Tides

Why does the sea climb up the beach and slide back, twice a day, forever?

The Moon's pull stretches the ocean into two bulges and Earth turns through them. Learn why there are two high tides a day, why they arrive later each day, and what makes a spring tide.

5 depths≈ 205 min of reading71 practice questions

The ladder

Choose how deep to go

Every depth is open. Start where it feels interesting — you can always move back.

Discoverabout 30 minTides: the sea's daily rise and fallMeet the tide: not a wave but the whole sea rising and falling. Find out how the Moon's pull makes two bulges, why most coasts get two high tides a day, why the tide is 50 minutes later each day, and what spring and neap tides are.Owner-accepted · not line-by-line reviewed
Understandabout 40 minHow the Moon builds two bulgesWork out why a pull towards the Moon makes a bulge away from it, where 24 h 50 min comes from, why the Sun's tide is only 46% of the Moon's, and why the same Moon gives Kochi one metre and Bhavnagar ten.Owner-accepted · not line-by-line reviewed
Investigateabout 45 minInvestigate: predicting, classifying and staying safePredict and check a day of tide heights, learn to tell semidiurnal, diurnal and mixed tides apart, meet the Hooghly bore and storm surges, see how tidal power and INCOIS's predictions work, and test the funnelling and resonance ideas with real numbers.Owner-accepted · not line-by-line reviewed
Go deeperabout 45 minDeepen: the mathematics and history behind a tide tableTrace the two-hundred-year path from Newton's equilibrium theory to Laplace's ocean waves and Kelvin's tide-predicting machine, meet the harmonic constituents that a real tide is built from, derive why a bay resonates at a quarter wavelength, and quantify Earth's own solid and atmospheric tides.Owner-accepted · not line-by-line reviewed
Extendabout 45 minExtend: deep time, deep space, and open questionsFollow tidal friction from a subtle offset in Earth's bulge to a shorter Cretaceous day, a measurably receding Moon, tidal heating on Io, Europa and Enceladus, and a set of open questions and careers built on this one idea.Owner-accepted · not line-by-line reviewed

Practise and apply

Question bank · 71 questions

8 sections from Foundation to Challenge, each with hints and a worked solution — or play a 10-question round for points and stars.

Foundation 17Core 25Stretch 19Challenge 10

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The web

Connected ideas

Follow a connection when you’re curious — some lead into completely different areas.

Tides

Builds onanother area
Gravity
Tides are gravity made visible: the Moon pulls the near ocean harder than the far ocean.
Related to
Phases of the Moon
Spring and neap tides follow the phases: the biggest tides come at new and full moon.
Usesanother area
Exploration: reasons and consequences
Sailing ships left harbour on the tide, and monsoon winds and currents set the whole calendar of Indian Ocean trade.
Used inanother area
Data handling
A tide table is a data set: measure the water twice a day for years, and the pattern lets you predict it.

Words to know

48 words are explained in these lessons

All vocabulary →

TideHigh tide (high water)Low tide (low water)Tidal rangeFlood tideEbb tideSlack waterTidal bulgeLunar day (tidal day)Spring tideNeap tideIntertidal zone+36 more