The EarthCambridge IGCSE Physics: Revision notes
Section 1
How does the Earth's rotation explain day and night?
The Earth rotates on its tilted axis once every approximately 24 hours (one solar day). This rotation is the reason we experience the cycle of day and night.
- As the Earth rotates, different parts of its surface face towards the Sun
- The side facing the Sun experiences daylight
- The side facing away from the Sun experiences darkness (night)
- The terminator line marks the boundary between day and night
- The apparent daily motion of the Sun across the sky (from east to west) is caused by the Earth rotating from west to east
- From our perspective on Earth's surface, it appears the Sun is moving, but actually we are rotating towards it
This continuous rotation ensures a regular, predictable cycle of day and night lasting approximately 24 hours for each complete rotation.
Imagine a lamp (the Sun) in the centre of a dark room and a spinning globe (Earth). As the globe rotates, different countries face towards and away from the lamp in turn, creating day and night.
Examiners want to see that you understand the Earth's rotation causes the apparent motion of the Sun, not that the Sun actually moves. Use the word 'apparent' when discussing the Sun's daily motion.
Section 2
Why do we experience seasons as Earth orbits the Sun?
The Earth orbits the Sun once every approximately 365 days (one year). The tilt of the Earth's axis (at approximately 23.5° to its orbital plane) causes the periodic nature of the seasons.
Mechanism of seasons:
- When the Northern Hemisphere is tilted towards the Sun, it receives more direct sunlight and experiences summer
- At the same time, the Southern Hemisphere is tilted away from the Sun and experiences winter
- Six months later, as the Earth completes half its orbit, the hemispheres swap positions
- The Southern Hemisphere is now tilted towards the Sun (summer) whilst the Northern Hemisphere is tilted away (winter)
- Spring and autumn occur when neither hemisphere is particularly tilted towards or away from the Sun
Why does axial tilt matter?
- The angle of incidence of the Sun's rays changes throughout the year due to axial tilt
- When tilted towards the Sun, rays hit more directly (smaller angle) → more concentrated energy → higher temperatures
- When tilted away, rays hit at a larger angle → energy is spread over a larger area → lower temperatures
- Days are also longer in summer (more hours of daylight) and shorter in winter (fewer hours of daylight)
Students often think seasons are caused by the Earth being closer to the Sun in summer and farther away in winter. This is incorrect—seasons are caused by axial tilt and the resulting angle of the Sun's rays, not distance from the Sun.
In June, the Northern Hemisphere is tilted towards the Sun. The Sun's rays strike at a smaller angle of incidence (more perpendicular), concentrating energy over a smaller area. Additionally, days are longer. Both factors result in higher temperatures and summer. In December, the Northern Hemisphere is tilted away, rays hit at a larger angle (more oblique), energy is spread over a larger area, days are shorter, and it is winter.
Section 3
What causes the Moon's phases and how do they recur?
The Moon orbits the Earth once every approximately one month (approximately 27–29 days). The periodic nature of the Moon's phases is explained by the Moon's changing position relative to the Earth and Sun.
How phases occur:
- The Moon does not produce its own light; it reflects sunlight
- As the Moon orbits the Earth, the angle between the Sun, Earth, and Moon continuously changes
- From Earth's perspective, we see different amounts of the illuminated (sunlit) side of the Moon
- When the Moon is between the Earth and Sun, we see mostly the dark side (new moon)
- When the Earth is between the Moon and Sun, we see mostly the illuminated side (full moon)
- At intermediate positions, we see partial illumination (crescent, quarter, and gibbous phases)
The cycle:
| Phase | Position | Illumination |
|---|---|---|
| New moon | Moon between Earth and Sun | Invisible (dark side facing Earth) |
| Waxing crescent | Moon moving away from Sun | Small illuminated crescent |
| First quarter | Moon at 90° from Sun | Half illuminated |
| Waxing gibbous | Moon approaching full position | More than half illuminated |
| Full moon | Earth between Moon and Sun | Fully illuminated |
| Waning gibbous | Moon moving away from full | More than half illuminated |
| Last quarter | Moon at 90° from Sun (opposite side) | Half illuminated |
| Waning crescent | Moon approaching new position | Small illuminated crescent |
This complete cycle repeats every approximately 29.5 days (lunar month).
Think of the Moon as a ball illuminated by a lamp (the Sun). As you walk around the ball whilst keeping the lamp fixed, different amounts of the ball's lit side face towards you, just as the Moon's phases change.
Remember: waxing means growing (increasing illumination), and waning means shrinking (decreasing illumination). The new moon is invisible, and the full moon is completely visible.
Section 4
What is average orbital speed and how do we calculate it?
