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Earth & the Solar System
Earth and Solar System - CIE IGCSE Physics
📚 CIE IGCSE Physics - Topic 6.1: The Earth and the Solar System
6.1.1 The Earth
Core Concepts
1. Earth's Rotation
The Earth rotates on its tilted axis once every approximately 24 hours, causing:
The Earth rotates on its tilted axis once every approximately 24 hours, causing:
- Apparent daily motion of the Sun (sunrise to sunset)
- Cycle of day and night
2. Earth's Orbit
The Earth orbits the Sun once every approximately 365 days (one year), causing:
The Earth orbits the Sun once every approximately 365 days (one year), causing:
- The periodic nature of seasons
- Different hemispheres receiving different amounts of sunlight throughout the year
3. Moon's Orbit
The Moon orbits the Earth once every approximately one month, causing:
The Moon orbits the Earth once every approximately one month, causing:
- Cycle of Moon phases (new moon to full moon and back)
- Spring and neap tides
🌍 Key Fact: Earth's axis is tilted at about 23.5 degrees - this tilt combined with orbit around Sun causes seasons
Supplement: Orbital Speed
Average Orbital Speed is defined as the total distance traveled in orbit divided by the orbital period.
v = 2πr / T
Where:
• v = average orbital speed (m/s)
• r = average radius of orbit (m)
• T = orbital period (s)
• π ≈ 3.14159
• v = average orbital speed (m/s)
• r = average radius of orbit (m)
• T = orbital period (s)
• π ≈ 3.14159
Example: Earth's Orbital Speed
Calculate Earth's average orbital speed around the Sun.
Orbital radius = 1.5 × 10¹¹ m
Orbital period = 365 days = 3.15 × 10⁷ s
Step 1: Write formula
v = 2πr / T
v = 2πr / T
Step 2: Substitute values
v = 2 × 3.14 × (1.5 × 10¹¹) / (3.15 × 10⁷)
v = 2 × 3.14 × (1.5 × 10¹¹) / (3.15 × 10⁷)
Step 3: Calculate
v = 9.42 × 10¹¹ / 3.15 × 10⁷ = 29,900 m/s
v = 9.42 × 10¹¹ / 3.15 × 10⁷ = 29,900 m/s
Answer: Earth orbits at approximately 30 km/s
6.1.2 The Solar System
Core: Solar System Composition
The Solar System Contains:
- One star: The Sun
- Eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
- Minor planets: Dwarf planets (Pluto) and asteroids
- Moons: Natural satellites orbiting planets
- Smaller bodies: Comets and natural satellites
🪐 Planet Order (My Very Excellent Mother Just Served Us Noodles):
Mercury → Venus → Earth → Mars → Jupiter → Saturn → Uranus → Neptune
Mercury → Venus → Earth → Mars → Jupiter → Saturn → Uranus → Neptune
Core: Planetary Characteristics
Inner vs Outer Planets:
- Inner Planets (Rocky): Mercury, Venus, Earth, Mars - small, dense, rocky surfaces
- Outer Planets (Gas Giants): Jupiter, Saturn, Uranus, Neptune - large, low density, gaseous
Gravitational Field Strength:
- At surface depends on planet's mass
- Decreases as distance from planet increases
Light Travel Time: Light takes time to travel between Solar System objects
Time = Distance / Speed of light
Example: Light Travel Time
Calculate how long sunlight takes to reach Earth.
Earth-Sun distance = 1.5 × 10¹¹ m
Speed of light = 3 × 10⁸ m/s
Step 1: Write formula
Time = Distance / Speed
Time = Distance / Speed
Step 2: Substitute values
Time = 1.5 × 10¹¹ / 3 × 10⁸
Time = 1.5 × 10¹¹ / 3 × 10⁸
Step 3: Calculate
Time = 500 seconds = 8 minutes 20 seconds
Time = 500 seconds = 8 minutes 20 seconds
Answer: Sunlight takes about 8 minutes to reach Earth
Core: Orbital Mechanics
Why Planets Orbit the Sun:
- Sun contains most of Solar System's mass (99.8%)
- Gravitational attraction keeps planets in orbit
Supplement: Advanced Concepts
Elliptical Orbits:
- Planets, minor planets, and comets have elliptical orbits
- Sun is not at center (except in circular orbits)
- Objects travel faster when closer to Sun
Orbital Speed and Distance:
- Sun's gravitational field strength decreases with distance
- Orbital speeds of planets decrease with distance from Sun
⚡ Conservation of Energy: In elliptical orbits, objects trade potential energy for kinetic energy - faster when closer to Sun, slower when farther away
Supplement: Solar System Formation
Accretion Model: Explains why inner planets are rocky and outer planets are gaseous
1. Interstellar Cloud
Cloud of gas and dust containing many elements
Cloud of gas and dust containing many elements
2. Gravity and Rotation
Gravity causes cloud to collapse and rotate, forming accretion disc
Gravity causes cloud to collapse and rotate, forming accretion disc
3. Planet Formation
• Inner region: Rocky planets form (high temperatures vaporize gases)
• Outer region: Gas giants form (cool enough to capture gases)
• Inner region: Rocky planets form (high temperatures vaporize gases)
• Outer region: Gas giants form (cool enough to capture gases)
Data Analysis Skills
📊 You should be able to:
- Analyze planetary data tables
- Interpret relationships between orbital distance and period
- Understand density and surface temperature patterns
- Calculate gravitational field strengths
Data Interpretation Example
Observation: Mercury orbits in 88 days, Neptune in 165 years
Pattern: Orbital period increases with distance from Sun
Reason: Weaker gravity and longer orbital paths farther from Sun
Key Relationships Summary
Orbital Speed: v = 2πr/T
Light Travel Time: t = d/c
Gravitational Field: Decreases with distance
Orbital Period: Increases with distance from Sun
Light Travel Time: t = d/c
Gravitational Field: Decreases with distance
Orbital Period: Increases with distance from Sun
Exam Focus Points
🎯 Must Know:
- Earth's rotation causes day/night cycle
- Earth's orbit around Sun causes seasons
- Moon's orbit causes phases
- Planet order and characteristics
- Orbital speed calculation
⚠️ Common Mistakes:
- Confusing rotation (day/night) with revolution (seasons)
- Forgetting units in orbital speed calculations
- Mixing up inner and outer planet properties
✅ Abel Masitsa
Why the Moon shows the same face to Earth
How to read the animation
- The Moon (small grey circle) revolves around Earth along the dashed orbit.
- The small dark mark on the Moon is a fixed feature on the Moon’s surface (a “face” marker).
- The Moon group both revolves around Earth and simultaneously rotates about its own center.
Key point: in the animation the Moon completes exactly one rotation while it completes one full orbit. That is why the same face always points to Earth—rotation period = orbital period.
Tip: use the controls below to speed up or slow the motion to observe the match.
12s
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🔑 The Relationship Made Simple
For the same face of the Moon to always face Earth:
The Moon must rotate exactly once during one full orbit around Earth.
If:
Rotation was faster → we would see different sides
Rotation was slower → we would also see different sides
Only 1 rotation per orbit = same face always facing us.
🧲 But WHY did it become like this?
Because of tidal locking:
Long ago, the Moon rotated faster.
Earth’s gravity pulled on the Moon’s uneven mass (the “bulge”).
This produced friction that slowed the Moon’s rotation.
It slowed until rotation matched orbit → then friction stopped.
This is the “balanced” state, so it remains locked.
📌 Short Version
The Moon rotates once in the same time it orbits once.
That is why the same side faces Earth always.
Gravity caused this synchronization → tidal locking.