What Causes Ocean Tides? The Moon, Two Bulges, and Why the Sea Breathes Twice a Day
✦ Key takeaways
- Tides come mainly from the Moon's gravity pulling on the oceans, with a smaller pull from the Sun.
- There are two water bulges: one facing the Moon and one on the opposite side, caused by the gravity gradient across Earth.
- Most coasts see roughly two highs and two lows each day, about 12 hours 25 minutes apart.
- Spring tides (strongest) occur when Sun and Moon align; neap tides (weakest) when they sit at right angles.
- Coastline shape and bay depth amplify the range — the Bay of Fundy reaches about 16 metres.
Stand on a beach for a few hours and you will see the water's edge move: it creeps up the sand, then steadily retreats, as if the whole ocean were breathing. That slow breathing is the tide, and behind it lies an elegant piece of astronomy linking Earth's water to the gravity of the distant Moon.
The Moon Tugs the Sea
Every mass attracts every other mass. The Moon, despite orbiting some 384,000 kilometres away, is large enough that its gravity pulls on the oceans. Water is liquid and free to move, so it responds to that pull far more readily than solid rock, gathering into a gentle bulge on the side of Earth facing the Moon. But gravity does not act equally across the whole planet. Water nearest the Moon is pulled harder than Earth's centre, and Earth's centre is pulled harder than the far-side water. It is this difference in pull across the globe — not its raw strength — that actually raises the tides.
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The Puzzle of the Second Bulge
The surprise is that there are two bulges, not one: one facing the Moon and one on the exact opposite side. Why should water pile up on the far side, where the Moon's pull is weakest? Picture three runners joined by elastic cords while the Moon tugs them: the front runner (the near water) is drawn away from the middle, and the middle (Earth's rock) is drawn away from the rear runner. The far water is effectively left behind, gathering into a second bulge that mirrors the first.
As Earth spins on its axis over twenty-four hours, every coastline sweeps through both bulges in turn. That is why most shores experience two high tides and two low tides a day — a pattern oceanographers call a semidiurnal tide.
Why the Timing Slips by 50 Minutes
If the Moon stood still, high tide would return exactly every twelve hours. But the Moon itself is orbiting Earth in the same direction it spins, so by the time our planet completes one rotation, the Moon has moved a little further along its path. Earth needs roughly fifty extra minutes to "catch up" and bring the coast back under the bulge. That is why high tide arrives about fifty minutes later each day, and why successive tides are 12 hours 25 minutes apart — the reason a tide table quietly slides forward day after day.
| Fact | Duration |
|---|---|
| Between two high tides | 12 hours 25 minutes |
| One full tidal cycle | 24 hours 50 minutes |
| Daily shift in timing | about 50 minutes |
| Moon's distance from Earth | roughly 384,000 km |
Spring Tides and Neap Tides
The Sun raises tides too, but its effect is only about 46% as strong as the Moon's, despite its enormous mass, simply because it lies more than 150 million kilometres away. When the Sun and Moon line up — at new moon or full moon — their pulls combine, making highs higher and lows lower. These are spring tides. When the Moon sits at a right angle to the Sun — at the quarter moons — each partly cancels the other, giving a smaller range known as neap tides. Note that "spring" here has nothing to do with the season; it describes water that "springs" up high, and the cycle repeats twice every lunar month.
| Type | Moon phase | Sun–Moon alignment | Tidal range |
|---|---|---|---|
| Spring tide | New or full moon | In a straight line | Large (strongest) |
| Neap tide | First or last quarter | Right angle, 90° | Small (weakest) |
How Coastlines Make the Difference
Out in the open ocean the two bulges are usually only half a metre to a metre tall. The dramatic ranges we see at the shore are shaped by coastal geometry: narrow bays and shallow estuaries funnel the incoming water and force it to pile up. In Canada's Bay of Fundy the difference between high and low water reaches about sixteen metres — the largest range on Earth — because the natural sloshing time of the bay nearly matches the tidal rhythm, amplifying it through resonance. This is why tide tables differ from port to port even though the astronomical cause is identical: the nearly enclosed Mediterranean sees a tide of only a few centimetres, while other coasts swing by several metres. Understanding this interplay of astronomy and geography is what makes tide prediction a precise science that fishermen, sailors, and harbour engineers rely on every single day.