What causes the tides, and how does the slow wobble of the Earth's axis change the sky over centuries?
The relative effects of the Sun and Moon in producing spring and neap tides, and the precession of the Earth's axis, its effects on the sky and its use in archaeoastronomy.
A focused answer to Edexcel GCSE Astronomy statements 3.5 to 3.7, covering how the gravitational pull of the Moon and Sun produces high and low tides and spring and neap tides, and how the slow precession of the Earth's axis changes the position of the celestial pole and the equinoxes, with its use in archaeoastronomy.
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What this dot point is asking
Edexcel statements 3.5 to 3.7 want you to understand the relative effects of the Sun and Moon in producing high, low, spring and neap tides, and to understand how the gradual precession of the Earth's axis affects the appearance of the Sun, Moon and stars from Earth, and its use in archaeoastronomy.
What causes the tides
The key comparison the exam wants is that the Moon dominates the tides despite the Sun's huge mass, because tidal force falls off very steeply with distance. As the Earth spins, a coastline passes through the bulges and the troughs between them, producing the daily rhythm of high and low water.
Spring and neap tides
"Spring" here means the water springs up high, not the season, so spring tides happen twice a month. The exam reliably asks you to match the tide type to the Sun-Moon alignment and the Moon phase: aligned (new or full) for spring, right angles (quarters) for neap. Drawing the three bodies in line versus at right angles is the clearest way to show it.
The precession of the Earth's axis
Because the pole drifts, Polaris is only temporarily the pole star; thousands of years ago Thuban was, and in the future Vega will be near the pole. The shifting equinoxes (the "precession of the equinoxes") mean a monument aligned to sunrise on a solstice or to a particular star slowly falls out of alignment over millennia. This is why archaeoastronomers must allow for precession when checking whether an ancient site such as Stonehenge once pointed accurately at an astronomical event, a direct link to Topic 7.
How Edexcel examines this
This is naked-eye Paper 1 content with reliable explanation marks. Tide questions reward stating that the Moon's gravity is the main cause (with the Sun smaller), then matching spring tides to the aligned Sun and Moon at new and full Moon (pulls adding) and neap tides to the right-angled geometry at the quarters (pulls partly cancelling). Precession questions reward the slow conical wobble of about 26000 years and a valid effect: the changing pole star or the shifting equinoxes that move monument alignments over millennia. The strongest synoptic link is to archaeoastronomy in Topic 7 (why an ancient alignment no longer points true). The two commonest errors are thinking spring tides relate to the season and confusing precession with a change in the tilt angle, so be precise that the tilt stays at degrees while the pointing direction drifts.
Try this
Q1. State when spring tides occur in terms of the Moon's phase. [1 mark]
- Cue. At new Moon and full Moon, when the Sun and Moon are aligned.
Q2. State the approximate period of the precession of the Earth's axis. [1 mark]
- Cue. About 26000 years.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 1AS0 20224 marksExplain the difference between spring tides and neap tides, and describe the positions of the Sun, Moon and Earth that produce each.Show worked answer →
Spring tides are the highest high tides and lowest low tides (the largest tidal range), and they occur when the Sun and Moon are lined up (at new Moon and full Moon), so their gravitational pulls add together (2 marks). Neap tides have the smallest tidal range, and they occur when the Sun and Moon are at right angles as seen from the Earth (at the first and third quarter Moon), so their pulls partly cancel (2 marks). Markers reward linking spring tides to the Sun and Moon aligned (pulls adding) at new and full Moon, and neap tides to the Sun and Moon at right angles (pulls partly cancelling) at the quarter Moons. The Moon's effect is larger than the Sun's.
Edexcel 1AS0 20213 marksExplain what is meant by the precession of the Earth's axis and describe one effect it has on astronomical observations over thousands of years.Show worked answer →
Precession is the very slow circular wobble of the direction of the Earth's rotation axis, which traces out a cone over a period of about 26000 years (1 mark). One effect is that the position of the north celestial pole gradually moves, so the "pole star" changes over time: Polaris is the pole star now, but thousands of years ago it was a different star (1 mark). Another acceptable effect is that the positions of the equinoxes drift around the sky (the precession of the equinoxes), changing the alignment of ancient monuments built to mark sunrise on a solstice (1 mark). Markers reward defining precession as the slow conical wobble (about 26000 years) and giving a valid effect such as the changing pole star or the shifting equinoxes and monument alignments.
Related dot points
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Sources & how we know this
- Pearson Edexcel Level 1/Level 2 GCSE (9-1) in Astronomy (1AS0) specification — Pearson (2017)