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How do tides work?

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Quick answer

Tides are the regular rise and fall of sea levels caused by the gravitational pull of the Moon and Sun, combined with Earth's rotation. The Moon's gravity stretches the oceans into two bulges, and as Earth rotates, most coasts experience two high and two low tides daily.

Tides work because the Moon's gravity pulls harder on the side of Earth facing it than on the far side, stretching the oceans into two bulges. As Earth rotates beneath those bulges, each coast passes through them — giving most places two high tides and two low tides a day.

Step by step

  1. The Moon's gravity pulls hardest on the ocean nearest to it, creating a bulge of water on that side.
  2. On the opposite side of Earth, the Moon pulls the solid Earth away from the water, leaving a second bulge.
  3. As Earth rotates, each location passes through both bulges, causing two high tides and two low tides each day.
  4. The Sun also exerts a gravitational pull, but it is less than half as effective as the Moon's because of its greater distance.
  5. When the Sun and Moon align at new and full moon, their pulls add up, producing larger spring tides.
  6. When the Sun and Moon are at right angles, their pulls partly cancel, resulting in smaller neap tides.

Key things to understand

  1. The Moon's gravity tugs the near-side ocean toward it, creating one bulge.
  2. On the far side, the Moon pulls the solid Earth away from the water, leaving a second bulge.
  3. Earth's rotation carries each location through both bulges, so most coasts see two highs and two lows daily.
  4. The Sun adds a smaller effect; when Sun and Moon align, tides are strongest (spring tides).

Labelled diagram

  1. Moon's gravity

    Pulls ocean water toward it, creating a bulge on the near side of Earth.

  2. Near-side bulge

    Water is pulled away from Earth's surface, forming a high tide.

  3. Far-side bulge

    Earth is pulled away from water, leaving a bulge on the opposite side.

  4. Earth's rotation

    Carries each location through both bulges, causing two high tides daily.

How the Moon creates two tidal bulges

Real-world examples

  • The Bay of Fundy in Canada has the world's highest tides, with a range of about 16 meters (52 feet).
  • The Bristol Channel in the UK experiences a tidal range of up to 15 meters due to its funnel shape.
  • In the Mediterranean Sea, tides are small because it is nearly enclosed and has little connection to the ocean.
  • The Thames Barrier in London closes during high tides to protect the city from flooding.

Common mistake

✗ Myth: The Moon's gravity only pulls on the water directly beneath it, creating a single high tide.

✓ Fact: The Moon's gravity pulls on the entire Earth, but the water responds more than the solid land. This creates two bulges: one on the side facing the Moon and one on the opposite side, resulting in two high tides per day.

People also ask

What causes tides?
Tides are caused by the gravitational pull of the Moon and, to a lesser extent, the Sun, combined with Earth's rotation. The Moon's gravity stretches the oceans into two bulges, and as Earth rotates, different places pass through these bulges, creating high and low tides.
Why are there two high tides a day?
There are two high tides a day because the Moon's gravity creates two ocean bulges: one on the side facing the Moon and one on the opposite side. As Earth rotates, most coastal locations pass through both bulges within about 24 hours and 50 minutes, resulting in two high tides and two low tides.
What are spring tides and neap tides?
Spring tides are higher-than-average tides that occur when the Sun and Moon align (at new and full moon), combining their gravitational pulls. Neap tides are lower-than-average tides that occur when the Sun and Moon are at right angles (first and third quarter moon), partially cancelling each other's effects.
Does the Sun affect tides?
Yes, but less than the Moon. When Sun and Moon line up, their pulls combine into larger 'spring' tides; at right angles they partly cancel into smaller 'neap' tides.
Why are some places' tides much bigger?
Coastline shape and ocean-basin resonance can amplify tides — bays like the Bay of Fundy funnel water into enormous tidal ranges.
Why isn't high tide at the same time each day?
The Moon moves along its orbit as Earth spins, so the tidal cycle shifts about 50 minutes later each day.

Part of the guide: Water & Weather Phenomena →