6.1.1 The Earth
- Syllabus
- 0625–2026–2027
- Topic
- 6.1.1
- Level
- —
Learning objectives
6.1.1.1Earth rotates on its tilted axis once• Know the Earth rotates on its tilted axis once in about 24 hours; use this to explain the Sun’s apparent daily motion and the day/night cycle6.1.1.2Earth orbits the Sun once in• Know: the Earth orbits the Sun once in approximately 365 days and use this to explain the periodic nature of the seasons6.1.1.3It takes approximately one month for• Know: it takes approximately one month for the Moon to orbit the Earth and use this to explain the periodic nature of the Moon’s cycle of phases6.1.1.4Average orbital speed• Define average orbital speed; recall/use: v = 2πr/T, where r is average orbital radius and T is orbital period
Explain Earth's rotation, day and night
Earth rotates once on its tilted axis in approximately 24 hours. The axis is the imaginary line through the North and South Poles.
At any moment, the hemisphere facing the Sun is illuminated and has daytime; the hemisphere facing away is in darkness and has night. As Earth rotates, each location moves into and then out of the illuminated half.
Earth rotates from west to east, so the Sun appears to move across the sky in the opposite direction, from east to west, rising in the east and setting in the west.
Because one rotation takes about 24 hours, the sequence of daylight and darkness and the Sun's apparent daily path repeat approximately every day.
Earth's rotation causes day and night. The tilt of the axis is important for seasons, but tilt is not the cause of the daily day/night cycle.
Explain Earth's year and the seasons
Earth completes one orbit of the Sun in approximately 365 days. This orbital period defines one year.
Earth's axis remains tilted in a nearly fixed direction as Earth moves around the Sun. During part of the orbit, one hemisphere is tilted towards the Sun; about half an orbit later it is tilted away.
| Hemisphere orientation | Sunlight received | Season tendency |
|---|---|---|
| tilted towards Sun | longer days and more direct rays | summer |
| tilted away from Sun | shorter days and less direct rays | winter |
| neither strongly towards nor away | intermediate day length and ray angle | spring or autumn |
The same orientations recur each orbit, so the seasons form a periodic cycle that repeats approximately every 365 days. The two hemispheres experience opposite seasons.
Seasons are not caused mainly by Earth being nearer to or farther from the Sun. They arise from axial tilt changing ray angle and daylight duration through the year.
Explain the Moon's cycle of phases
The Moon takes approximately one month to complete one orbit of Earth.
The Sun always illuminates half of the Moon. As the Moon moves around Earth, the viewing angle changes, so observers see different fractions of the illuminated half.
The visible sequence repeats: new Moon, waxing crescent, first quarter, waxing gibbous, full Moon, waning gibbous, last quarter and waning crescent, then new Moon again.
Because the Moon returns to approximately the same orbital arrangement after about one month, its cycle of phases is periodic with approximately a monthly timescale.
Ordinary Moon phases are not caused by Earth's shadow. Earth's shadow on the Moon produces a lunar eclipse, which is a different event.
Calculate average orbital speed
Average orbital speed is the total distance travelled around an orbit divided by the orbital period.
For an approximately circular orbit, the distance travelled in one orbit is 2πr, so v = 2πr/T, where r is the average orbital radius measured from the centre of the orbit and T is the time for one complete orbit.
| Step | Action |
|---|---|
| 1 | identify r and the full orbital period T |
| 2 | convert distance and time to units that match the required speed |
| 3 | calculate the circumference 2πr |
| 4 | divide by T and state the speed unit |
For Earth, r ≈ 1.5 × 10¹¹ m and T ≈ 365 × 24 × 60 × 60 s. Therefore v ≈ 2π(1.5 × 10¹¹)/(3.15 × 10⁷) ≈ 3.0 × 10⁴ m/s.
The same relation can be rearranged: T = 2πr/v or r = vT/(2π).
Use the orbital radius, not the radius of the planet, unless the orbit is at the planet's surface. Average speed is not average velocity: velocity direction changes continuously around an orbit.