[{"data":1,"prerenderedAt":994},["ShallowReactive",2],{"layer:tides:discover":3},{"layer":4,"contentHash":975,"dependencyHashes":976,"approval":987,"releaseId":993},{"schemaVersion":5,"conceptId":6,"locale":7,"depth":8,"revision":5,"title":9,"subtitle":10,"summary":11,"objectives":12,"estimatedMinutes":18,"plate":19,"blocks":40,"sourceIds":970,"reviewStatus":971,"authoring":972},1,"tides","en","discover","Tides: the sea's daily rise and fall","Why the whole ocean leans towards the Moon, twice a day, forever","Meet 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.",[13,14,15,16,17],"Tell the difference between a tide, a wave and a tsunami, and use the words high water, low water, tidal range, flood and ebb correctly.","Explain why the Moon's pull makes two tidal bulges and why Earth turning through them gives two high tides a day.","Predict roughly when tomorrow's high tide will be, using the 50-minutes-later rule.","Say what spring and neap tides are and link them to the phases of the Moon.","Describe how tidal range varies around India's coasts and how coastal life and coastal work are timed by the tide.",30,{"title":20,"rows":21},"Lesson plate",[22,25,28,31,34,37],{"label":23,"value":24},"Depth","Discover",{"label":26,"value":27},"Reading time","≈ 30 minutes",{"label":29,"value":30},"Prior knowledge","None — curiosity about the sea is enough",{"label":32,"value":33},"Chapters","10",{"label":35,"value":36},"Labs","Tide lab (bulges, spring–neap, tide clock), sort, match",{"label":38,"value":39},"Numbers used","metres, hours and minutes",[41,45,51,57,63,68,73,103,108,113,116,145,158,174,179,184,190,194,209,214,220,245,249,267,271,300,305,311,327,342,364,369,374,377,381,407,411,419,425,430,435,438,517,522,527,530,575,601,605,610,613,640,696,701,706,709,738,759,763,809,944,960],{"id":42,"type":43,"markdown":44},"intro-beach","prose","Walk down to a beach in the morning, plant a stick in the wet sand right where the water stops, and go away for six hours. When you come back, one of two surprising things will have happened. Either the sea has climbed right up to your stick and swallowed it, or it has crept far away and left your stick standing in dry sand a long walk from the water.\n\nNobody emptied the sea. No river ran away with it. The whole ocean simply **leaned** — and it leans back again a few hours later, and then again, and again, day after day, for as long as there has been an ocean.\n\nThat slow, dependable breathing of the sea is called the **tide**, and this lesson is about the astonishing fact behind it: **the tide is the Moon, 384,000 kilometres away, pulling on the water of your beach.**",{"id":46,"type":47,"variant":48,"title":49,"markdown":50},"how-to-use","callout","observation","How to use this lesson","Read the chapters in order the first time; each one sets up the next. Whenever you meet a **prediction**, decide on your answer before you read on — being surprised is what makes an idea stick. The labs are safe to play with as long as you like.",{"id":52,"type":53,"title":54,"eyebrow":55,"navLabel":56},"ch1","chapter","A sea that climbs the beach and climbs back down","Chapter 01","1 The sea moves",{"id":58,"type":43,"markdown":59,"help":60},"tide-not-waves","Stand at the edge of the sea and the first thing you notice is **waves**: water heaping up, curling over and rushing at your feet every few seconds. Waves are made by the **wind**. A storm far out at sea shoves the surface into ridges, and those ridges travel until they trip on the shallows and break.\n\nThe tide is something else entirely, and much harder to see while you are watching. The tide is the **whole water level** going up and down. It takes about six hours to rise and about six hours to fall, so it is far too slow to watch the way you watch a wave. You notice it by coming back later, or by looking at something that does not move: a rock, a jetty, a fishing boat's mooring rope, the green weed on a harbour wall.\n\nHere is the difference in one sentence. **A wave moves the water up and down in one place for a few seconds. A tide moves the whole sea up and down over several hours.**",{"simplerExplanation":61,"anotherExample":62},"Waves are quick ripples made by wind. The tide is the slow rise and fall of the entire sea, which takes about six hours each way.","Think of a bathtub. Splashing your hand makes waves. Opening the tap raises the whole water level. The tide is the tap, not the splash.",{"id":64,"type":47,"variant":65,"title":66,"markdown":67},"def-tide","definition","Tide, in one line","A **tide** is the regular rise and fall of the sea, usually **twice a day**, caused mainly by the pull of the **Moon's gravity** on the ocean.\n\nYour NCERT geography book puts it almost exactly this way: *the rhythmic rise and fall of ocean water twice in a day is called a tide*.",{"id":69,"type":47,"variant":70,"title":71,"markdown":72},"misc-tide-wave","misconception","“A big tide means big waves”","It sounds right, but the two are separate. A day with an enormous tidal range can be glassy calm, and a day of thundering surf can have a tiny tide. Waves come from **wind**; tides come from the **Moon and Sun**.\n\nA **tsunami** is a third thing again. It is not a tide at all, even though it is sometimes wrongly called a tidal wave. A tsunami is one huge wave made by an earthquake or a landslide under the sea, and it arrives without any timetable. Tides arrive exactly on time, every day, and are printed in books a year in advance.",{"id":74,"type":75,"caption":76,"columns":77,"rows":82},"table-three-movements","table","Three different ways the sea moves — do not mix them up",[78,79,80,81],"Movement","What makes it","How long it lasts","Can you predict it?",[83,88,93,98],[84,85,86,87],"Wave","Wind blowing over the surface","A few seconds from crest to crest","Roughly, from weather forecasts",[89,90,91,92],"Tide","The pull of the Moon, helped by the Sun","About 6 hours up, 6 hours down","Yes — exactly, years ahead",[94,95,96,97],"Tsunami","An earthquake or landslide under the sea","Minutes between crests, rare","No — only a warning once it starts",[99,100,101,102],"Ocean current","Wind, heat and saltiness of the sea","Flows for months and years","Broadly, from long records",{"id":104,"type":47,"variant":105,"title":106,"markdown":107},"tryit-stones","try_it","The stone-line test (any coast, any day)","You need a beach, a watch and six stones. It works on a river mouth or a harbour wall too.\n\n1. Put a stone exactly where the water reaches at its furthest up the beach. Note the time.\n2. Every hour, put another stone at the new water line. Use the furthest reach of the water, not where any single wave stops.\n3. After five or six hours, look back at your line of stones.\n\nThe gaps tell a story: the sea moves fastest in the middle hours and hardly at all in the first and last hour. Stay well away from cliffs, rocks and creeks while you do this, and always with an adult.",{"id":109,"type":53,"title":110,"eyebrow":111,"navLabel":112},"ch2","High water, low water, and the height between them","Chapter 02","2 High and low",{"id":114,"type":43,"markdown":115},"high-low-range","Sailors and fishers have precise words for all this, and they are worth learning because every tide table in the world uses them.