CIE IGCSE Physics (0625) The Earth Study Notes - New Syllabus
CIE IGCSE Physics (0625) The Earth Study Notes
LEARNING OBJECTIVE
- Understanding the concepts of The Earth
Key Concepts:
- Earth’s Rotation, Apparent Motion of the Sun, and Day/Night Cycle
- Earth’s Orbit & The Moon’s Orbit
- Average Orbital Speed
Earth's Rotation, Apparent Motion of the Sun, and Day/Night Cycle
Earth’s Rotation, Apparent Motion of the Sun, and Day/Night Cycle
The Earth is a planet that rotates on its axis, which is tilted at approximately 23.5°, once every 24 hours. This rotation causes:
- The cycle of day and night.
- The apparent daily motion of the Sun from east to west.
Earth’s Rotation
- Earth rotates from west to east (anticlockwise when viewed from above the North Pole).
- This rotation takes approximately 24 hours and defines a solar day.
Apparent Motion of the Sun
- The Sun appears to move across the sky from east to west due to Earth’s rotation.
- This is not due to the actual motion of the Sun but because of the observer’s changing position on the rotating Earth.
Day and Night Cycle
- One half of the Earth is illuminated by the Sun (day), while the other half is in darkness (night).
- As Earth rotates, locations move into and out of sunlight, creating the periodic cycle of day and night.
Note: The apparent daily movement of the Sun and the regular occurrence of day and night are both caused by the Earth’s rotation on its tilted axis every 24 hours.
Example:
Why does the Sun set later in summer than in winter in most parts of the world?
▶️ Answer/Explanation
The Earth’s axis is tilted at approximately 23.5°. During summer in a given hemisphere (e.g., Northern Hemisphere), that hemisphere is tilted towards the Sun.
This causes the Sun to take a longer arc across the sky, rising earlier and setting later, resulting in longer daylight hours.
In contrast, during winter, the hemisphere is tilted away from the Sun, and the Sun takes a shorter path, causing it to set earlier.
Conclusion: The tilt of Earth’s axis and its rotation together explain the change in sunset times across seasons.
Earth’s Orbit & The Moon’s Orbit
Earth’s Orbit and the Seasons
- Earth’s Revolution: The Earth orbits the Sun once every approximately 365.25 days.
- Orbit Shape: The orbit is nearly circular (slightly elliptical), but the shape is not the main cause of seasons.
- Axial Tilt: Earth’s axis is tilted at about 23.5° relative to the plane of its orbit.
- Reason for Seasons: As the Earth orbits the Sun, different parts of the Earth receive varying amounts of sunlight during the year due to the tilt.
Explanation of the Periodic Nature of Seasons:
- When the Northern Hemisphere is tilted toward the Sun (around June), it experiences summer due to longer days and more direct sunlight.
- At the same time, the Southern Hemisphere is tilted away from the Sun and experiences winter.
- About six months later (around December), the situation reverses – the Southern Hemisphere experiences summer and the Northern Hemisphere has winter.
- This change occurs in a predictable, yearly cycle, resulting in the four seasons: spring, summer, autumn, and winter.
Note:
It is the tilt of the Earth’s axis and not the distance from the Sun that causes the seasons.
Example:
Why is it summer in Australia when it is winter in England?
▶️ Answer/Explanation
The Earth is tilted on its axis. When the Earth reaches the part of its orbit where the Southern Hemisphere is tilted toward the Sun, countries like Australia receive more direct sunlight and have longer days, leading to summer.
At the same time, the Northern Hemisphere is tilted away from the Sun. Places like England receive less direct sunlight and shorter days, resulting in winter.
Conclusion: The seasons are opposite in the two hemispheres due to Earth’s axial tilt during its orbit around the Sun.
The Moon’s Orbit and Phases
- Orbital Period: The Moon takes approximately 27.3 days to complete one orbit around the Earth (sidereal month).
- Phases Cycle: The complete cycle of Moon phases, called a lunar month, takes about 29.5 days (synodic month), which is what we observe from Earth.
- Cause of Phases: The Moon does not emit its own light – we see it because it reflects sunlight.
- As the Moon orbits Earth, we see different portions of its sunlit side:
- New Moon: Moon is between Earth and Sun; dark side faces Earth.
- First Quarter: Half of the Moon’s face is illuminated (right half visible from Northern Hemisphere).
- Full Moon: Earth is between the Sun and Moon; full face is lit and visible.
- Last Quarter: Half of the Moon is lit (left half visible from Northern Hemisphere).
Why the Phases Are Periodic:
- The relative positions of the Sun, Earth, and Moon change as the Moon orbits Earth.
- This causes the visible portion of the Moon’s sunlit surface to change in a regular, repeating pattern every ~29.5 days.
Example:
Why do we see a full moon only once every month?
▶️ Answer/Explanation
The Moon takes about 29.5 days to complete a full cycle of phases. A full moon occurs when the Earth is between the Sun and the Moon, and the side of the Moon facing Earth is fully lit by sunlight.
Since the positions of the Sun, Earth, and Moon align this way only once each orbit, we see a full moon approximately once every month.
Average Orbital Speed
Average Orbital Speed
Average orbital speed is the constant speed an object would need to travel in a circular orbit to complete one full revolution in a given time.
Formula:
\( v = \dfrac{2\pi r}{T} \)
- Where:
- \( v \) = average orbital speed (m/s)
- \( r \) = average radius of the orbit (m)
- \( T \) = orbital period (s)
Explanation:
- The object travels a circular path with circumference \( 2\pi r \).
- If it completes this orbit in time \( T \), its average speed is the distance divided by time.
Example:
Calculate the average orbital speed of the Moon if the average radius of its orbit around Earth is \( 3.84 \times 10^8 \, \text{m} \) and the orbital period is 27.3 days.
▶️ Answer/Explanation
Step 1: Convert the time into seconds:
\( T = 27.3 \, \text{days} = 27.3 \times 24 \times 3600 = 2.36 \times 10^6 \, \text{s} \)
Step 2: Use the formula:
\( v = \dfrac{2\pi r}{T} = \dfrac{2\pi \times 3.84 \times 10^8}{2.36 \times 10^6} \approx 1022 \, \text{m/s} \)
\(\boxed{v \approx 1022 \, \text{m/s}}\)