🌍 Earth – Short Study Notes

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### 🧭 Age of Earth

#### 📌 Radioactive Dating

• Determined by radioactive dating, method developed by Ernst Rutherford (1905)
• Helps calculate age using decay of radioactive elements

#### 🔬 Types of Dating Methods

• Uranium-Lead Method → used for oldest rocks
• Potassium-Argon Method, Rubidium-Strontium Method → long-term dating
• Radiocarbon & Carbon (C) Dating → used for latest rocks
• Chlorine-36 Method → specialized dating technique

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### 🌐 Shape of Earth

#### 🟢 Geoid / Oblate Spheroid

• Earth is slightly flattened at poles and bulged at equator
• Not a perfect sphere → due to rotation

#### ⚖️ Forces Involved

• Centripetal Force → acts towards centre (inside)
• Centrifugal Force → acts away from centre

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### 📏 Radius of Earth

• Equatorial Radius → 6378 km
• Polar Radius → 6357 km
• Mean Radius → 6371 km

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### 🔄 Rotation of Earth

#### ⏱️ Basic Facts

• Earth rotates on its own axis
• Time → 23 hr 56 min 4 sec
• Direction → West to East

#### 🌍 Speed Variation

• Maximum at Equator
• Minimum at Poles

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### ☀️ Revolution of Earth

#### 🌀 Orbit Details

• Earth revolves around Sun in elliptical orbit
• Time → 365 days 6 hr 9 min 9 sec

#### 🚀 Speed

• Orbital Speed → 29.8 km/sec
• Maximum → Mercury
• Minimum → Neptune

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### 🌞 Distance from Sun

#### 🔥 Perihelion (Nearest Point)

• Date → January 3
• Distance → 14,75,00,000 km

#### ❄️ Aphelion (Farthest Point)

• Date → July 4
• Distance → 15,25,00,000 km

#### 🌙 Moon Distances

• Perigee → Moon closest to Earth
• Apogee → Moon farthest from Earth

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### 📐 Inclination

• Axial Tilt → 23.5°
• Orbital Inclination → 66.5°

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### 🌎 Division of Earth

#### ➖ Horizontal Division

• Equator → divides into Northern & Southern Hemisphere

#### ➗ Vertical Division

• Prime Meridian → divides into Eastern & Western Hemisphere

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🧠 Memory Tricks

• “Geo-flat” → Geoid = flat at poles
• “23-56 Rule” → Rotation ≈ 24 hrs (23:56 exact)
• “Jan Near, July Far” → Perihelion (Jan), Aphelion (July)
• “Equator = Energy” → Max speed at Equator
• “23.5 Tilt = Seasons” → axial tilt causes seasons