\n\n- **High tide** (or high water) is the moment when the sea stops rising and is at its highest. The water covers most of the beach.\n- **Low tide** (or low water) is the moment when it stops falling and is at its lowest. Rocks, mud and sand that were under the sea are suddenly in the open air.\n- The **tidal range** is the height difference between the two. It is just a subtraction: high water height minus low water height.\n- The water coming in is the **flood tide**; the water going out is the **ebb tide**. In between, for a few quiet minutes, the water barely moves at all: **slack water**.\n\nNotice that the range is the interesting number. Two high tides can be exactly as high as each other and still be quite different tides, because what matters for a fishing boat, a crab, or a child on a sandbank is **how far the water travels**.",{"id":117,"type":118,"tone":119,"items":120},"spec-tide-words","spec","blue",[121,125,129,133,137,141],{"label":122,"big":123,"value":124},"High water","Highest","The top of the tide. The sea pauses, then starts to fall.",{"label":126,"big":127,"value":128},"Low water","Lowest","The bottom of the tide. Widest beach, most rock pools.",{"label":130,"big":131,"value":132},"Tidal range","High − low","The height between them. In Mumbai about 4.4 m at spring tides.",{"label":134,"big":135,"value":136},"Flood tide","Coming in","Water rising, roughly 6 hours long, fastest in the middle.",{"label":138,"big":139,"value":140},"Ebb tide","Going out","Water falling. On a river it can run out faster than a person walks.",{"label":142,"big":143,"value":144},"Slack water","Still","The quiet pause at the very top and very bottom of the tide.",{"id":146,"type":147,"title":148,"problem":149,"steps":150,"help":155},"we-range","worked_example","Working out a tidal range","A tide board at Haldia, on the Hooghly, reads **5.9 m** at high water in the morning and **0.4 m** at low water that afternoon. What is the tidal range?",[151,152,153,154],"The range is simply high water minus low water.","5.9 − 0.4 = **5.5 m**.","That is taller than a two-storey house. All of that water arrives and leaves again in about six hours each way.","Check the units: both heights are measured from the same fixed zero mark on the board, so subtracting them is fair.",{"simplerExplanation":156,"anotherExample":157},"Range = how high the water got − how low it got. 5.9 − 0.4 = 5.5 m.","At Kochi the same sums give about 1.3 − 0.3 = 1.0 m: the sea there barely moves up and down compared with the Hooghly.",{"id":159,"type":160,"itemId":161,"prompt":162,"check":163,"hints":168,"feedback":171},"pr-range","practice","tides.discover-range","At Visakhapatnam a tide gauge shows high water at **1.7 m** and low water at **0.2 m**. What is the tidal range, in metres?",{"kind":164,"answer":165,"tolerance":166,"unit":167},"number",1.5,0.01,"m",[169,170],"Range = high water − low water.","1.7 − 0.2 = ?",{"correct":172,"incorrect":173},"Right: 1.7 − 0.2 = **1.5 m**. That is a gentle tide, typical of India's east coast.","Subtract the low from the high: 1.7 − 0.2 = 1.5 m.",{"id":175,"type":47,"variant":176,"title":177,"markdown":178},"aha-range-varies","aha","The same Moon, wildly different tides","Here is something strange. The Moon pulls on every sea on Earth by almost exactly the same amount. Yet the tidal range at Kochi is about **1 metre**, at Mumbai about **4.4 metres**, at Bhavnagar in Gujarat about **10 metres**, and in the Bay of Fundy in Canada about **16 metres** — sixteen times Kochi's tide.\n\nSo the size of a tide cannot be about the Moon alone. It must also be about the **shape of the coast** the water arrives at. Chapter 8 comes back to this.",{"id":180,"type":53,"title":181,"eyebrow":182,"navLabel":183},"ch3","The Moon is doing it","Chapter 03","3 The Moon's pull",{"id":185,"type":43,"markdown":186,"help":187},"moon-pulls","Gravity is not something only Earth has. **Everything** with mass pulls on everything else. You pull on the Moon; the Moon pulls on you. Earth's pull is the one you feel, because Earth is enormous and very close, which is why you stay on the ground and not stuck to the ceiling.\n\nBut the Moon's pull reaches us too. It is weak — far too weak to lift you off a chair, far too weak to pull a cricket ball out of the air. So how can it possibly heave the ocean about?\n\nTwo reasons.\n\n**First, the ocean is free to move.** Rock is stiff and stays where it is. Water flows. Give water the gentlest sideways nudge and it will slide, and if you keep nudging in the same direction for six hours the water keeps sliding and piles up.\n\n**Second, the ocean is vast.** The Moon is not tugging at one bucket; it is tugging at every bucketful in the Indian Ocean at once, all leaning the same way. Add up an unimaginable number of tiny nudges and you get a sea that climbs a beach.",{"simplerExplanation":188,"anotherExample":189},"The Moon's pull is tiny, but water can flow. A tiny push that keeps going for six hours over a whole ocean moves an enormous amount of water.","One person leaning on a stuck bus does nothing. A hundred people leaning gently for a long time move it. The Moon has the whole ocean leaning for it.",{"id":191,"type":47,"variant":70,"title":192,"markdown":193},"misc-moon-magnet","“The Moon is a magnet that lifts the water up”","The Moon is not magnetic, and magnets have nothing to do with tides — they do not pull on water at all.\n\nWhat the Moon has is **gravity**, the same force that keeps your feet on the floor. And it does not really *lift* the ocean. It mostly nudges water **sideways**, along the surface, towards the place under the Moon. The water slides along and heaps up there, a little like sweeping dust into a pile. Piling it up sideways is much easier than lifting it.",{"id":195,"type":196,"component":197,"componentVersion":5,"config":198,"objective":203,"textAlternative":204,"help":205},"lab-bulges","interactive","tide-lab",{"modes":199,"places":201,"challenges":202},[200],"bulges",[],3,"See how the Moon's pull stretches Earth's ocean into two bulges, and turn Earth inside them.","This lab shows Earth as a blue circle with a thin ocean around it, and the Moon off to one side. You can drag the Moon around Earth and spin Earth.\n\nWith the Moon switched off, the ocean is an even ring: the same depth everywhere.\n\nSwitch the Moon on and the ring stretches into a slightly squashed oval, with **two** bulges of deeper water: one on the side facing the Moon and one on the exactly opposite side. A label reads: *high tide where the water is deep, low tide where it is shallow, halfway between the bulges*.\n\nDrag the Moon to a new position and both bulges follow it, always staying in line with the Moon. Spin Earth instead and the bulges stay put while the continents turn underneath them. A marker on a coast shows the pattern: high, low, high, low, high — two of each in one turn.