OCEAN · FIELD GUIDE
What Causes the Tides? The Moon, the Sun & Spring Tides
Twice a day the sea climbs up the beach and slides back again, like the planet breathing. Here's what's actually pulling the water — and why the timing shifts a little every day.
The tide is the slow, steady breathing of the sea — and once you see what's driving it, the daily rhythm and the monthly cycle both fall into place.
The Moon does most of the pulling
Gravity from the Moon tugs hardest on the water directly beneath it, drawing the ocean up into a bulge. Less obviously, there's a matching bulge on the opposite side of the planet: there the Moon's pull is weakest, so that water is effectively flung outward as the Moon draws the solid Earth toward itself. Two bulges, on opposite sides.
As the Earth spins on its axis, your stretch of coast sweeps through both of them in a day — giving most places two high tides and two low tides. That's why a tide isn't a once-a-day event but a steady in-out, in-out.
Why the timing drifts
If the Moon held still, the cycle would be a tidy 24 hours. But the Moon is moving — about 13 degrees further around its orbit each day — so the spinning Earth has to turn a bit extra to line back up with it. That bit extra is about 50 minutes, which is why high tide arrives roughly 50 minutes later each day. The clock is no help; the tide table is everything.
Spring tides and neap tides
The Sun adds a smaller pull of its own, and the two bodies move in and out of step over the month:
- Spring tides — around the new and full Moon, the Sun and Moon line up and their gravity combines. The highs are higher, the lows are lower, the range is at its biggest. ("Spring" here means the water springs up — nothing to do with the season.)
- Neap tides — around the half Moons, the Sun pulls at right angles to the Moon and partly cancels it. The range shrinks to its gentlest.
The coast shapes the rest
The bulges set the rhythm, but the actual height at any one beach depends on the shape of the seabed and the shoreline, which can funnel and amplify the water enormously. That's why the same tide that barely moves a metre on an open ocean island can rise more than fifteen metres in a place like the Bay of Fundy — and why a local prediction always beats a rule of thumb.
A last, important caveat: tide tables assume calm weather. In a storm, wind and low pressure can heap extra water against the coast — a storm surge — pushing the real level well above the predicted high. On a spring high tide, that's the recipe for the worst coastal flooding, so give the sea a wide margin whenever the weather turns.
Frequently asked questions
Why are there two high tides a day, not one?
The Moon's gravity pulls the ocean into a bulge on the side of Earth facing it — but there's a second bulge on the far side, where the water is in effect 'left behind' as the Moon tugs the solid Earth toward itself. As the planet spins through both bulges each day, most coasts pass through two highs and two lows. The cycle takes about 24 hours and 50 minutes rather than a flat 24, because the Moon has moved along its orbit in the meantime.
Why does high tide come later each day?
Because the Moon doesn't stay still. While Earth spins once, the Moon moves about 13 degrees further along its monthly orbit, so the planet has to turn a little extra to 'catch up' to the Moon's pull. That extra turn takes roughly 50 minutes, which is why high tide slips about 50 minutes later each day — and why a tide table, not the clock, is the only reliable guide.
What are spring and neap tides?
Spring tides are the biggest tides of the month — the highest highs and lowest lows. They happen around the new and full Moon, when the Sun and Moon line up and their gravity pulls together. 'Spring' has nothing to do with the season; it means the tide 'springs' up. Neap tides are the gentlest, with the smallest range, and happen at the half-Moons when the Sun's pull is at right angles to the Moon's and partly cancels it.
Does the Sun affect the tides too?
Yes, but less than the Moon. The Sun is vastly bigger but also vastly farther away, and tides depend on the difference in gravity across the Earth, which falls off steeply with distance. So the Sun's tidal pull is a bit under half the Moon's. When they align (new and full Moon) you get the larger spring tides; when they oppose (half Moons) you get the smaller neaps.
Why are some places' tides so much bigger than others?
The bulges are only the starting point. The real range at any coast depends on the shape of the seabed and the coastline, which can funnel and amplify the water. The Bay of Fundy in Canada sees over 15 metres because its length and depth resonate with the tidal rhythm, while many open ocean islands barely see a metre. That's why a local prediction beats any global rule of thumb.
Can the tide be higher than the prediction says?
Yes. Published tide tables are astronomical — they assume calm weather. Strong onshore winds and low air pressure in a storm can pile extra water against the coast, a storm surge, pushing the real level well above the predicted high tide. That combination, especially on a spring high tide, is what causes the worst coastal flooding, so always allow a wide margin in rough weather.
SEE IT LIVE
Everything in this guide is on the live ocean map.