Science
How do tides work?
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.
See it in motion.
Watch a 2-minute animated lesson that shows exactly how tides works.
Step by step
- 1The Moon's gravity tugs the near-side ocean toward it, creating one bulge.
- 2On the far side, the Moon pulls the solid Earth away from the water, leaving a second bulge.
- 3Earth's rotation carries each location through both bulges, so most coasts see two highs and two lows daily.
- 4The Sun adds a smaller effect; when Sun and Moon align, tides are strongest (spring tides).
Frequently asked questions
- Why are there two high tides a day?
- There are two ocean bulges — one facing the Moon and one on the opposite side — and Earth's rotation carries each place through both each day.
- 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.