\n\nThree challenges ask you to place the marker at high tide, at low tide, and at the second high tide of the day.",{"hints":206},[207,208],"Look for two bulges, not one.","Try spinning Earth slowly and counting the high tides your marker passes through in one full turn.",{"id":210,"type":53,"title":211,"eyebrow":212,"navLabel":213},"ch4","Why there are two high tides, not one","Chapter 04","4 Two bulges",{"id":215,"type":43,"markdown":216,"help":217},"two-bulges","Most people, told that the Moon pulls the sea towards it, expect **one** heap of water on the Moon's side — and therefore one high tide a day. But almost every coast gets **two** high tides a day. There is a second bulge on the side of Earth facing *away* from the Moon. Why would water pile up on the far side, where the Moon's pull is weakest?\n\nThe answer is beautiful, and it is about **difference**, not strength.\n\nGravity gets weaker with distance. So the Moon does not pull equally on all of Earth:\n\n- The **near ocean** is closest, so it is pulled **hardest**.\n- The **solid Earth** in the middle is further away, so it is pulled a bit **less**.\n- The **far ocean** is furthest of all, so it is pulled **least**.\n\nNow watch what that does. The near ocean is pulled ahead of the solid Earth, so it slides towards the Moon and heaps up: bulge one. But the solid Earth is itself pulled ahead of the far ocean — so the planet moves out from under that water, and the far ocean is **left behind**: bulge two.\n\nNothing is throwing the far water outwards. It is simply the slowest one in a race where everything is being pulled the same way but by different amounts.",{"simplerExplanation":218,"anotherExample":219},"The Moon pulls the near sea most, the solid Earth a bit less, and the far sea least. So the near sea is pulled away from Earth, and Earth is pulled away from the far sea. Water heaps up at both ends.","Three friends jog towards an ice-cream cart, the fastest one in front. They were side by side at the start; a minute later they are strung out in a line. Nobody pushed them apart — they just moved at different speeds.",{"id":221,"type":222,"title":223,"items":224},"steps-three-balls","steps","The three-balls race: why a pull can stretch things",[225,229,233,237,241],{"title":226,"tag":227,"text":228},"Start together","Three balls","Imagine three balls floating in space in a line pointing at the Moon, evenly spaced, released at the same moment.",{"title":230,"tag":231,"text":232},"The Moon pulls all three","Different strengths","The near ball is pulled hardest, the middle one a little less, the far one least — because gravity fades with distance.",{"title":234,"tag":235,"text":236},"They all fall Moonwards","Same direction","Every ball moves towards the Moon. Not one of them moves away from it.",{"title":238,"tag":239,"text":240},"But they spread out","The stretch","The near ball pulls ahead of the middle one; the middle one pulls ahead of the far one. The line of three gets longer.",{"title":242,"tag":243,"text":244},"Now look from the middle","Two bulges","Sit on the middle ball and it looks as though the near ball moved towards the Moon and the far ball moved away. That is exactly what the ocean does.",{"id":246,"type":47,"variant":70,"title":247,"markdown":248},"misc-flung","“The far bulge is water being flung off by spinning”","This one is printed in a lot of books, and it is wrong. Earth's daily spin does not fling the far bulge outwards.\n\nTwo clues that the spin cannot be the cause. First, if spinning flung water outwards, the fling would be largest at the equator all the time and would not care where the Moon was — but the far bulge always sits exactly opposite the Moon and follows it around. Second, the Moon itself has tides raised on it by Earth, and it is **not** spinning fast at all.\n\nThe far bulge is there because the far water is pulled **less** than the solid Earth is, so the planet is pulled out from under it. The cause is the **difference** in the Moon's pull from one side of Earth to the other.",{"id":250,"type":251,"prompt":252,"options":253,"explanation":266},"predict-two-highs","prediction","Earth turns once in about 24 hours, and the two bulges stay lined up with the Moon. If you stand still on a coast for a whole day, how many high tides pass you?",[254,257,260,263],{"id":255,"label":256},"a","One, when the Moon is overhead",{"id":258,"label":259},"b","Two, about twelve hours apart",{"id":261,"label":262},"c","Four, one every six hours",{"id":264,"label":265},"d","None — the bulges turn with you","**Two.** As Earth turns, your coast is carried through the near bulge (high tide), then round to the shallow water between the bulges (low tide), then through the far bulge (a second high tide), then the other shallow patch (a second low tide), and back to the start.\n\nOne turn, two bulges, two high tides and two low tides. The bulges do not turn with you: they stay pointing at the Moon while the planet rotates underneath them.",{"id":268,"type":47,"variant":176,"title":269,"markdown":270},"aha-earth-turns","The bulges stay still; you are the one moving","This is the part that flips the picture round. The two heaps of water do not race round the planet chasing the Moon. They more or less **stay where they are**, lined up with the Moon, while **Earth turns through them**.\n\nHigh tide is not water arriving at you. It is **you arriving at the water**, carried there by the spinning planet at over 1,000 kilometres per hour.",{"id":272,"type":196,"component":273,"componentVersion":5,"config":274,"objective":298,"textAlternative":299},"lab-match-words","match-pairs",{"prompt":275,"mode":276,"pairs":277},"Match each tide word to what it means.","connect",[278,281,284,286,288,290,292,295],{"a":279,"b":280},"High tide","The sea at its highest; most of the beach covered",{"a":282,"b":283},"Low tide","The sea at its lowest; widest beach and rock pools",{"a":130,"b":285},"High water height minus low water height",{"a":134,"b":287},"The hours when the water is coming in",{"a":138,"b":289},"The hours when the water is going out",{"a":142,"b":291},"The still pause at the top or bottom of a tide",{"a":293,"b":294},"Tidal bulge","A heap of deeper water lined up with the Moon",{"a":296,"b":297},"Intertidal zone","Ground covered at high tide, bare at low tide","Match eight tide words to their plain meanings by connecting the cards.","A matching game with eight pairs. High tide goes with 'the sea at its highest, most of the beach covered'. Low tide goes with 'the sea at its lowest, widest beach and rock pools'. Tidal range goes with 'high water height minus low water height'. Flood tide goes with 'the hours when the water is coming in'. Ebb tide goes with 'the hours when the water is going out'. Slack water goes with 'the still pause at the top or bottom of a tide'. Tidal bulge goes with 'a heap of deeper water lined up with the Moon'. Intertidal zone goes with 'ground covered at high tide and bare at low tide'.",{"id":301,"type":53,"title":302,"eyebrow":303,"navLabel":304},"ch5","Why the tide is late every single day","Chapter 05","5 The 50 minutes",{"id":306,"type":43,"markdown":307,"help":308},"fifty-minutes","Ask any fisher on any coast in India and they will tell you the same thing: **the tide comes later every day**. If high tide was at half past five this morning, tomorrow it will be closer to twenty past six. About **50 minutes later**, day after day, all year.\n\nWhy? Because the Moon does not stay still while Earth turns.