SSC CGL - Detailed Guide 2025

Self-Paced Course

Universe and Solar System

Reference: Lucent GK, NCERT Class 6–12

The Universe includes all of space, matter, energy, stars, galaxies, and planets.
Age of the universe: ~13.8 billion years.
Origin theory: Big Bang Theory is the most accepted explanation.
Our galaxy is the Milky Way (called Akash Ganga in Hindi).
Nearest galaxy: Andromeda.

The Solar System is part of the Milky Way.
It consists of the Sun at the center and 8 planets, their moons (satellites), asteroids, comets, meteoroids, and dwarf planets.

Order	Planet	Key SSC Points
1	Mercury	Closest to Sun, smallest, no moons.
2	Venus	Hottest planet, retrograde rotation (clockwise), no moons, Earth’s twin.
3	Earth	Only planet with life, 1 moon, 71% water surface.
4	Mars	Known as Red Planet, has 2 moons (Phobos & Deimos).
5	Jupiter	Largest planet, has Great Red Spot, 79+ moons including Ganymede (largest moon).
6	Saturn	Known for prominent rings, 83+ moons, Titan is its largest moon.
7	Uranus	Rotates on its side, bluish due to methane, 27 moons.
8	Neptune	farthest from Sun, strong winds, 14 moons.

A satellite is any object that revolves around a planet.
Natural satellite: Moon (Earth), Phobos & Deimos (Mars), etc.
Artificial satellite: Man-made object in orbit (e.g., INSAT, GSAT).
Earth's Moon
- Only natural satellite of Earth.
- Takes 27.3 days to revolve and rotate around Earth (synchronous rotation).
- Causes tides on Earth due to gravitational pull.
- No atmosphere, extreme temperatures.

Small, rocky objects found mostly between Mars and Jupiter in the Asteroid Belt.
Largest asteroid: Ceres (also a dwarf planet).
Others: Vesta, Pallas, Hygiea.

Term	Description
Meteoroid	A small rock or debris moving in space.
Meteor	When a meteoroid enters Earth’s atmosphere and burns due to friction – "shooting star".
Meteorite	If part of a meteor survives and hits Earth.

A. Rotation (Earth spinning on its axis)
- Takes 24 hours (actually 23 hrs 56 mins)
- Direction: West to East.
- Effect: Day and Night
  - Sun appears to rise in the east and set in the west.
  - Equator rotates fastest, poles slowest.
B. Revolution (Earth orbiting around the Sun)
- Takes 365.25 days.
- Path is elliptical, not circular.
- Inclination of Earth's axis: 23.5°

Seasons
- Caused by Earth's tilt (23.5°) and revolution.
- Different parts of Earth receive varying sunlight:
  - Summer Solstice (June 21): Northern Hemisphere tilts towards the Sun → Longest day.
  - Winter Solstice (Dec 22): Southern Hemisphere tilts towards the Sun → Shortest day in Northern Hemisphere.
  - Equinoxes (March 21 & Sept 23): Equal day and night.
Year Length
- One complete revolution = 365¼ days → Hence, leap year every 4 years (adds 1 day to Feb).

Topic	Important Point
Venus	Hottest planet, retrograde rotation
Mars	Red Planet, 2 moons
Jupiter	Largest planet
Saturn	Ringed planet
Uranus	Tilted axis
Neptune	Coldest, farthest
Earth’s rotation	Causes day and night
Earth’s revolution + axial tilt	Causes seasons
Asteroid belt	Between Mars and Jupiter
Halley's comet	Appears every 76 years
Moon	1 natural satellite of Earth, causes tides

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