What does Edexcel GCSE Astronomy Topic 3 cover?

Topic 3 covers the relative sizes and distances of the Earth, Moon and Sun, the Sun's mean diameter, how Eratosthenes and Aristarchus measured the system, the relative effects of the Sun and Moon in producing spring and neap tides, the precession of the Earth's axis, and the causes and appearance of solar and lunar eclipses including the umbral contact terms.

How did Eratosthenes measure the size of the Earth?

Eratosthenes used the angle of the Sun's shadow at two cities a known distance apart. The shadow angle equals the angle between the cities at the Earth's centre, so the distance between them is that fraction of the full circumference. From an angle of about 7.2 degrees and a separation of about 800 km he found a circumference near 40000 km.

What is the difference between spring and neap tides?

Spring tides have the largest tidal range and occur at new and full Moon, when the Sun and Moon are aligned so their gravitational pulls add. Neap tides have the smallest range and occur at the quarter Moons, when the Sun and Moon are at right angles so their pulls partly cancel. The Moon's tidal effect is larger than the Sun's.

Why do eclipses not happen every month?

The Moon's orbit is tilted by about 5 degrees to the plane of the Earth's orbit (the ecliptic), so at most new and full Moons the Moon passes slightly above or below the exact line and the shadow misses. An eclipse only occurs when a new or full Moon happens near a node, where the two orbital planes cross.

How should I revise Topic 3 for Edexcel GCSE Astronomy?

Drill the Eratosthenes circumference calculation, learn the Aristarchus methods (the half-Moon right angle for the Sun's distance, eclipse timing for relative sizes), match spring and neap tides to the Sun-Moon alignment, understand precession as a 26000 year wobble, and learn the eclipse geometries (Moon between for solar, Earth between for lunar) and the umbral contacts.

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Edexcel GCSE Astronomy Topic 3 The Earth-Moon-Sun system: a complete overview of sizes, distances, tides, precession and eclipses

A deep-dive Edexcel GCSE Astronomy guide to Topic 3 The Earth-Moon-Sun system. Covers relative sizes and distances, the measurements of Eratosthenes and Aristarchus, tides, the precession of the Earth's axis, and solar and lunar eclipses, with the exam patterns Pearson repeats.

Generated by Claude Opus 4.813 min read1AS0 Topic 3Updated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

What Topic 3 actually demands
Sizes, distances and early measurements
Tides and precession
Eclipses
How Topic 3 is examined
Check your knowledge

What Topic 3 actually demands

The Earth-Moon-Sun system is a calculation-rich naked-eye topic: the scale of the system and its historical measurement, the tides, precession, and the geometry of eclipses. It rewards careful geometry and confident standard-form arithmetic.

This guide walks through the dot points of the topic, then sets out the exam patterns Pearson repeats. Each dot point has a matching page with practice questions; this overview ties them together.

Sizes, distances and early measurements

The Sun is largest (mean diameter $1.4 \times 10^6\,\text{km}$ , about 109 Earths), then the Earth (13000 km), then the Moon (3500 km). The Sun is about 400 times further than the Moon. Eratosthenes found the Earth's circumference from the shadow angle at two cities a known distance apart; Aristarchus used the half Moon (a right-angled triangle) for the Sun's relative distance and eclipse timing for relative sizes.

Tides and precession

Tides come mainly from the Moon's gravity (the Sun's effect is smaller). Spring tides (largest) occur at new and full Moon (pulls add); neap tides (smallest) at the quarter Moons (pulls partly cancel). Precession is the slow (about 26000 year) conical wobble of the Earth's axis, changing the pole star and shifting the equinoxes (and monument alignments).

Eclipses

A solar eclipse (new Moon) has the Moon between the Sun and Earth; a lunar eclipse (full Moon) has the Earth between the Sun and Moon. Types are total, annular and partial (solar) and total and partial (lunar). Eclipses are not monthly because the Moon's orbit is tilted about 5 degrees. The umbral contacts (first to fourth) time the stages.

How Topic 3 is examined

A typical Edexcel profile for the Earth-Moon-Sun system:

Calculation. The Eratosthenes circumference, and scale or ratio work in standard form.
Explanation. The Aristarchus methods, spring and neap tides, precession effects.
Geometry. The eclipse arrangements and why eclipses are not monthly.
Definitions. The umbral contact terms.

Check your knowledge

A mix of calculation and explanation questions covering Topic 3. Attempt them under timed conditions, then check against the solutions.

Eratosthenes found a shadow angle of 7.2 degrees between cities 800 km apart. Calculate the Earth's circumference. (2 marks)
State what Aristarchus used the half Moon to estimate. (1 mark)
State when spring tides occur and why they are large. (2 marks)
State the approximate period of the precession of the Earth's axis. (1 mark)
Describe the positions of the Sun, Earth and Moon during a total solar eclipse. (2 marks)
Explain why eclipses do not happen every month. (2 marks)
State what second umbral contact marks. (1 mark)

Sources & how we know this

Pearson Edexcel Level 1/Level 2 GCSE (9-1) in Astronomy (1AS0) specification — Pearson (2017)