🌍 The Earth – CIE IGCSE Physics Notes
1️⃣ Earth’s Rotation and the Day–Night Cycle
✅ Key Facts
- The Earth is a planet.
- It rotates on its axis.
- One complete rotation takes approximately 24 hours.
- The Earth’s axis is tilted at about 23.5° to the vertical.
- The rotation is from west to east (anticlockwise when viewed from above the North Pole).
🔑 Important Definitions
Axis
An imaginary line through the centre of the Earth from the North Pole to the South Pole.
Rotation
The spinning of a body about its axis.
🌞 Apparent Daily Motion of the Sun
Because the Earth rotates:
- The Sun appears to:
- Rise in the east
- Move across the sky
- Set in the west
⚠️ Important:
The Sun is not actually moving around the Earth — this is called apparent motion.
🌗 Day and Night
As the Earth rotates:
- The side facing the Sun experiences day
- The side facing away experiences night
Since one rotation takes 24 hours:
- Each location experiences a 24-hour cycle
- This causes the periodic cycle of day and night
✍️ Exam Tip
If asked to explain day and night:
✔️ Mention rotation
✔️ Mention 24 hours
✔️ Mention that half the Earth faces the Sun
✔️ State that it is periodic
Example answer structure:
The Earth rotates on its axis once every 24 hours. The side facing the Sun experiences daylight, while the opposite side experiences night. This rotation causes the daily cycle of day and night.
2️⃣ Earth’s Orbit and the Seasons
✅ Key Facts
- The Earth orbits the Sun
- One orbit takes approximately 365 days (1 year)
- The orbit is slightly elliptical
- The axis remains tilted at 23.5° during orbit
🔑 Important Definitions
Orbit
The path of one object around another due to gravitational attraction.
Revolution
The motion of a body around another body.
🌦 Why Seasons Occur
Seasons are caused by:
- The Earth's tilted axis
- The Earth's orbit around the Sun
They are NOT caused by:
❌ The Earth being closer to the Sun
❌ Changes in the Sun’s temperature
☀️ How the Tilt Causes Seasons
When a hemisphere is tilted towards the Sun:
- Sunlight is more direct
- Days are longer
- It is summer
When tilted away from the Sun:
- Sunlight is less direct
- Days are shorter
- It is winter
This cycle repeats every 365 days → seasons are periodic
🌍 Opposite Seasons
When it is:
- Summer in the Northern Hemisphere
→ It is winter in the Southern Hemisphere
✍️ Exam Tip
When explaining seasons:
✔️ Always mention tilt
✔️ Always mention orbit
✔️ Always mention change in sunlight intensity
Common mistake: Saying seasons are caused by distance from the Sun ❌
3️⃣ The Moon and Its Phases
✅ Key Facts
- The Moon is Earth’s natural satellite
- It orbits the Earth
- One orbit takes approximately 1 month (~27–29 days)
🔑 Important Definitions
Satellite
A body that orbits another larger body.
Natural satellite
A satellite formed naturally (e.g., the Moon).
🌙 Why Moon Phases Occur
The Moon does not produce its own light.
It reflects sunlight.
As the Moon orbits Earth:
- We see different portions of the sunlit side.
- This creates the cycle of phases.
🌘 Main Phases of the Moon
- New Moon
- Crescent
- First Quarter
- Gibbous
- Full Moon
- Waning phases
This cycle repeats approximately every month → periodic
⚠️ Important Concept
The Moon is always half illuminated by the Sun.
Phases change because:
- Our viewing angle changes.
✍️ Exam Tip
If asked to explain Moon phases:
✔️ Mention orbit
✔️ Mention reflection of sunlight
✔️ Mention changing visible portion
Example structure:
As the Moon orbits the Earth once every month, we see different amounts of its sunlit surface. This causes the periodic cycle of phases.
4️⃣ Supplement: Average Orbital Speed
📘 Definition
Average orbital speed is the total distance travelled in one orbit divided by the time taken.
📌 Formula
Where:
- ( v ) = average orbital speed (m/s)
- ( r ) = average radius of orbit (m)
- ( T ) = orbital period (s)
- ( 2πr ) = circumference of orbit
🔎 Understanding the Formula
The object travels one full circle:
Distance = circumference = (2\pi r)
Time taken = (T)
So speed = distance ÷ time
🧮 Example Calculation
If a satellite orbits Earth at:
⚠️ Unit Reminder
Always convert:
- Days → seconds
- Hours → seconds
- Kilometres → metres
✍️ Exam Tips for Calculations
✔️ Write formula first
✔️ Substitute values clearly
✔️ Include units
✔️ Give answer to correct significant figures
Common mistake:
- Forgetting to convert days into seconds
1 day = 24 × 60 × 60 = 86,400 s
🔁 Summary Table
| Motion | Time Taken | What It Causes |
|---|---|---|
| Earth rotation | ~24 hours | Day and night |
| Earth orbit | ~365 days | Seasons |
| Moon orbit | ~1 month | Moon phases |
🎯 Common Exam Questions
You may be asked to:
- Explain why we have day and night
- Explain why seasons occur
- Explain the phases of the Moon
- Calculate orbital speed
- State the tilt of the Earth's axis (23.5°)
- Define rotation, revolution, orbit, satellite
🚨 Common Mistakes to Avoid
❌ Saying seasons are due to distance from the Sun
❌ Forgetting to mention tilt
❌ Confusing rotation with orbit
❌ Forgetting unit conversions in orbital speed questions
🧠 Quick Concept Links (Helpful for Revision)
- Rotation → Time
- Orbit → Year
- Tilt → Seasons
- Reflection → Moon phases
- Circular motion → Orbital speed formula
🌞 The Solar System – CIE IGCSE Physics Notes
1️⃣ Structure of the Solar System
🌟 (a) The Sun
The Solar System contains:
- One star → the Sun
- It contains most of the mass of the Solar System (~99.8%)
- It produces energy by nuclear fusion
- It provides light and heat to the planets
🪐 (b) The Eight Planets (in order from the Sun)
You must know their names in correct order:
- Mercury
- Venus
- Earth
- Mars
- Jupiter
- Saturn
- Uranus
- Neptune
🧠 Memory Trick
My Very Easy Method Just Speeds Up Naming
🌑 (c) Minor Planets
Minor planets orbit the Sun and include:
- Dwarf planets such as Pluto
- Asteroids (mainly in the asteroid belt between Mars and Jupiter)
🌙 (d) Moons
- Moons orbit planets.
