Earth, Moon & Sun
Section: Space Physics | Syllabus: Cambridge IGCSE Physics (0625)
Day and Night
The Earth rotates on its axis - an imaginary line running through the North and South Poles - once every approximately 24 hours. Because light travels in straight lines, only the half of the Earth facing the Sun is in daylight at any one moment, which is what produces the cycle of day and night.
- The Earth's axis is tilted at approximately 23.5° to its orbital plane around the Sun
- The Earth spins from west to east, which is why the Sun appears to move across the sky from east to west
Apparent Daily Motion of the Sun
- The Sun appears to rise in the east and set in the west - but it is the Earth's rotation that causes this, not the Sun actually moving around the Earth
- At sunrise, a location on Earth is just rotating into sunlight, so the Sun appears on the eastern horizon
- At midday, the Sun appears highest in the sky
- At sunset, the location is rotating out of sunlight, so the Sun appears on the western horizon
The Earth's rotation on its axis produces the cycle of day and night
Seasons
The Earth orbits the Sun once every approximately 365 days (one year). Because the Earth's axis stays tilted in the same direction throughout this orbit, different hemispheres receive different amounts of sunlight at different points in the year - this is what produces the seasons.
- When a hemisphere is tilted towards the Sun: days are longer and sunlight arrives more directly → summer
- When a hemisphere is tilted away from the Sun: days are shorter and sunlight arrives at a shallower angle, spread over a larger area → winter
- When the northern hemisphere has summer, the southern hemisphere has winter, and vice versa
- Locations at or near the Equator receive sunlight at roughly the same angle all year round, so they experience little seasonal variation
- The further a location is from the Equator, the more extreme its seasonal differences
The tilt of the Earth's axis - not its distance from the Sun - causes the seasons
Phases of the Moon
The Moon orbits the Earth approximately once a month. The Moon does not produce its own light - it only reflects sunlight from the Sun. As the Moon orbits the Earth, an observer on Earth sees different fractions of its sunlit side, and this is what creates the phases of the Moon.
- A full cycle of phases, from one new moon to the next, takes about 29.5 days
| Phase | Description |
|---|---|
| New Moon | Moon is between the Earth and Sun - its lit side faces away from Earth, so it appears dark |
| Waxing Crescent | A small sliver on the right side is visible and growing |
| First Quarter | The right half of the Moon is visible |
| Waxing Gibbous | More than half is visible and still growing |
| Full Moon | Earth is between the Sun and Moon - the entire lit side faces Earth |
| Waning Gibbous | More than half is visible but now shrinking |
| Last Quarter | The left half of the Moon is visible |
| Waning Crescent | A small sliver on the left side is visible, shrinking back towards new moon |
The changing angle between the Sun, Moon and Earth produces the cycle of phases
- A day is the time for the Earth to rotate once on its axis (~24 hours)
- A month is roughly the time for the Moon to orbit the Earth once and complete a cycle of phases (~29.5 days)
- A year is the time for the Earth to orbit the Sun once (~365 days)
Average Orbital Speed
Extended Only
Since planetary orbits are approximately circular, the average orbital speed of an object can be calculated from the circumference of its orbit and the time taken to complete one orbit.
- The distance travelled in one complete orbit is the circumference of a circle, 2π r; dividing this by the orbital period, T, gives the average orbital speed
- v = (2π r/T)
- v = average orbital speed (m/s)
- r = average orbital radius - the average distance from the object being orbited (m)
- T = orbital period - the time taken for one complete orbit (s)
v = 2πr / T for a body in a circular orbit
Worked Example: Orbital Speed of Earth
Museli wants to calculate the Earth's average orbital speed around the Sun, given r = 1.5 × 10^11 m and T = 1 year.
- Convert T to seconds: T = 365 × 24 × 3600 = 3.15 × 10^7 s
- Substitute into v = (2π r/T): v = 2 × π × 1.5 × 10^113.15 × 10^7 ≈ 3.0 × 10^4 m/s = 30 km/s
- Always convert the orbital period into seconds before substituting into the equation - 1 year ≈ 3.15 × 10^7 s
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