Narration · approximately 40 seconds

Tides happen because the Moon's gravity pulls on Earth's oceans. The Moon tugs hardest on the water nearest to it, drawing it into a bulge, and this is the high tide facing the Moon. But there is also a second bulge on the opposite side of Earth, because the Moon pulls the solid Earth more strongly than it pulls the far ocean, effectively leaving that water behind. So the oceans are stretched into two bulges at once.
77 words · written for a clear narration pace

The Flash Answer

Tides happen because the Moon's gravity pulls on Earth's oceans. The Moon tugs hardest on the water nearest to it, drawing it into a bulge, and this is the high tide facing the Moon. But there is also a second bulge on the opposite side of Earth, because the Moon pulls the solid Earth more strongly than it pulls the far ocean, effectively leaving that water behind. So the oceans are stretched into two bulges at once. As the Earth spins on its axis each day, any given coastline passes through both bulges and both the gaps between them, giving roughly two high tides and two low tides a day. The Sun's gravity adds to the effect, making tides especially large or small at certain times of the month.

Gravity that pulls harder up close

Tides begin with gravity, and specifically with the fact that gravity gets weaker with distance. The Moon pulls on everything on Earth, but it does not pull on all of it equally. The side of Earth facing the Moon is about 12,700 kilometres closer than the far side, and over that distance the Moon's pull is measurably stronger on the near side than on the far side. It is this difference in pull across the width of the planet — not the raw strength of the pull — that creates tides.

The solid Earth is rigid and moves as a single lump, but the oceans are free to flow, so they respond to the uneven pull by shifting. Water on the side nearest the Moon is tugged toward it and heaps up into a bulge. That heaping of water is a high tide, and if the oceans only did this we would have one high tide a day. But there is a famous second bulge on the far side, and understanding it is the key to the whole phenomenon.

Key fact

Tides come from the difference in the Moon's gravity across Earth — a stronger pull on the near side than the far side — not simply from the Moon's overall pull.

The puzzle of the second bulge

The most counterintuitive part of tides is that there is a high tide on the side of Earth facing away from the Moon at the same time as the one facing toward it. If the Moon pulls water toward itself, why would water also pile up on the opposite side? The answer is that the Moon pulls the whole Earth, and it pulls different parts by different amounts.

Think of three things in a row: the near ocean, the solid Earth in the middle, and the far ocean. The Moon pulls the near ocean most strongly, the solid Earth less strongly, and the far ocean least of all. Relative to the solid Earth in the middle, the near water is pulled toward the Moon — making the near bulge — while the far water is pulled least, so the Earth is drawn away from under it, leaving it behind as a bulge on the far side. In effect the planet is being stretched gently along the Earth–Moon line, and the oceans bulge out at both ends of that stretch.

So the oceans take on a slightly egg-shaped, stretched form, with a bulge pointing toward the Moon and another pointing directly away from it. These two bulges are the two daily high tides, and the thinner regions at right angles between them are the low tides.

  1. The Moon pulls unevenly

    Its gravity is stronger on the near side of Earth than on the far side.

  2. Near water heaps up

    Ocean facing the Moon is pulled toward it, forming a bulge — a high tide.

  3. Far water is left behind

    The Moon pulls the solid Earth more than the far ocean, so that water bulges outward too.

  4. Earth spins through the bulges

    As the planet rotates, each coast passes through both bulges and both low points each day.

Two highs and two lows a day

The two bulges stay roughly lined up with the Moon while the Earth rotates underneath them once a day. Picture standing on a coastline: as your part of the planet turns, you are carried into the near bulge (high tide), then out to a thin region (low tide), then into the far bulge (high tide again), and finally out to the other thin region (low tide) before the cycle repeats. That is why most coasts experience about two high tides and two low tides in each daily cycle.

The timing is not exactly 24 hours, and the reason is neat. While the Earth spins, the Moon is also moving along in its own orbit, so the Earth has to turn a little extra each day to catch up with the Moon's new position. This makes the tidal cycle about 24 hours and 50 minutes long, which is why high tide arrives roughly 50 minutes later each day — a shift anyone who lives by the sea learns to expect.

~2 highs / dayMost coastlines see about two high tides and two low tides in each daily cycle of roughly 24 hours 50 minutes, as Earth spins through the two ocean bulges.
A coastline at high tide with the Moon visible in the sky above the sea
The Moon rising over a tidal coastline. Original image generated for Flash Science.

