The Earth and the Solar System
The Origin of the Earth
The most widely accepted scientific explanation for Earth’s origin is the Big Bang Theory. Approximately 13.8 billion years ago, all matter and energy in the universe was compressed into an extremely hot, dense point that exploded向外扩张. This expansion continues today.
Formation of the Solar System
About 4.6 billion years ago, a rotating cloud of gas and dust (solar nebula) began to collapse under its own gravity. At the center, the Sun formed. The remaining material slowly orbited and clumped together to form planets, including Earth.
Evolution of the Earth
- Hadean Eon (4.6–4 billion years ago): Volcanic, no atmosphere, meteorite bombardment
- Archean Eon (4–2.5 billion years ago): First oceans, single-celled life appears
- Proterozoic Eon (2.5 billion – 541 million years ago): Oxygen rises, first multicellular life
- Phanerozoic Eon (541 million years ago–present): Cambrian explosion, complex life, dinosaurs, humans
Structure of the Earth
The Earth has three main layers:
Crust
- Outermost layer, thinnest (5–70 km thick)
- Two types: Continental crust (lighter, granite-like, ~30 km thick) and Oceanic crust (denser, basalt-like, ~5–10 km thick)
- Contains oxygen, silicon, aluminum, iron
Mantle
- Below the crust, extends to ~2,900 km depth
- Semi-solid, hot, rocky material — convection currents here drive plate tectonics
- Contains iron and magnesium (mafic minerals)
Core
- Outer Core: Liquid iron-nickel, creates Earth’s magnetic field
- Inner Core: Solid iron-nickel, extremely hot (~5,000°C)
- The magnetic field (magnetosphere) protects Earth from harmful solar radiation
Atmosphere
Earth’s atmosphere is a mixture of gases:
| Gas | Percentage |
|---|---|
| Nitrogen (N₂) | 78% |
| Oxygen (O₂) | 21% |
| Argon (Ar) | 0.93% |
| Carbon Dioxide (CO₂) | 0.04% |
| Others (Ne, He, CH₄) | Trace |
Layers of the Atmosphere
- Troposphere (0–12 km): Weather occurs here, all life, highest density
- Stratosphere (12–50 km): Ozone layer resides here — absorbs UV radiation
- Mesosphere (50–80 km): Coldest layer, meteors burn up here
- Thermosphere (80–700 km): Aurora borealis occurs here, ISS orbits here
- Exosphere (700–10,000 km): Fades into space — contains satellites
Ozone Layer
The ozone layer (in stratosphere) is critical for life on Earth. It absorbs ultraviolet (UV) radiation. Without it, UV would cause skin cancer, damage crops, and disrupt ecosystems. CFCs (chlorofluorocarbons) are known to deplete ozone — this led to the Montreal Protocol (1987).
The Solar System
The Sun
- A middle-aged star, 4.6 billion years old
- Composed mostly of hydrogen (71%) and helium (27%)
- Surface temperature: ~5,500°C | Core temperature: ~15 million°C
- Energy from nuclear fusion (hydrogen → helium)
Planets (In order from Sun)
| Planet | Type | Special Features |
|---|---|---|
| Mercury | Terrestrial | Closest, smallest, no atmosphere |
| Venus | Terrestrial | Hottest (greenhouse), rotates backwards |
| Earth | Terrestrial | Only planet with liquid water and life |
| Mars | Terrestrial | Red planet, Olympus Mons (largest volcano) |
| Jupiter | Gas giant | Largest, Great Red Spot (storm) |
| Saturn | Gas giant | Famous rings, least dense (floats on water) |
| Uranus | Ice giant | Rotates on its side, blue-green |
| Neptune | Ice giant | Farthest, coldest, fastest winds |
Terrestrial planets (rocky): Mercury, Venus, Earth, Mars Jovian planets (gas giants): Jupiter, Saturn Ice giants: Uranus, Neptune
Moon
Earth has one natural satellite — the Moon. Key facts:
- Diameter: ~3,474 km (about 1/4 of Earth’s)
- No atmosphere, no water
- synchronous rotation (same side always faces Earth)
- Famous features: Sea of Tranquility (Apollo 11 landing), craters, highlands
Phases of the Moon
The Moon doesn’t emit light — we see it illuminated by reflected sunlight. As the Moon orbits Earth, the Sunlit portion visible from Earth changes:
New Moon → Waxing Crescent → First Quarter → Waxing Gibbous → Full Moon → Waning Gibbous → Last Quarter → Waning Crescent → New Moon
CTET mnemonic: New Moon = 0% lit (we see dark side). Full Moon = 100% lit.
Seasons
Earth’s tilted axis (23.5°) causes seasons:
- When the Northern Hemisphere tilts toward the Sun → Summer (longer days)
- When the Southern Hemisphere tilts toward the Sun → Winter in North
- Equinox (March 21, September 23): Day and night equal lengths — Earth’s axis is perpendicular to the Sun’s rays
Key Phenomena
Solar Eclipse
Moon comes between Earth and Sun → blocks sunlight partially or fully. Total solar eclipse occurs when the Moon completely covers the Sun — only visible from a narrow path on Earth.
Lunar Eclipse
Earth comes between the Sun and Moon → Earth’s shadow falls on Moon. During a total lunar eclipse, the Moon turns reddish (called a “Blood Moon”).
CTET Exam Focus
- Earth’s three layers: crust, mantle, core — composition and thickness
- Atmosphere composition (N₂ at 78%, O₂ at 21%)
- Ozone layer and CFC threat (Montreal Protocol)
- Solar system: Order of planets, terrestrial vs jovian types
- Moon phases: New → Full → New cycle (29.5 days)
- Seasons: Tilt of Earth’s axis (23.5°)
- Difference between solar and lunar eclipse
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