Average orbital speed is the distance an object travels along its orbital path divided by the time taken to complete one full orbit. It is defined by the equation:
v = 2πr / T
Where:
- v = average orbital speed (in metres per second, m/s)
- r = orbital radius (distance from the centre of the object being orbited, in metres)
- T = orbital period (time for one complete orbit, in seconds)
- 2πr = circumference of the circular orbit
Understanding the equation:
- The numerator 2πr represents the total distance travelled in one complete orbit (the circumference of a circle)
- The denominator T is the time taken to complete that orbit
- Dividing distance by time gives speed
- This is the average speed because we assume a circular orbit and constant speed
Calculating orbital speeds:
For the Earth orbiting the Sun:
- Orbital radius: r ≈ 1.5 × 10¹¹ m
- Orbital period: T ≈ 365.25 days = 365.25 × 24 × 3600 = 3.16 × 10⁷ seconds
- v = (2π × 1.5 × 10¹¹) / (3.16 × 10⁷)
- v ≈ 3.0 × 10⁴ m/s (or 30 km/s)
For the Moon orbiting the Earth:
- Orbital radius: r ≈ 3.8 × 10⁸ m
- Orbital period: T ≈ 27.3 days = 27.3 × 24 × 3600 ≈ 2.36 × 10⁶ seconds
- v = (2π × 3.8 × 10⁸) / (2.36 × 10⁶)
- v ≈ 1.0 × 10³ m/s (or 1 km/s)
If a satellite orbits at a radius of 6.6 × 10⁶ m with a period of 2.4 × 10⁴ seconds: v = (2π × 6.6 × 10⁶) / (2.4 × 10⁴) = (4.15 × 10⁷) / (2.4 × 10⁴) ≈ 1.7 × 10³ m/s. Always ensure your period is in seconds and radius in metres before substituting.
Examiners often test this equation by asking you to rearrange it or substitute values. Always show your working, including unit conversions (e.g., days to seconds), and state your final answer with correct units (m/s or km/s).
Section 5
How are Earth's rotation, orbit, and the Moon's orbit interconnected?
The three motions—Earth's rotation, Earth's orbit, and the Moon's orbit—operate on different timescales and create the observable cycles we experience:
Timescale comparison:
| Motion | Period | Observable effect |
|---|---|---|
| Earth's rotation | ~24 hours | Day and night cycle |
| Moon's orbit | ~27–29 days | Lunar phases |
| Earth's orbit | ~365 days | Seasonal changes |
Interconnections:
- The day/night cycle (24 hours) is determined by Earth's rotation rate
- Lunar phases repeat roughly every 29.5 days because this is how long the Moon takes to orbit and return to the same position relative to the Sun and Earth
- Seasons repeat every 365 days as Earth completes its orbit, with the same hemisphere tilted towards the Sun at the same time each year
- All three cycles are periodic and predictable, allowing us to maintain calendars and predict future positions
- The three distinct timescales (day, month, year) are the basis of our modern timekeeping system
Why this matters for exams:
Understanding that these are three independent but interdependent cycles helps you explain real-world observations:
- Why there are roughly 365 days in a year but the Moon phases repeat monthly
- Why summer in the Northern Hemisphere occurs at the same calendar date every year despite the lunar phases changing
- How orbital speed equations apply to all three bodies
In exam questions, clearly distinguish between the three different timescales: explain day/night using rotation (~24 hours), phases using the Moon's orbit (~29.5 days), and seasons using Earth's orbit (~365 days). Mixing these up is a common error.
Must Know
-
Earth's rotation on its tilted axis (once every ~24 hours) causes the apparent daily motion of the Sun and the day/night cycle; the side facing the Sun experiences daylight and the opposite side experiences night
-
Earth's orbit around the Sun (once every ~365 days) combined with axial tilt (~23.5°) causes the periodic nature of seasons; when a hemisphere is tilted towards the Sun, it receives more direct sunlight and experiences summer, whilst the tilted-away hemisphere experiences winter
-
The Moon's orbit around the Earth (once every ~27–29 days) causes lunar phases because we see different amounts of the Moon's illuminated side as it orbits; the cycle from new moon through full moon back to new moon repeats periodically every ~29.5 days
-
Average orbital speed is defined by the equation v = 2πr / T, where v is speed (m/s), r is orbital radius (m), and T is orbital period (seconds); the numerator 2πr is the orbital circumference and dividing by T (period) gives average speed
-
Always convert time to seconds before using the orbital speed equation; ensure radius is in metres; these three cycles operate on different timescales (day, month, year) but are all periodic and predictable
That's the notes covered.
Carry on to the next subtopic.