\n\nEarth spins once in 24 hours. But in those 24 hours the Moon has travelled a good way along its month-long orbit — roughly the width of 24 full moons across the sky. So when your coast comes back round to where the Moon *was* yesterday, the Moon is no longer there. Earth has to turn a little further to catch up, and that little extra turn takes about 50 minutes.\n\nAstronomers call this stretched-out day the **lunar day** or **tidal day**: **24 h 50 min** long, instead of the ordinary 24-hour solar day. Two high tides fit into it, so high tides are about **12 h 25 min** apart.",{"simplerExplanation":309,"anotherExample":310},"In one day the Moon moves on a bit. Your coast has to turn an extra 50 minutes to get back under it, so every tide is about 50 minutes later than yesterday's.","Try to touch a friend walking slowly round you in a circle while you spin on the spot. Each time you come round, they have moved on, so you have to turn a bit further to face them again.",{"id":312,"type":313,"items":314},"formulas-lunar-day","formulas",[315,318,321,324],{"expression":316,"caption":317},"1 lunar day = 24 h 50 min","One turn of Earth relative to the Moon: 50 minutes longer than a clock day.",{"expression":319,"caption":320},"high to high = 12 h 25 min","Half a lunar day. Two high tides fit into every lunar day.",{"expression":322,"caption":323},"high to low = 6 h 13 min","A quarter of a lunar day: the water takes just over six hours to fall.",{"expression":325,"caption":326},"tomorrow = today + 50 min","The daily slip. Over a fortnight that adds up to about 12 hours.",{"id":328,"type":147,"title":329,"problem":330,"steps":331,"help":337},"we-tomorrow","When is tomorrow's morning high tide?","High water at a fishing harbour is at **05:10** today. The tide there is the usual twice-a-day sort. At about what time will the next high tide be, and the morning high tide tomorrow?",[332,333,334,335,336],"Two high tides come one lunar day apart, and a lunar day is 24 h 50 min. So consecutive high tides are half of that apart: **12 h 25 min**.","Next high tide: 05:10 + 12 h 25 min. Add the hours first: 05:10 + 12 h = 17:10. Then the minutes: 17:10 + 25 min = **17:35**.","Tomorrow morning's high tide is one more step on: 17:35 + 12 h 25 min = 29:60, which is past midnight, so take off 24 hours: **06:00**.","Check against the 50-minute rule: 05:10 + 50 min = 06:00. ✓ The two ways agree.","So the pattern for this harbour is 05:10, 17:35, 06:00, 18:25, and so on — everything sliding later by about 50 minutes a day.",{"simplerExplanation":338,"hints":339},"Add 12 h 25 min for the next high tide, or add 50 minutes to get the same tide tomorrow.",[340,341],"12 h 25 min is half of 24 h 50 min.","If the total goes past 24:00, subtract 24 hours and it is the next day.",{"id":343,"type":160,"itemId":344,"prompt":345,"check":346,"hints":358,"feedback":361},"pr-shift","tides.discover-shift","High tide at Chennai is at 08:40 on Monday. Roughly when is the matching high tide on Tuesday?",{"kind":347,"options":348,"correct":357},"choice",[349,351,353,355],{"id":255,"label":350},"08:40 — tides keep the same time every day",{"id":258,"label":352},"About 09:30",{"id":261,"label":354},"About 07:50",{"id":264,"label":356},"About 20:40",[258],[359,360],"Tides come later each day, not earlier.","Add about 50 minutes.",{"correct":362,"incorrect":363},"Right: 08:40 + 50 min = **09:30**. The Moon has moved on, so Earth must turn a little further to bring Chennai back under it.","Tides slip about 50 minutes **later** each day, because the Moon moves along its orbit while Earth turns. 08:40 + 50 min = 09:30.",{"id":365,"type":47,"variant":366,"title":367,"markdown":368},"nuance-moonrise","nuance","The Moon is late too — and by the same amount","Watch moonrise for a week and you will see the Moon itself coming up about 50 minutes later each evening. That is not a coincidence: it is the very same effect.\n\nBoth the Moon's rising and the tide slip by the same 50 minutes a day, because both are counting the same thing — how long Earth takes to turn back round to face the Moon.",{"id":370,"type":53,"title":371,"eyebrow":372,"navLabel":373},"ch6","Spring tides and neap tides","Chapter 06","6 Spring and neap",{"id":375,"type":43,"markdown":376},"spring-neap","The Moon does most of the work, but the **Sun raises tides too**. The Sun is unimaginably more massive than the Moon, but also enormously further away, and for tides distance matters hugely. The result is that the Sun's tide is a bit less than **half** the Moon's — about 46% as strong.\n\nSo there are two tides being made at once, and what you get depends on whether they help each other or fight each other.\n\n- At **new moon** and **full moon**, the Sun, Earth and Moon are in a line. The two tides pull the same way and add up. You get the biggest tides of the fortnight: **spring tides**. Highest highs, lowest lows, widest range.\n- At the **first quarter** and **last quarter** moons, the Sun is off at right angles to the Moon. The Sun's tide partly cancels the Moon's. You get the feeblest tides of the fortnight: **neap tides**. The sea hardly bothers to move.\n\nBecause the Moon takes about a month to go through all its phases, spring tides come round roughly every **14.8 days** — twice a month, at new moon and at full moon.",{"id":378,"type":47,"variant":70,"title":379,"markdown":380},"misc-spring-season","“Spring tides happen in spring”","Nothing to do with the season. A spring tide can happen in January, in the middle of the monsoon, or on your birthday, and there are about two of them every month all year round.\n\nThe word comes from the old sense of **springing up** — water leaping up the beach, the way we say a spring of water bubbles up out of the ground. **Neap** is an old English word that meant something like *scanty* or *lacking*.",{"id":382,"type":75,"caption":383,"columns":384,"rows":389},"table-phases-tides","The fortnightly rhythm: phase, alignment and the tide you get",[385,386,387,388],"Moon phase","Where the Sun is","Tides add or cancel?","What you see",[390,395,400,404],[391,392,393,394],"New moon (amavasya)","Same side as the Moon","Add","Spring tide: very high highs, very low lows",[396,397,398,399],"First quarter (half moon)","At right angles","Partly cancel","Neap tide: small range, sleepy sea",[401,402,393,403],"Full moon (purnima)","Opposite side to the Moon","Spring tide again, just as big",[405,397,398,406],"Last quarter (half moon)","Neap tide again",{"id":408,"type":47,"variant":176,"title":409,"markdown":410},"aha-both-alignments","Why full moon gives a spring tide too","It surprises people that the Sun and Moon on **opposite** sides of Earth give big tides, just as they do when they are on the same side.\n\nRemember that each of them makes **two** bulges, one towards and one away. So whether the Sun is on the Moon's side or the far side, its pair of bulges lines up with the Moon's pair either way, and the heaps stack on top of each other. What matters for a spring tide is that the three bodies are in a **straight line** — not which end of the line the Sun is at.",{"id":412,"type":196,"component":197,"componentVersion":5,"config":413,"objective":417,"textAlternative":418},"lab-spring-neap",{"modes":414,"places":415,"challenges":416},[375],[],4,"Move the Moon round its phases and watch the tidal range grow at new and full moon and shrink at the quarters.","This lab shows Earth with its two tidal bulges, the Moon that you can move round its orbit, and the Sun fixed to one side. A bar at the bottom shows the tidal range you would get.