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

Published by Patrick Mutisya · 14 days ago

Cambridge IGCSE Physics 0625 – 6.1.1 The Earth

6.1.1 The Earth

Learning Objective

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.

Key Concepts

  • Rotation – The Earth spins about an imaginary line called its axis.

  • Axis tilt – The axis is inclined at about \$23.5^{\circ}\$ to the plane of the Earth's orbit (the ecliptic).

  • Period of rotation – One complete turn takes roughly 24 hours (a sidereal day is 23 h 56 min, but the solar day is 24 h).

  • Apparent daily motion of the Sun – Because of Earth’s rotation, the Sun appears to rise in the east, move across the sky, and set in the west each day.

  • Day‑night cycle – The side of Earth facing the Sun experiences daylight; the opposite side experiences night.

Earth’s Rotation

The Earth rotates from west to east. The angular velocity \$\omega\$ can be expressed as

\$\omega = \frac{2\pi}{T}\$

where \$T\$ is the period of rotation (≈ 24 h). Substituting gives \$\omega \approx 7.27 \times 10^{-5}\ \text{rad s}^{-1}\$.

Axis Tilt and Its Consequences

The tilt of \$23.5^{\circ}\$ means that different parts of the Earth receive varying amounts of solar radiation throughout the year, giving rise to the seasons. For the daily cycle, the tilt causes the Sun’s apparent path to change with latitude:

  1. At the equator the Sun’s path is nearly vertical, leading to roughly equal day and night lengths.

  2. At higher latitudes the Sun’s path is lower in the sky, resulting in shorter days in winter and longer days in summer.

Apparent Daily Motion of the Sun

Because the Earth rotates eastward, an observer on the surface perceives the Sun to move westward across the sky. This apparent motion can be described as:

  • Sunrise – the Sun appears on the eastern horizon.

  • Solar noon – the Sun reaches its highest altitude for that day.

  • Sunset – the Sun disappears below the western horizon.

Day‑Night Cycle Explained

When a point on Earth’s surface rotates into the Sun’s illumination, it experiences daylight. As rotation continues, that point rotates away from the Sun’s line of sight and experiences night. The cycle repeats every 24 hours.

Summary Table

FeatureValue / DescriptionRelevance to Daily Motion
Axis tilt\$23.5^{\circ}\$ to the eclipticDetermines Sun’s apparent height in the sky at different latitudes.
Rotation period≈ 24 h (solar day)Sets the length of one complete day‑night cycle.
Direction of rotationWest to eastCauses the Sun to appear to move east‑to‑west.
Angular velocity\$\omega \approx 7.27 \times 10^{-5}\ \text{rad s}^{-1}\$Quantifies the rate of Earth’s spin.

Suggested diagram: Earth showing its axis tilt, direction of rotation, and the apparent path of the Sun across the sky for an observer at the equator versus a higher latitude.

Key Points to Remember

  • The Earth rotates once every \overline{24} h, producing the day‑night cycle.

  • The axis is tilted \$23.5^{\circ}\$, influencing the Sun’s apparent altitude and the length of daylight.

  • Because the Earth rotates eastward, the Sun appears to move westward across the sky each day.