The Tides ofAtlantic College
St Donat's · Vale of Glamorgan · Wales
51°24′7″N 3°31′50″W
How do tides work?
The St Donat’s coastline has a rock platform that stretches 150 metres out to sea. Barnacled and ancient, it sits fully exposed — a landscape you can walk across, investigate, and find fossils in. Come back in six hours and it will be gone, under four to eight metres of water. This happens twice a day, every day, with mechanical precision.
Our story begins with the Moon
384,000 kilometres away and about 1/80th the mass of Earth, the Moon pulls hard enough to move whole oceans.
All forces are interactions
Gravity is the attractive force between any two masses, the closer they are, the stronger the effect. The strength of the force depends on the masses of the two objects, the distance between them, and a Universal Gravitation Constant in the following way.
It's a celestial dance
Rather than crashing into each other, the moon accelerates towards Earth and the Earth also accelerates towards the moon in an elliptical orbit. They are locked in celestial dance based on their initial motion when the solar system was formed.
The tidal force raises two ocean bulges
Imagine a circle of particles around the Earth. Gravity is distance-dependent; it’s stronger the closer you are. As the particles accelerate toward the moon, the near particles accelerate faster than the far ones. The result? The circle stretches into an ellipse. This is how the tidal force acts on fluids on Earth.
Back on the coast
These two photographs were taken six hours apart. Left is the platform exposed, the sea distant. Right is the slipway, the same sky but the sea has climbed four metres and swallowed everything. The Moon hasn't moved much. Earth has simply rotated, sweeping this shore through the tidal bulge and out the other side.
Two highs, two lows a day
Our Moon moves very slowly, orbiting Earth at about 13.2° per day relative to fixed stars and takes 27.3 days to make exactly one full 360-degree revolution around Earth (siderial month).
But the peaks are too even
The peaks here look like they reach the same height. The real sea does not. Something else is pulling on it.
The pull of the Sun
If we look at data from Tide Forecast for Llantwit Major, you will notice a repeating pattern: tidal ranges (the difference between high and low tide) grow, shrink, and grow again in a roughly 14-day cycle. This two-week rhythm is the clue. While the Moon dominates our tides, this pattern reveals the Sun's influence. The Sun is 27 million times more massive than the Moon, and its distance 390 times further away. The Moon's orbit relative to the Sun is 29.5 days (synodic month). Because Earth is also constantly moving along its orbit around the Sun, the Moon must travel slightly further to line up in the same position.
The two effects add and subtract
The Moon's rhythm runs for 12 hours and 25 minutes, which pushes the arrival of high tides roughly 50 minutes later each day. The Sun's rhythm is an exact 12 hours. The two signals beat against each other, swelling and fading every 14.8 days. The Sun's tidal pull is 46% of the Moon's.
Spring tides
At new Moon and full Moon, the Sun, Earth and Moon line up. Both tidal pulls point in the same direction. They add together. High tides climb higher, low tides drop lower — the range between them is at its maximum.
A spring tide and a strong wind
When a large spring tide meets windy conditions, the two combine to throw up turbulent waves against this coast.
Neap tides
At quarter Moon, the Sun and Moon pull at right angles to each other. Their tidal bulges point in different directions — they partially cancel. High tides are lower than usual, low tides are higher. The range shrinks to its minimum.
A neap tide and a weak wind
The same shore is calm, and the water moves far less between high and low.
The role of the Bristol channel
St Donat’s sits on the Bristol Channel, a long wedge of sea that narrows between Wales and England toward the River Severn. In the open ocean the range is about 2 m. Here, the channel does something different.
The funnel forces the water up
As the tide moves up the narrowing channel, the same water is squeezed into less and less space. The open-ocean tide of a metre or two grows to 9–10 metres here.
Among the largest tides on Earth
Further up the channel at Avonmouth the mean spring range reaches 14.5 metres — second only to the Bay of Fundy in Canada on the other side of the Atlantic. On the biggest tides the flood can pile into a single wave that runs up the Severn.
The tide floods faster than it ebbs
Watch the water climb the slipway: on a spring tide it rises almost 10 metres, and floods in faster than it drains away. In shallow water the crest outruns the trough, so the wave leans forward — the flood is short and steep, the ebb long and slow. We capture that lean with a third term, the overtide.
Watch out for the returning tide
On a gently shelving flat the water’s edge can advance quick — and because the flood is faster than the ebb, it returns sooner than you expect. This is how the tide here can cut people off.
One tide, three factors
Add the Moon, the Sun and the shape of the coast, and the tide can be predicted years ahead — to the centimetre. Here is the full pattern over one month, full moon to full moon.
St Donat's · Vale of Glamorgan
The Tides of Atlantic College Explained
- The Moon raises two bulges; Earth’s spin gives two high tides a day, about 12 h 25 min apart — the M₂ tide.
- The Sun adds a weaker pull — spring tides when it lines up with the Moon, neap tides when it sits at a right angle — the S₂ tide.
- The Bristol Channel funnels the tide to 9–10 m here, among the greatest ranges on Earth.
- Shallow water adds an overtide, M₄ — four highs a day — that makes the flood faster than the ebb.
The tide you see on the beach is a sum of many constituents: M₂ (lunar) and S₂ (solar) give two a day; M₄ is a lunar overtide — four a day. Summed, they give h(t) = M₂ + S₂ + M₄.
A note on accuracy
Where this model stops
This is a teaching model — an astronomical prediction built from a handful of harmonic constituents. The real sea is messier. Some of what it leaves out:
- The Moon’s elliptical orbit. Its distance swings from ~363,000 to ~406,000 km; because tidal force falls off as 1/distance³, that’s a ±~18% change every 27.5 days. Real springs are unequal — a perigean “king tide” outranks an apogean one. We assume a circle, so ours come out level.
- The daily inequality. The changing declination of the Moon and Sun makes one of the day’s two highs taller than the other. We draw them equal.
- Weather. This is astronomy only — a deep low and an onshore gale can pile the real water a metre or more above prediction. A storm surge.
- The lag. The biggest spring tide arrives a day or two after new or full Moon, not on it — the “age of the tide.” We line the constituents up perfectly and skip it.
- The bulge itself. The real tide isn’t a bulge the Earth turns under — it’s a wave sloshing around ocean basins, steered by coastlines and the planet’s spin. The amplitudes here come from the Nash Point gauge, not from first principles.
What it does get right is the rhythm — two tides a day, the fortnightly spring–neap swing, and why this coast has one of the largest ranges on Earth. More on tidal constituents →
Created and produced by Jaya Ramchandani (welearnwegrow.bio) in collaboration with Claude Design · Constituent data from UKHO tidal analysis for Nash Point gauge