\n\nWith the Moon at **new moon**, between Earth and the Sun, the Sun's bulges sit on top of the Moon's and the range bar is at its longest. The label reads *spring tide*.\n\nMove the Moon a quarter of the way round to **first quarter** and the Sun is now at right angles. The two sets of bulges fight, the oval becomes nearly a circle and the range bar shrinks to about a third of its spring length. The label reads *neap tide*.\n\nAt **full moon**, opposite the Sun, the bar is long again: another spring tide. At **last quarter** it is short again.\n\nFour challenges ask you to find a spring tide, find a neap tide, predict the range at a given phase, and say how many days pass between one spring tide and the next (about 14.8).",{"id":420,"type":421,"conceptId":422,"relation":423,"explanation":424},"conn-phases","connection","phases-of-the-moon","related_to","The Moon's phases and the fortnightly spring–neap rhythm are the same clock: new and full moon mean Sun, Earth and Moon in a line, and that line means big tides.",{"id":426,"type":421,"conceptId":427,"relation":428,"explanation":429},"conn-gravity","gravity","helps_understand","Tides are gravity made visible. Knowing that gravity weakens with distance is what explains the two bulges and why the Sun's tide is smaller than the Moon's.",{"id":431,"type":53,"title":432,"eyebrow":433,"navLabel":434},"ch7","Life in the strip between the tides","Chapter 07","7 Life in between",{"id":436,"type":43,"markdown":437},"intertidal","The strip of shore that the tide covers and uncovers twice a day is called the **intertidal zone**, and it is one of the toughest neighbourhoods on Earth. Twice a day it is the sea floor; twice a day it is dry land in the sun. Anything living there must survive both, and must keep time.\n\nAnd they do keep time. Barnacles shut their shells the moment the water leaves and open them again the moment it returns. Fiddler crabs come out to feed on the falling tide and retreat into their burrows before the water comes back. Mudskippers — fish that walk — spend low tide hopping about on the mud of the Sundarbans. Shorebirds time their arrival on a mudflat for the hours when it is bare and full of worms. Many of these animals keep on following the tidal rhythm even in a laboratory tank where there is no tide at all: they have a clock inside them set to **12 h 25 min**, not to 24 hours.",{"id":439,"type":440,"title":441,"prompt":442,"options":443},"explorer-shore-life","explorer","Who lives by the tide clock?","Pick a shore creature to see how the tide rules its day.",[444,457,469,481,493,505],{"id":445,"label":446,"chain":447,"badge":453,"note":456},"barnacle","Barnacle",[448,449,450,451,452],"High tide: shell opens","Legs comb the water for food","Tide falls","Shell snaps shut, water sealed inside","Waits, glued to the rock",{"text":454,"tone":455},"Needs both states","yes","Barnacles are stuck head-down to a rock for their whole adult life, so they cannot follow the water. Instead they carry a private sea with them: when the tide leaves, the shell plates close and trap a mouthful of sea water so the animal stays wet for six hours. When the tide returns, the plates open and feathery legs sweep out to comb plankton from the water. On a rocky shore you can often read the tide from the height of the barnacle band alone, because barnacles can only live between the levels that the water reaches and leaves.",{"id":458,"label":459,"chain":460,"badge":466,"note":468},"crab","Fiddler crab",[461,462,463,464,465],"Tide falls, mud is bare","Crabs emerge in thousands","Feed on the mud, wave claws","Tide turns","Back into the burrow, plug the door",{"text":467,"tone":455},"Feeds at low tide","On the mudflats of the Sundarbans, Chilika and Gujarat's creeks, fiddler crabs pour out of their burrows as soon as the water leaves, sifting the mud for scraps. The males wave one giant claw. Long before the water returns they go back down and pull a plug of mud over the entrance, trapping a bubble of air. Kept in a tank with no tide at all, they still come out and go in on the tidal rhythm — the clock is inside them.",{"id":470,"label":471,"chain":472,"badge":478,"note":480},"mangrove","Mangrove tree",[473,474,475,476,477],"High tide: roots in salt water","Special roots take air from above","Low tide: roots breathe freely","Seeds drop into the ebb","Carried off to new mud",{"text":479,"tone":455},"Lives in the tide","Mangroves are trees that have solved a problem no other tree will touch: standing in salty water twice a day. Many push up snorkel-like roots called pneumatophores that stick out of the mud and take in air when the tide is out. Their tangled roots trap mud, build new land and break the force of storm waves. The Sundarbans, shared by India and Bangladesh, is the largest mangrove forest in the world and exists entirely because of the tide.",{"id":482,"label":483,"chain":484,"badge":490,"note":492},"horseshoe","Horseshoe crab",[485,486,487,488,489],"Waits offshore","Comes ashore on a spring tide","Lays eggs high on the sand","Tide falls, eggs left safe","Next spring tide: hatchlings swim off",{"text":491,"tone":455},"Times eggs to spring tides","Horseshoe crabs are older than the dinosaurs and India has them on the Odisha coast, including the beaches near Chandipur and Balasore. They come ashore to lay eggs on the highest tides of the month, so the eggs end up buried above the reach of ordinary tides and stay safe from fish. They hatch around the next spring tide, when the water comes back high enough to carry the young out to sea. Miss the tide and the plan fails — so the whole species runs on the Moon's calendar.",{"id":494,"label":495,"chain":496,"badge":502,"note":504},"bird","Shorebird",[497,498,499,500,501],"Tide falls, mudflat appears","Flocks land and probe the mud","Feed for a few hours","Tide rises","Fly to a roost above the water line",{"text":503,"tone":455},"Feeds only at low tide","Sandpipers, plovers and godwits — some of them flying in from Siberia — crowd onto Indian mudflats at Chilika, the Gulf of Kutch and the Sundarbans. Their feeding ground only exists for a few hours each tide, so they must eat fast and know exactly when to leave. Their long bills are tools for finding worms and shellfish in mud that has just come out of the water and has not yet dried.",{"id":506,"label":507,"chain":508,"badge":514,"note":516},"weed","Seaweed",[509,510,511,512,513],"Under water: floats upright","Sunlight makes food","Tide falls: flops onto rock","Slimy coat holds water in","Tide returns: stands up again",{"text":515,"tone":455},"Survives drying out","Seaweeds have no roots that drink; they take everything they need from the water around them. At low tide they collapse into slippery heaps, which is exactly the point: piled up and coated in slime, they lose far less water than they would standing up. Some shore seaweeds can lose most of their water and recover completely when the tide comes back. That is why rocks near the low-water mark are the slipperiest place on any coast — walk there with great care.",{"id":518,"type":47,"variant":519,"title":520,"markdown":521},"example-chandipur-crab","example","Chandipur: a beach that walks away","At **Chandipur in Odisha**, the sea goes out at low tide and keeps going — the water can retreat as much as **5 kilometres** from the shore, leaving a vast plain of wet sand where an hour before there were boats.