strong that even light cannot pass (forming a black hole).
• Radiometric Dating — Process used to ascertain the age of rocks, carbon, or fossils; works on the principle that the amount of radioactive carbon depletes with time, allowing scientists to determine the age of the Earth.
• Umbra — The darkest portion during an Eclipse; in this zone the Sun is totally eclipsed.

Midnight Sun — Occurs during summer months in the Northern places of the Arctic Circle and southern places of the Antarctic Circle, where the sun is visible even at midnight.

• Equinox — Occurs twice a year; on this day, the day and night are equal throughout the globe as the Sun is exactly above the Equator.
• Spring Equinox — 21st March
• Autumnal Equinox — 23rd September (Sun heading towards Tropic of Capricorn)
• International Date Line (IDL) — Passes through the Pacific Ocean, roughly following 180° Latitude and 180° Longitude, dividing the Earth into Eastern and Western hemispheres.
• Horse Latitudes — Also known as the "Belt of Calm"; the region between 30°–35° N and 30°–35° S latitudes. Also called the High Pressure Subtropical Belt.
• Earth Hour — A global event that started as a light-off event in Sydney, Australia in 2007, organised by the World Wide Fund (WWF) for nature. Communities turn off non-essential lights for one hour from 8:30 pm – 9:30 pm at the end of March, symbolising human commitment towards the Earth.

Ozone Layer — Lies in the stratosphere; blocks harmful UV rays from the Sun. A hole has developed in the Ozone layer above Antarctica near the south pole.

• Meteorology — The branch of science concerned with the processes of the atmosphere; e.g., forecasting of weather.
• Isobars (see Iso-lines table in Section 5).

Iso-lines are imaginary lines on a map joining places of equal values of a given quantity — frequently tested in SSC exams.

Cartography — The study and practice of map making. The person who draws maps is called a cartographer.

• Contours — Imaginary lines connecting places of similar heights on a map.
• Planimeter (Platometer) — An instrument used to measure the area of 2-dimensional shapes (e.g., area of maps).
• Magnetic Meridian — An equivalent imaginary line connecting the magnetic South and North poles; can be taken as the horizontal component of the magnetic force.

Moho Discontinuity (Mohorovičić Discontinuity) — The boundary between the Earth's crust and mantle; lies 35 km below continents and 7 km beneath oceanic crust.

• Continental Drift Theory — Proposed by Alfred Wegener in 1912; states that continents have changed their initial positions and that initially there was only one continent — Pangaea.