Know that the Earth is a planet that rotates on its axis, which is tilted, once in approximately 24 hours, and use this to explain observations of the apparent daily motion of the Sun and the periodic cycle of day and night
6.1.1 The Earth
Learning objective
Know that the Earth is a planet that rotates on its tilted axis once in approximately 24 hours, and use this information to explain the apparent daily motion of the Sun and the regular cycle of day and night.
Key idea: The Earth turns eastward on an axis that is tilted 23.5° to the ecliptic. This rotation makes the Sun appear to move westward across the sky, producing a 24‑hour day‑night cycle. The tilt, together with Earth’s yearly orbit, also explains the changing height of the Sun in the sky (the basis of the seasons) – a point revisited in the next syllabus section (6.1.2).
1. Earth’s rotation
- Direction: Earth rotates from west to east (counter‑clockwise when viewed from above the North Pole).
- Axis: An imaginary line joining the North and South Poles – a tilted axis (23.5° to the ecliptic).
- Period of rotation
- Solar day – time between two successive noons ≈ 24 h.
- Sidereal day – time for one complete turn relative to the stars = 23 h 56 min 04 s.
- The solar day is slightly longer because Earth moves about 1° in its orbit while it is rotating.
- Why the Sun appears to move westward: Because Earth rotates eastward, an observer on the surface sees the Sun (and the stars) sweep across the sky from east to west.
2. Tilt of the axis
- Value: 23.5° to the plane of Earth’s orbit (the ecliptic).
- Effect on the Sun’s apparent path
- At the equator the Sun’s track is almost vertical; day and night are roughly equal all year.
- At higher latitudes the Sun’s track is lower. In summer the Sun reaches a higher altitude → longer daylight; in winter it stays lower → shorter daylight.
- Link to the daily motion: The same tilted axis that produces the seasonal change also determines the angle at which sunlight strikes a location, influencing the Sun’s apparent height at any time of day.
3. Apparent daily motion of the Sun
Because Earth rotates eastward:
- Sunrise – Sun first appears on the eastern horizon.
- Solar noon – Sun reaches its highest altitude for that day.
- Sunset – Sun disappears below the western horizon.
This east‑to‑west motion is an *apparent* effect; the Sun does not orbit Earth.
4. Day‑night cycle
Any point on Earth’s surface experiences daylight when it is turned toward the Sun and night when it is turned away. One complete rotation (≈ 24 h) therefore produces one full day‑night cycle.
5. Earth’s orbital motion (context)
- Orbital period: Earth revolves around the Sun once every ≈ 365 days.
- Relevance: Combined with the tilted axis, this yearly orbit creates the annual cycle of seasons – the Sun’s apparent height varies throughout the year as well as during a single day.
6. The Moon (brief note)
- Orbital period: The Moon completes an orbit around Earth in ≈ 27.3 days.
- Result: This motion gives rise to the regular monthly cycle of lunar phases, which the syllabus expects students to be able to explain using the same ideas of rotation and orbit.
7. Summary table
| Feature | Value / Description | Relevance to daily motion |
|---|---|---|
| Tilted axis | 23.5° to the ecliptic | Determines the Sun’s apparent height and, together with the orbit, the seasonal variation. |
| Rotation period | Solar day ≈ 24 h (sidereal day 23 h 56 min 04 s) | Sets the length of one complete day‑night cycle. |
| Direction of rotation | West → East | Produces the apparent east‑to‑west motion of the Sun. |
| Orbital period | ≈ 365 days | With the tilt, produces the annual cycle of seasons. |
| Moon’s orbital period | ≈ 27.3 days | Explains the monthly cycle of lunar phases. |
8. Suggested diagram
9. Further reading (optional)
For students who wish to explore the quantitative side, the angular velocity of Earth’s spin is given by
\[
\omega = \frac{2\pi}{T}
\]
where \(T = 86\,400\ \text{s}\) for a solar day, giving \(\omega \approx 7.27 \times 10^{-5}\ \text{rad s}^{-1}\). This detail is not required for the core syllabus.
Key points to remember
- The Earth rotates eastward once every 24 hours, producing the regular day‑night cycle.
- The rotation axis is tilted 23.5° to the ecliptic; this tilt influences the Sun’s apparent altitude and, together with Earth’s yearly orbit, creates the seasons.
- Because of eastward rotation, the Sun appears to move westward across the sky each day.
- Earth’s orbital period is about 365 days, and the Moon’s orbital period is about 27.3 days**, giving the monthly lunar‑phase cycle.