\n\nWhy there? Because the sea bed is astonishingly flat. To uncover 5 km of it, the water only has to drop about 3 metres — that is a slope of roughly 1 metre down for every 1667 metres out. A tiny fall in water level uncovers an enormous amount of ground.\n\nHorseshoe crabs, which have lived on Earth for hundreds of millions of years, breed on this coast. Visitors walk out across the sand and are warned, every single day, to walk back before the tide turns.",{"id":523,"type":53,"title":524,"eyebrow":525,"navLabel":526},"ch8","Tides around India's coasts","Chapter 08","8 India's coasts",{"id":528,"type":43,"markdown":529},"india-coasts","India has more than 7,500 kilometres of coastline, and the tide behaves completely differently along it.\n\nGo to **Kochi** in Kerala and the sea rises and falls by about a metre: barely more than the depth of a bathtub, a tide you could easily fail to notice. Go to **Mumbai** and it is about four and a half metres at spring tides. Go north to **Bhavnagar**, at the top of the **Gulf of Khambhat** in Gujarat, and the range can reach about **ten metres** — as tall as a three-storey building, the biggest tide on the Indian coast.\n\nThe reason is the **shape of the coast**. The Gulf of Khambhat is a long, narrowing, shallowing funnel. A tide rolling in from the Arabian Sea is squeezed sideways by the narrowing shores and upwards by the shallowing bottom, and the same amount of water has nowhere to go but up. Kerala's coast, by contrast, is open and straight: the tide sweeps past without being squeezed at all.\n\nThe same trick, on a grander scale, makes the **Bay of Fundy** in Canada the biggest tide in the world at about 16 metres — because the bay's shape happens to rock back and forth in almost exactly the rhythm of the tide, like a child on a swing being pushed at just the right moment.",{"id":531,"type":532,"title":533,"note":534,"scale":535,"rungs":536},"ladder-ranges","ladder","Typical spring tidal ranges, from a calm sea to the world's biggest","Approximate spring-tide figures; the exact value depends on the day and on where along the coast you stand.","linear",[537,541,544,548,551,555,559,563,567,571],{"label":538,"value":539,"display":540},"Open ocean, far from land",0.5,"about 0.5 m",{"label":542,"value":5,"display":543},"Kochi, Kerala","about 1 m",{"label":545,"value":546,"display":547},"Chennai, Tamil Nadu",1.2,"about 1.2 m",{"label":549,"value":165,"display":550},"Visakhapatnam, Andhra Pradesh","about 1.5 m",{"label":552,"value":553,"display":554},"Paradip, Odisha",2.3,"about 2.3 m",{"label":556,"value":557,"display":558},"Mumbai, Maharashtra",4.4,"about 4.4 m",{"label":560,"value":561,"display":562},"Haldia, Hooghly estuary",5.5,"about 5.5 m",{"label":564,"value":565,"display":566},"Kandla, Gulf of Kutch",6.5,"about 6.5 m",{"label":568,"value":569,"display":570},"Bhavnagar, Gulf of Khambhat",10,"about 10 m",{"label":572,"value":573,"display":574},"Bay of Fundy, Canada",16,"about 16 m",{"id":576,"type":196,"component":197,"componentVersion":5,"config":577,"objective":599,"textAlternative":600},"lab-tide-clock",{"modes":578,"places":580,"challenges":416},[579],"tide-clock",[581,584,587,590,593,596],{"id":582,"label":583,"rangeM":5},"kochi","Kochi (about 1 m)",{"id":585,"label":586,"rangeM":546},"chennai","Chennai (about 1.2 m)",{"id":588,"label":589,"rangeM":557},"mumbai","Mumbai (about 4.4 m)",{"id":591,"label":592,"rangeM":561},"haldia","Haldia, Hooghly (about 5.5 m)",{"id":594,"label":595,"rangeM":569},"bhavnagar","Bhavnagar (about 10 m)",{"id":597,"label":598,"rangeM":573},"fundy","Bay of Fundy (about 16 m)","Run a tide clock for six coasts and see how differently the same Moon moves the sea at each of them.","This lab is a tide clock: a round dial whose hand goes once round in a lunar day of 24 h 50 min, with a picture of a beach that fills and empties as the hand turns.\n\nPick a place from the list — Kochi, Chennai, Mumbai, Haldia on the Hooghly, Bhavnagar in the Gulf of Khambhat, or the Bay of Fundy in Canada — and the beach picture is drawn with that place's typical spring tidal range. The figures are typical values, not a tide table for any particular day.\n\nAt Kochi, with about 1 metre of range, the water line barely shifts. At Mumbai, with about 4.4 metres, boats that floated beside a jetty settle onto mud. At Bhavnagar, with about 10 metres, the water climbs a wall three storeys tall. At the Bay of Fundy, with about 16 metres, a whole harbour empties.\n\nFour challenges ask you to stop the clock at high water, at low water, at the halfway point of the flood, and to say which of two places has the larger range.",{"id":602,"type":47,"variant":366,"title":603,"markdown":604},"nuance-not-everywhere-two","Not every coast gets two equal tides","Most of India's coast gets two highs and two lows a day, and along the west coast the two are usually much the same size. On parts of the east coast the morning and evening tides can be noticeably **unequal** — one high clearly bigger than the other. A few places in the world get only **one** high tide a day.\n\nThe two-bulge picture explains the basic rhythm; the details depend on the shape of each ocean basin. You will meet the three kinds of tide — semidiurnal, diurnal and mixed — in the Investigate layer.",{"id":606,"type":53,"title":607,"eyebrow":608,"navLabel":609},"ch9","What people do with the tide","Chapter 09","9 People and tides",{"id":611,"type":43,"markdown":612},"people-tides","For anyone who works on a coast, the tide is not scenery. It is the timetable that everything else has to fit around.\n\n**Fishing.** Many small boats can only be launched and landed near high water; at low tide there is no water under them. Fish and prawns move in and out with the tide, so nets are set to catch the flood or the ebb. In the creeks of Gujarat, Maharashtra and the Sundarbans, stake nets are fixed in the channel and simply wait for the tide to bring the fish.\n\n**Ports.** A loaded cargo ship sits deep in the water. Where a harbour is shallow, such as Kolkata up the Hooghly, big ships can only come in on a rising tide and must leave on one too. Pilots plan a ship's arrival down to the quarter hour.\n\n**Salt.** India makes most of its salt from the sea, especially in Gujarat and Tamil Nadu. Salt makers let a high tide flood shallow pans, close the gates, and let the Sun do the rest for weeks until the salt is left behind.\n\n**Sailing.