Earth & Universe — NDA Geography Notes

The Universe

• Definition: All existing matter and energy — including space, time, galaxies, stars, planets, gas, and dust. *
• Age: ≈ 13.8 billion years
• Origin: Big Bang Theory
• All matter and energy were once concentrated in an extremely hot, dense point (singularity).A massive explosion (~13.8 billion years ago) led to the continuous expansion of the Universe.
• A massive explosion (~13.8 billion years ago) led to the continuous expansion of the Universe.
• Within ~300,000 years, atoms formed and the universe became transparent to radiation (Cosmic Microwave Background emerged).

Galaxy

• Definition: A vast system of stars, gas, dust, and dark matter* bound together by gravity.
• Types (by shape): Spiral, Elliptical, Irregular.
• Our Galaxy: Milky Way (Akash Ganga) → Spiral Shape [NDA 2008-II]
• Our solar system lies in the Orion Arm (Orion Spur) of the Milky Way Galaxy.
• Nearest dwarf galaxies: Large & Small Magellanic Clouds*
• Nearest major galaxy: Andromeda (M31) — about 2.5 million light-years away

Star

• Definition: A self-luminous celestial body that produces energy through nuclear fusion (Hydrogen → Helium).
• Sun – Nearest star to Earth.
• Brightest star in our night sky: Sirius (Dog Star) — appears brightest due to closeness (8.6 light-years).
• Nearest star system: Alpha Centauri system (three stars: Alpha A, Alpha B, Proxima Centauri).
• Closest star (after the Sun): Proxima Centauri [CDS 2019-I] ~4.24 light-years away.

Constellation

• Definition: A group of stars forming a recognizable pattern in the night sky. [CDS 2019-I]
• Ex:
• Ursa Major (The Great Bear, Saptarishi) [CDS 2008-II]Ursa Minor (The Little Bear, contains the pole star at the end)
• Orion (The Hunter)
• Zodiac Signs: 12 Zodiac Signs are constellations. They lie along the Ecliptic Plane (apparent path of the Sun).
• Pole Star (Dhruv Tara): Appears almost fixed in the northern sky, located near the celestial north pole → used to find direction (north).

Our Solar System

• The Sun, 8 planets, their satellites (moons), dwarf planets, asteroids, comets, meteoroids, and other celestial bodies form the Solar System.
• Age ≈ 4.6 billion years.

The Sun

• Nearest star;
• ~99.86% of solar system mass;
• Distance from Earth ≈1 AU
• Light time ≈ 8 min 20 s
• Composition (approx.): H ~71%, He ~27%, others ~2%.
• Energy source: Nuclear fusion (H → He).

Planets of the Sun

Definition of Planet (IAU – 2006): A planet is a celestial body that:

1. Orbits the Sun
   1. Has enough mass so that its gravity makes it nearly spherical in shape*
   1. Has cleared its orbit of other smaller objects

Eight Planets (Pluto: Reclassified in 2006 as a Dwarf Planet.)

1. Mercury:
   • Closest to Sun; not the hottest (no atmosphere);
   • Extreme temperature range; (Day (facing Sun): +430°C, Night (away from Sun): −180°C; Difference ≈ 610°C)
2. Venus:
   • Earth’s Twin (similar size + composition)
   • Brightest planet (after Sun & Moon): Thick yellowish sulphuric acid clouds reflect sunlight (high albedo) [CDS 2009-I] [CDS 2022-II]
   • Appears as Morning Star / Evening Star
   • Veiled Planet →Surface completely hidden by clouds
   • Atmosphere: ~96.5% CO₂ + thick H₂SO₄ clouds→traps solar heat (greenhouse effect) [CDS 2023-II]
   • Hottest Planet→ Even hotter than Mercury despite being farther from Sun
   • Retrograde rotation (spins East → West) → Only Venus & Uranus show backward rotation
3. Earth:
4. Known as “Blue Planet” (≈71% surface water)
5. Densest planet or highest density [NDA 2021-I]
6. Shape – Geoid (slightly flattened at poles)
7. Life supported by moderate temperature, liquid water, oxygen, and the ozone layer
8. Magnetosphere protects from harmful solar radiation
9. Mars:
10. Known as Red Planet (iron oxides).
11. Olympus Mons – largest volcano.
12. Evidence of past water, polar ice caps.
13. Thin atmosphere composed of carbon dioxide (~95%), with traces of nitrogen and argon.

[CDS 2023-II]

5. Jupiter:
   • Largest planet
   • Great Red Spot-giant storm
   • Fastest rotation (~10 h)
   • Strong magnetic field
6. Saturn:
   • Most prominent ring system (ice + rock particles)
7. Uranus:
   • Axial tilt ~98° → “rolls on its side”
   • retrograde rotation
   • Faint rings
   • Methane gives cyan/greenish hue (methane)
8. Neptune
   • Farthest planet.
   • Deep blue colour due to presence of methane gas (which absorbs red light, reflects blue)[CDS 2012-II]
   • Fastest winds

Planet Groups

1. Inner/Terrestrial (Rocky): [CDS 2012-II] [CDS 2020-II]

• Mercury, Venus, Earth, Mars.
• Rocky, small, dense, few/no moons
• Outer/Jovian: [CDS 2023-I]

• Jupiter, Saturn, Uranus, Neptune.
• Gas/ice giants, massive, thick atmospheres (H₂ + He), many moons, all have rings
• Separated by the Asteroid Belt (between Mars & Jupiter).

Order of the Planets

• Order by distance from Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
• Order by size (diameter, largest→smallest): Jupiter, Saturn, Uranus, Neptune, Earth, Venus, Mars, Mercury  

[NDA 2009-I] [NDA 2021-II] [CDS 2016-I]

• Order by mass (largest→smallest): Jupiter, Saturn, Neptune, Uranus, Earth, Venus, Mars, Mercury

Other Celestial Bodies

• Dwarf Planets: Pluto, Eris, Haumea, Makemake (Kuiper Belt); Ceres (Asteroid Belt).
• Asteroids: Rocky bodies, mainly between Mars & Jupiter.
• Comets:  Icy bodies on elongated orbits; from regions like the Kuiper Belt/Oort Cloud. Develop tails (gas and dust) when near the Sun due to sublimation. [CDS 2009-II] [CDS 2012-II]

• Meteoroid: Small rocky or metallic body in space.
• Meteor: Meteoroid entering Earth’s atmosphere, producing a bright streak. (shooting star)
• Meteorite: Meteoroid that survives its journey and lands on Earth.
• Black Hole: [NDA 2019-I] [CDS 2009-I] [CDS 2014-I]

The Moon

• A natural satellite — a celestial body that orbits a planet due to the planet’s gravity.
• The Moon is Earth’s only natural satellite.

• Formation Theory: Giant Impact Hypothesis or Big Splat*. A Mars-sized body collided with Earth→. The Moon formed from the debris.
• Surface: Very low albedo (~0.12); no significant atmosphere → extreme temperature, no liquid water on surface; no life.
• Great dark plains on the Moon are called “Maria” (Latin for seas) —formed by ancient volcanic activity and lava flows. [NDA 2009-I]
• Mean distance ≈ 384,400 km.
• Diameter ≈ ¼ of Earth.

• The Moon’s rotation and revolution periods are equal (≈27.3 days), so the same side of the Moon always faces Earth. (Synchronous Rotation).
• Sidereal* Month: The time taken by the Moon to complete one full revolution around Earth with respect to distant stars*. ≈ 27.3 days
• Synodic Month: The time taken by the Moon to go from one Full Moon to the next Full Moon, or from one same lunar phase to the next, with respect to the Sun. 29.5 days*