The Sun's supporting role

The Moon is the main author of the tides, but it is not the only one. The Sun is vastly more massive than the Moon, yet also vastly farther away, and because tides depend on the difference in pull across the Earth, the Sun's tidal effect works out to be a bit less than half the Moon's. Still, that is more than enough to matter, and the Sun and Moon together shape the tides we actually see.

When the Sun, Earth and Moon line up — at new moon and full moon — the Sun's tidal pull adds to the Moon's, and the two bulges reinforce each other to give especially large tides called spring tides, which have nothing to do with the season. When the Sun and Moon are at right angles, around the first and last quarter moons, their effects partly cancel, giving the gentlest tides of the month, known as neap tides. So the rhythm of the tides carries the fingerprint of both the day, from Earth's spin, and the month, from the Moon's phases.

Why tides differ so much from place to place

The simple picture of two smooth bulges explains why tides happen, but real coastlines add enormous variety, and this is where honesty about the model matters. The bulges are an idealisation. In reality the continents get in the way, the oceans slosh in complicated patterns, and the shape of coasts and sea floors funnels and amplifies the water. The Bay of Fundy in Canada sees tides of over 15 metres, while some seas barely have a noticeable tide at all.

Local geography, the depth of the water, and the way each ocean basin naturally rocks back and forth all combine to set the true height and timing of the tide at any given beach. This is why tide tables are worked out for specific ports rather than from the Moon alone. The underlying cause is always the same — the difference in the Moon's and Sun's gravity across the Earth — but the world's coastlines turn that steady cause into a rich variety of local rhythms.

✗ The myth

The Moon simply pulls the sea upward beneath it

Tides are often pictured as the Moon lifting the water directly under it, which would give just one high tide a day.

✓ The evidence

Two bulges, from a stretch

The Moon pulls the near ocean, the solid Earth and the far ocean by different amounts, stretching the planet along the Earth-Moon line. That makes two bulges at once — near and far — so most coasts get two high tides a day, not one.

Key takeaways

  • Tides come from the difference in the Moon's gravity across Earth, stronger on the near side than the far side.
  • The oceans are stretched into two bulges — one facing the Moon and one directly opposite — giving two high tides.
  • As Earth spins each day, every coast passes through both bulges and both low points, so most see two highs and two lows.
  • The tidal cycle is about 24 hours 50 minutes because the Moon moves on in its orbit, so tides shift ~50 minutes later daily.
  • The Sun adds a bit under half the tidal effect, boosting tides at new and full moon (spring tides) and easing them at the quarters (neap tides).
  • Coastlines and sea-floor shapes turn the simple bulges into a huge range of local tide heights and timings.

Frequently asked questions

Why are there two high tides a day instead of one?

The Moon pulls the near ocean, the solid Earth and the far ocean by different amounts, stretching the planet along the Earth-Moon line. This makes two bulges at once — one toward the Moon and one away from it — so as Earth spins, each coast passes through both, giving two high tides.

Does the Sun affect the tides too?

Yes. The Sun's tidal effect is a bit under half the Moon's. When the Sun and Moon align at new and full moon, their pulls combine for large spring tides; when they are at right angles near the quarter moons, the effects partly cancel for gentle neap tides.

Why does high tide come later each day?

While Earth spins, the Moon moves along in its orbit, so Earth must turn a little extra to line up with the Moon again. This makes the tidal cycle about 24 hours 50 minutes, so high tide arrives roughly 50 minutes later each day.

Why are tides much bigger in some places than others?

The two-bulge picture is idealised. Real continents, water depth and the shape of coasts and sea floors funnel and amplify the water differently everywhere, so tides range from over 15 metres in the Bay of Fundy to almost nothing in some seas. Tide tables are calculated per location.

Sources & further reading

This explainer was prepared through desk research using the sources below; established facts are distinguished from open questions in the text.

  1. NOAA — Tides and Water Levels: what causes tides, National Oceanic and Atmospheric Administration.
  2. NASA — Moon in Motion: tides and tidal forces, NASA Science.
  3. Encyclopaedia Britannica — Tide: cause, spring and neap tides, Britannica.
  4. NOAA National Ocean Service — Why does the ocean have tides?, NOAA.
  5. Royal Museums Greenwich — What causes tides and why there are two a day, Royal Observatory.

More Flash Science explainers are being published regularly. Questions or corrections? Email contact@flashscience.org.

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