** Before engines, every ship left harbour on the ebb and came in on the flood, because a sailing ship cannot fight a tidal stream. Whole trading calendars for the Indian Ocean were built around tides and monsoon winds.",{"id":614,"type":222,"title":615,"items":616},"steps-fishing-day","A day timed by the tide on a creek in Gujarat",[617,621,625,629,633,636],{"title":618,"tag":619,"text":620},"Before dawn, low water","Boat aground","The boat sits on the mud. Nets are mended, ice is loaded, nothing can leave.",{"title":622,"tag":623,"text":624},"Flood tide begins","Water rising","Water creeps back up the creek. Crabs disappear into their burrows and birds leave the mud.",{"title":626,"tag":627,"text":628},"Boat floats","Two hours before high water","There is enough water under the keel. The boat leaves on the last of the flood.",{"title":630,"tag":631,"text":632},"High water, slack","The quiet pause","For a few minutes nothing flows. Stake nets are checked and emptied.",{"title":138,"tag":634,"text":635},"Water falling","The creek pours back out to sea, carrying fish and prawns into waiting nets.",{"title":637,"tag":638,"text":639},"Return before low water","The deadline","The boat must be back in before the channel empties, or it waits at sea for six hours.",{"id":641,"type":196,"component":642,"componentVersion":5,"config":643,"objective":694,"textAlternative":695},"lab-sort-tide","sort-game",{"prompt":644,"bins":645,"items":652,"seconds":693},"Does this happen at high tide or at low tide?",[646,649],{"id":647,"label":648},"high","At high tide",{"id":650,"label":651},"low","At low tide",[653,657,661,665,669,673,677,681,685,689],{"id":654,"label":655,"bin":650,"why":656},"s1","Rock pools are open to the air and full of small creatures","Rock pools are left behind by the falling sea; at high tide they are simply part of the sea.",{"id":658,"label":659,"bin":647,"why":660},"s2","A loaded cargo ship can cross a shallow harbour bar","The extra depth of high water is what lets a deep-loaded ship pass over a shallow patch.",{"id":662,"label":663,"bin":650,"why":664},"s3","Fiddler crabs pour out onto the mud to feed","The crabs feed on bare mud and retreat into their burrows before the water returns.",{"id":666,"label":667,"bin":647,"why":668},"s4","Sea water is let into a salt pan through a gate","Salt makers use the highest water to flood the pans, then shut the gate and let the Sun evaporate it.",{"id":670,"label":671,"bin":650,"why":672},"s5","The widest stretch of beach is walkable","Low water uncovers the intertidal zone: the widest beach of the day.",{"id":674,"label":675,"bin":647,"why":676},"s6","Boats moored in a creek are floating and tugging at their ropes","There is water under them at high tide; at low tide they sit on the mud.",{"id":678,"label":679,"bin":647,"why":680},"s7","A tidal bore runs up a river against the current","A bore is the leading edge of a big flood tide pushing up a river mouth.",{"id":682,"label":683,"bin":650,"why":684},"s8","Seaweed lies flopped over in slippery heaps on the rocks","Out of the water, seaweed collapses into heaps that keep it wet until the tide returns.",{"id":686,"label":687,"bin":650,"why":688},"s9","Shorebirds crowd onto a mudflat to probe for worms","The mudflat only exists as feeding ground while the water is off it.",{"id":690,"label":691,"bin":650,"why":692},"s10","People are warned not to walk out onto the sandbank","The danger of being cut off starts at low tide, when the bank looks safe and the water is far away.",0,"Sort ten coastal events into the tide that makes them happen.","A sorting game with two bins, 'at high tide' and 'at low tide', and ten cards.\n\nAt low tide: rock pools are open to the air; fiddler crabs feed on bare mud; the widest beach is walkable; seaweed lies in slippery heaps; shorebirds probe a mudflat; and people are warned about walking out onto a sandbank, because that is when it looks safe.\n\nAt high tide: a loaded cargo ship can cross a shallow harbour bar; sea water is let into a salt pan; moored boats float and tug at their ropes; and a tidal bore runs up a river.",{"id":697,"type":53,"title":698,"eyebrow":699,"navLabel":700},"ch10","The tide is a timetable — and a warning","Chapter 10","10 Staying safe",{"id":702,"type":47,"variant":703,"title":704,"markdown":705},"careful-cutoff","careful","How people get caught out by the tide","The tide is completely predictable, which is exactly why it catches people: it looks harmless while it is doing the dangerous thing.\n\n- **Walking out at low tide.** On a flat shore such as Chandipur, the water may be kilometres away. Walking back 5 km takes about **75 minutes** even at a brisk pace, and the flood tide does not wait. Go out only with an adult, only with the day's tide times, and turn back long before low water ends.\n- **Sandbanks and rock islands.** Water can fill the channel *behind* you first, so you are cut off while the sea in front still looks far away.\n- **Creeks and river mouths.** An ebb tide in a narrow creek runs faster than an adult can swim. Never swim across a tidal creek.\n- **Slippery weed.** The lowest rocks are the slipperiest, because they are only uncovered for an hour or two.\n\nIf you get caught: move towards the shore, not along the beach, and call for help early.",{"id":707,"type":43,"markdown":708},"tide-tables","Everyone who works on the coast uses a **tide table**: a printed or online list of the times and heights of high and low water for a particular port, for every day of the year. In India these predictions come from government bodies including the **Survey of India** and **INCOIS**, the Indian National Centre for Ocean Information Services in Hyderabad, and you will find a laminated copy pinned up in almost every fishing harbour office.\n\nThe astonishing part is that a tide table can be printed **years ahead** and still be right to within a few minutes. That is only possible because the tide is driven by the motions of the Moon and the Sun, which are the most predictable things we know. Measure a port's tides carefully for a few years, work out the pattern, and you can run it forwards forever.\n\nReading one is easy. Find today's date, read across for the time and height of each high and low water, and check the little Moon symbol: a full or new moon next to the date means the biggest tides of the fortnight are on the way.",{"id":710,"type":75,"caption":711,"columns":712,"rows":715},"table-tide-table","A tide table, read like a train timetable (example layout)",[713,122,714,126,714],"Date","Height",[716,722,728,732],[717,718,719,720,721],"Mon 6","05:10 and 17:35","4.3 m","11:20 and 23:45","0.5 m",[723,724,725,726,727],"Tue 7","06:00 and 18:25","4.4 m","12:10 and —","0.4 m",[729,730,725,731,727],"Wed 8","06:50 and 19:15","00:35 and 13:00",[733,734,735,736,737],"Thu 9","07:40 and 20:05","4.2 m","01:25 and 13:50","0.6 m",{"id":739,"type":160,"itemId":740,"prompt":741,"check":742,"hints":753,"feedback":756},"pr-read-table","tides.discover-read-table","Using the table above, a boat needs high water to leave harbour on Wednesday the 8th, and the crew would like a morning departure. What time should they aim for?",{"kind":347,"options":743,"correct":752},[744,746,748,750],{"id":255,"label":745},"00:35",{"id":258,"label":747},"06:50",{"id":261,"label":749},"13:00",{"id":264,"label":751},"19:15",[258],[754,755],"Look along the Wednesday row for the high water column.","Morning means before midday.",{"correct":757,"incorrect":758},"Right: **06:50** is Wednesday's morning high water. The other high water that day, 19:15, is in the evening; 