- Example: Earth has one moon.
- Jupiter has many moons.
☄️ (e) Smaller Solar System Bodies
Include:
- Comets (icy objects with long elliptical orbits)
- Asteroids
- Natural satellites (moons)
2️⃣ Rocky vs Gaseous Planets
🪨 Inner Planets (Terrestrial Planets)
- Mercury, Venus, Earth, Mars
- Small
- Rocky
- High density
- Closer to the Sun
- Warmer
🌬 Outer Planets (Gas Giants)
- Jupiter, Saturn, Uranus, Neptune
- Large
- Mostly gas (hydrogen and helium)
- Lower density
- Colder
- Many moons
🌌 Accretion Model of Solar System Formation
🌫 Step 1: Interstellar Cloud
The Solar System formed from:
- A cloud of gas and dust
- Contained many elements
- Gravity caused collapse
🔄 Step 2: Rotation and Accretion Disc
As the cloud collapsed:
- It began to rotate
- Flattened into a disc
- The Sun formed at the centre
- Planets formed from clumps of matter (accretion)
⚖ Why Inner Planets Are Rocky
Near the Sun:
- Temperatures were very high
- Only heavy materials (metals and rock) could condense
Far from the Sun:
- It was cooler
- Light gases (hydrogen, helium) could remain
- Gas giants formed
✍️ Exam Tip
If asked to explain differences:
✔️ Mention gravity
✔️ Mention accretion
✔️ Mention temperature differences
3️⃣ Gravitational Field Strength
🌍 Definition
Gravitational field strength (g):
Force per unit mass
Unit: N/kg
(a) Depends on Mass of Planet
Greater planet mass → stronger gravitational field.
Example:
- Jupiter has stronger gravity than Earth.
(b) Decreases with Distance
As distance from planet increases:
- Gravitational field strength decreases.
This follows an inverse square relationship (for higher level understanding).
4️⃣ Calculating Time for Light to Travel
⚡ Speed of Light
Formula
Example
Distance from Earth to Sun:
≈ 8 minutes 20 seconds
✍️ Exam Tip
Always:
✔️ Use standard form
✔️ Show substitution
✔️ Include units
5️⃣ Why Planets Orbit the Sun
The Sun contains most of the Solar System's mass.
More mass → stronger gravity.
Therefore:
- The Sun’s gravitational pull keeps planets in orbit.
6️⃣ Force Keeping Planets in Orbit
The force is:
Gravitational attraction between the Sun and the planet.
This provides the centripetal force needed for circular or elliptical motion.
7️⃣ Elliptical Orbits
Planets, comets and minor planets follow:
- Elliptical orbits
The Sun is:
- At one focus of the ellipse
- Not at the centre (unless orbit is nearly circular)
8️⃣ Analysing Planetary Data
You may be given tables of:
- Orbital distance
- Orbital period
- Density
- Surface temperature
- Gravitational field strength
Trends to Know
As distance from Sun increases:
- Orbital period increases
- Orbital speed decreases
- Temperature decreases
Rocky planets:
- Higher density
Gas giants:
- Lower density
✍️ Exam Tip
When analysing tables:
✔️ Look for patterns
✔️ Compare inner vs outer planets
✔️ State trends clearly
9️⃣ Gravitational Field Strength of the Sun
As distance from Sun increases:
- Gravitational field strength decreases
- Orbital speeds decrease
Outer planets move slower than inner planets.
🔟 Why Planets Move Faster When Closer to the Sun
In elliptical orbit:
- When closer → gravitational potential energy decreases
- Kinetic energy increases
- Speed increases
When further away:
- Potential energy increases
- Kinetic energy decreases
- Speed decreases
This follows:
Conservation of energy
📊 Summary Table
| Feature | Inner Planets | Outer Planets |
|---|---|---|
| Size | Small | Large |
| Composition | Rocky | Gaseous |
| Density | High | Lower |
| Temperature | Higher | Lower |
| Orbital Speed | Faster | Slower |
🚨 Common Exam Mistakes
❌ Saying Pluto is a planet
❌ Forgetting order of planets
❌ Forgetting Sun is at a focus of ellipse
❌ Confusing gravity with magnetic force
❌ Not using standard form in calculations
🎯 Common Exam Questions
- Name the planets in order
- Explain formation of Solar System
- Explain difference between rocky and gaseous planets
- Calculate light travel time
- Explain why planets move faster near the Sun
- Analyse planetary data table
- Describe gravitational field changes