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Why does the sea come in and go out twice a day?
Ocean Tides lab · Last updated: 16 October 2026
You explore why the sea creeps up the beach and then slips back, not once but twice a day. They see how the Moon's gravity tugs on Earth's oceans and why there are two bulges of water, not one. Tides matter to fishing boats, ports and the creatures living on every shore.
What you'll discover
- The Moon's gravity pulls more strongly on the side of Earth nearest to it than on the far side.
- This difference raises two bulges of ocean, one facing the Moon and one on the opposite side.
- As Earth spins through both bulges each day, most coasts get two high tides and two low tides.
The science in plain words
Tides are caused mainly by the Moon's gravity. Gravity gets weaker with distance, so the Moon pulls hardest on the ocean on the side of Earth facing it, less hard on Earth's centre and least on the ocean on the far side. It is this difference in pull, rather than the pull itself, that matters.
On the near side, the water is pulled towards the Moon a little more than Earth's centre, so it bulges out towards the Moon. On the far side, Earth's centre is pulled towards the Moon more than the water is, so that water is left behind and forms a second bulge facing away. Earth turns once a day underneath these two bulges, so a place on the coast passes through high water twice and low water twice. Because the Moon also moves along its orbit, the cycle takes about 24 hours and 50 minutes, so high tide comes roughly 50 minutes later each day.
The Sun raises tides too, though they are a little under half as strong as the Moon's. At new and full Moon, when the Sun and Moon line up, their effects add to make extra-high spring tides. At the quarter Moons they partly cancel, giving gentler neap tides. Coastlines and the shape of the seabed make the real tides at each beach very different.
Where it fits
- India, NCERT: Class 9 science, Chapter 9: Gravitation
- United States: Grades 6 to 8
Common questions
What causes tides?
Tides are caused mainly by the Moon's gravity, with a smaller effect from the Sun. The Moon pulls more strongly on the near side of Earth than the far side, raising two bulges of ocean. As Earth spins, coasts move through these bulges and the sea rises and falls.
Why are there two high tides a day?
The Moon raises two bulges of water: one on the side facing the Moon and one on the opposite side, where the water is pulled less than Earth's centre. Earth rotates through both bulges in about a day, so most coasts get two high tides.
What are spring tides and neap tides?
Spring tides happen at new and full Moon, when the Sun and Moon line up and their pulls combine, making high tides higher and low tides lower. Neap tides happen at the quarter Moons, when the pulls are at right angles and the tidal range is smaller.
Related labs
- Moon Phases: A lamp, a globe and a Moon on real gravity: why does the Moon change shape?
- Orbital Mechanics: Newton's cannonball: how fast must you fire it sideways so it never lands?
- Ocean Currents: Ice at one end, warmth at the other: how does a whole ocean start to move?
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Try it yourself
The Ocean Tides lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.