Know that microwave radiation of a specific frequency is observed at all points in space around us and is known as cosmic microwave background radiation (CMBR)

6.1 The Earth – Rotation, Tilt and Seasons

1. Rotation

• Earth rotates once about its axis every 24 h (≈ 86 400 s).
• Result: the regular alternation of day and night.
• Linear speed at the equator:

  
  v = 2πR / T ≈ 2π (6.37 × 10⁶ m) / 86 400 s ≈ 465 m s⁻¹

2. Axial tilt

• Earth’s axis is tilted 23.5° to the plane of its orbit (the ecliptic).
• The tilt, together with the orbital motion, causes the Sun’s apparent height in the sky to change over the year – giving the seasons.
• When the Northern Hemisphere is tilted towards the Sun the days are longer and the Sun appears higher → summer; the opposite gives winter.

3. Seasons – quick exam checklist

• Identify the season in a given hemisphere by the tilt direction.
• Remember: the length of a year (orbital period) is 365 days 6 h 9 min ≈ 1 yr.
• Key point for AO1: “The Earth’s rotation causes day/night, its tilt (23.5°) causes the seasons.”

6.2 The Solar System

1. Overview

The Sun is at the centre of a system of eight planets, dwarf planets, moons, asteroids and comets. All objects move in (approximately) elliptical orbits due to the Sun’s gravity.

2. Planets – key facts (core IGCSE content)

3. Dwarf planets, moons, asteroids & comets

• Dwarf planets – bodies that orbit the Sun and are massive enough to be round, but have not cleared their orbital zone. Examples: Pluto, Eris, Ceres.
• Moons – natural satellites. Earth has one (the Moon); Jupiter has >70, Saturn >80, etc.
• Asteroids – rocky bodies mainly in the asteroid belt between Mars and Jupiter.
• Comets – icy bodies from the Kuiper Belt or Oort Cloud that develop a coma and tail when near the Sun.

6.3 Extension (Enrichment) – Cosmic Microwave Background Radiation (CMBR)

This material is not required for the core IGCSE exam but is useful for deeper understanding or extension work.

1. What is the CMBR?

• A faint, uniform glow of electromagnetic radiation that fills the whole Universe.
• Observed in every direction – the same at all points in space.
• Remnant “after‑glow” of the hot, dense early Universe (the Big Bang).

2. Key observational facts

• Discovered in 1965 by Arno Penzias and Robert Wilson using a horn antenna.
• Has an almost perfect black‑body spectrum with an average temperature 2.725 K.
• Peak wavelength (Wien’s law)

  
  λ_max = b/T ≈ 2.898 × 10⁻³ / 2.725 ≈ 1.06 mm

• Corresponding peak frequency

  
  ν_max ≈ c/λ_max ≈ 2.8 × 10¹¹ Hz ≈ 160 GHz

• Falls in the microwave region of the electromagnetic spectrum (≈ 300 MHz – 300 GHz).
• Isotropic to about one part in 10⁵; tiny anisotropies map the density fluctuations that later formed galaxies.

3. How is it detected?

• Ground‑based radio telescopes with very sensitive microwave receivers (e.g., Atacama Cosmology Telescope).
• High, dry sites or space platforms are chosen to minimise atmospheric absorption.
• Satellites – COBE, WMAP and Planck – have measured the spectrum and temperature with high precision.

4. Numerical summary (quick revision)

5. Using Wien’s law – exam skill (AO2)

Wien’s displacement law for wavelength: λ_max = b/T, where b = 2.898 × 10⁻³ m·K.

Example – Find λmax for T = 2.7 K
λmax = (2.898 × 10⁻³ m·K) / 2.7 K
≈ 1.07 × 10⁻³ m = 1.07 mm

Then obtain the frequency: νmax = c/λmax ≈ 3.00 × 10⁸ m s⁻¹ / 1.07 × 10⁻³ m ≈ 2.8 × 10¹¹ Hz (≈ 160 GHz).

6. Why it supports the Big‑Bang model (AO1)

• The hot‑big‑bang theory predicted a relic radiation field; its discovery confirmed the prediction.
• The observed black‑body spectrum matches the theoretical Planck curve for 2.7 K.
• Measured anisotropies provide the seeds for later galaxy formation.

4 Practical Skills linked to Space Physics (AO3)

Activity 1 – Modelling Earth’s tilt and the seasons

1. Materials: a bright lamp (Sun), a white ball (Earth), a protractor, a marker.
2. Fix the ball on a stand so it can rotate. Tilt the axis to 23.5°.
3. Rotate the ball once while keeping the lamp fixed; observe which hemisphere receives more light at different positions.
4. Record observations and label the four seasons for each hemisphere.
5. Safety: do not look directly at the lamp when it is on; keep the lamp away from flammable materials.

Activity 2 – Measuring the apparent diameter of the Moon

1. Use a simple “pinhole camera” (a cardboard box with a small circular hole) and a sheet of white paper as a screen.
2. During a clear night, point the camera at the Moon and measure the diameter of the projected image with a ruler.
3. Calculate the Moon’s angular size using θ ≈ (image diameter / distance to screen) (radians) and convert to degrees (1 rad ≈ 57.3°).
4. Compare your result with the accepted value ≈ 0.5°.
5. Safety: never look directly at the Sun through the pinhole camera.

Link to assessment objectives

• AO1 – Knowledge: recall facts about Earth’s rotation, tilt, planetary order, and (if chosen) the CMBR.
• AO2 – Application: use formulas such as v = 2πr/T for Earth’s equatorial speed, or θ = s/D for angular size.
• AO3 – Practical/experimental: design, carry out and analyse the two activities above; present results in a simple table or graph.

Exam Revision Checklist (IGCSE 0625 – Space Physics)

• State that the Earth rotates once every 24 h → day/night.
• State the axial tilt (23.5°) and explain how it produces the seasons.
• List the eight planets in order, give one key feature for each, and know the difference between terrestrial and giant planets.
• Define dwarf planets, moons, asteroids and comets.
• Be able to calculate:

  • Linear speed at the equator (v = 2πR/T).
  • Orbital speed of the Moon (v = 2πr/T, with r ≈ 3.84 × 10⁸ m, T ≈ 2.36 × 10⁶ s → v ≈ 1.02 km s⁻¹).
  • Peak wavelength of a black‑body using Wien’s law (extension only).

• Remember the practical safety points when working with bright lamps or the Sun.
• If you attempt the extension material, be ready to state:

  • What the CMBR is and why it is called “microwave” (λ ≈ 1 mm, ν ≈ 160 GHz).
  • How its discovery supports the Big‑Bang theory.