00:35 and 13:00 are low waters.","The high water column for Wednesday shows 06:50 and 19:15. The morning one is 06:50. Be careful not to pick a low water time.",{"id":760,"type":761,"prompt":762},"reflect-own-coast","reflection","Suppose your family runs a small boat from a creek where the tide range is 4 metres and the creek dries out completely at low water. Write down the three questions you would need answered before setting out tomorrow morning, and say where you would find each answer.",{"id":764,"type":765,"title":766,"terms":767},"glossary-discover","glossary","Tide words worth keeping",[768,771,774,777,780,782,784,786,788,791,795,798,800,803,806],{"term":89,"meaning":769,"example":770},"The regular rise and fall of the sea, usually twice a day, caused mainly by the Moon's gravity.","“The tide is coming in” means the water is rising.",{"term":772,"meaning":773},"High tide (high water)","The moment when the sea is at its highest and stops rising.",{"term":775,"meaning":776},"Low tide (low water)","The moment when the sea is at its lowest and stops falling.",{"term":130,"meaning":778,"example":779},"The height difference between high water and low water.","High 5.9 m, low 0.4 m, so the range is 5.5 m.",{"term":134,"meaning":781},"The hours when the water is coming in and the level is rising.",{"term":138,"meaning":783},"The hours when the water is going out and the level is falling.",{"term":142,"meaning":785},"The short, quiet pause at the top and the bottom of a tide, when the water hardly flows.",{"term":293,"meaning":787},"A heap of slightly deeper water. Earth has two, both lined up with the Moon.",{"term":789,"meaning":790},"Lunar day (tidal day)","The time Earth takes to turn back round to face the Moon: 24 h 50 min, 50 minutes longer than a clock day.",{"term":792,"meaning":793,"example":794},"Spring tide","The biggest tides of the fortnight, at new moon and full moon, when the Sun and Moon pull in line.","Nothing to do with the season.",{"term":796,"meaning":797},"Neap tide","The smallest tides of the fortnight, at the quarter moons, when the Sun and Moon pull at right angles.",{"term":296,"meaning":799},"The strip of shore that is covered at high tide and bare at low tide.",{"term":801,"meaning":802},"Tide table","A list of the times and heights of high and low water for a port, printed a year or more in advance.",{"term":804,"meaning":805},"Tidal bore","A wave of the incoming tide that travels up a river against the current.",{"term":807,"meaning":808},"Estuary","The wide mouth of a river where it meets the sea and the water is part fresh, part salt, and full of tide.",{"id":810,"type":811,"title":812,"questions":813},"quiz-discover","quiz","Check yourself: the sea's daily rise and fall",[814,827,840,853,866,879,892,905,918,931],{"itemId":815,"prompt":816,"options":817,"correct":258,"why":826},"tides.discover-q-what","What is a tide?",[818,820,822,824],{"id":255,"label":819},"A large wave caused by a storm at sea",{"id":258,"label":821},"The regular rise and fall of the whole sea, usually twice a day",{"id":261,"label":823},"A current that flows in one direction all year",{"id":264,"label":825},"A wave caused by an earthquake","A tide is the rise and fall of the **water level** itself, taking about six hours each way. Storm waves, ocean currents and tsunamis are all different things.",{"itemId":828,"prompt":829,"options":830,"correct":255,"why":839},"tides.discover-q-cause","What causes the tides?",[831,833,835,837],{"id":255,"label":832},"The Moon's gravity, helped by the Sun's",{"id":258,"label":834},"The wind blowing across the ocean",{"id":261,"label":836},"Rivers emptying into the sea",{"id":264,"label":838},"The Moon's light warming the water","Gravity, not light and not wind. The Moon does most of the work because it is close; the Sun adds a tide about 46% as strong.",{"itemId":841,"prompt":842,"options":843,"correct":261,"why":852},"tides.discover-q-two-bulges","Why are there **two** tidal bulges?",[844,846,848,850],{"id":255,"label":845},"One is made by the Moon and one by the Sun",{"id":258,"label":847},"The spinning Earth flings water off the far side",{"id":261,"label":849},"The Moon pulls the near ocean more than Earth, and Earth more than the far ocean",{"id":264,"label":851},"One bulge is high tide and the other is low tide","It is all about **difference**. The near water is pulled ahead of the planet; the planet is pulled ahead of the far water. Both ends end up heaped up.",{"itemId":854,"prompt":855,"options":856,"correct":258,"why":865},"tides.discover-q-count","Most coasts get how many high tides in a day?",[857,859,861,863],{"id":255,"label":858},"One",{"id":258,"label":860},"Two",{"id":261,"label":862},"Four",{"id":264,"label":864},"It changes every day","Earth turns through both bulges in one lunar day, giving two high tides and two low tides, about 12 h 25 min apart.",{"itemId":867,"prompt":868,"options":869,"correct":261,"why":878},"tides.discover-q-late","High tide today was at 09:00. Roughly when is it tomorrow?",[870,872,874,876],{"id":255,"label":871},"09:00",{"id":258,"label":873},"About 08:10",{"id":261,"label":875},"About 09:50",{"id":264,"label":877},"About 21:00","Tides slip about 50 minutes later each day, because the Moon has moved along its orbit and Earth must turn further to catch up. 09:00 + 50 min = 09:50.",{"itemId":880,"prompt":881,"options":882,"correct":261,"why":891},"tides.discover-q-spring","Spring tides happen when…",[883,885,887,889],{"id":255,"label":884},"…it is springtime",{"id":258,"label":886},"…the Moon is at first or last quarter",{"id":261,"label":888},"…the Sun, Earth and Moon are in a line, at new and full moon",{"id":264,"label":890},"…there is a storm at sea","In line means the Sun's tide adds to the Moon's, giving the biggest range of the fortnight. The word means *springing up*, not the season.",{"itemId":893,"prompt":894,"options":895,"correct":258,"why":904},"tides.discover-q-neap","At a neap tide, what is the tidal range like?",[896,898,900,902],{"id":255,"label":897},"The biggest of the month",{"id":258,"label":899},"The smallest of the fortnight",{"id":261,"label":901},"Exactly zero",{"id":264,"label":903},"The same as always","At the quarter moons the Sun pulls at right angles to the Moon and partly cancels its tide, so the range is at its smallest — small, but never zero.",{"itemId":906,"prompt":907,"options":908,"correct":258,"why":917},"tides.discover-q-range","High water is 6.2 m and low water is 1.1 m. What is the range?",[909,911,913,915],{"id":255,"label":910},"7.3 m",{"id":258,"label":912},"5.1 m",{"id":261,"label":914},"6.2 m",{"id":264,"label":916},"1.1 m","Range = high − low = 6.2 − 1.1 = **5.1 m**. Adding them has no meaning.",{"itemId":919,"prompt":920,"options":921,"correct":258,"why":930},"tides.discover-q-place","Why is the tidal range at Bhavnagar so much bigger than at Kochi?",[922,924,926,928],{"id":255,"label":923},"The Moon is closer to Gujarat",{"id":258,"label":925},"The Gulf of Khambhat is a narrowing funnel that squeezes the tide upwards",{"id":261,"label":927},"Kerala has more rivers",{"id":264,"label":929},"Gujarat is further north","The Moon pulls almost equally on both. The difference is the **shape of the coast**: a narrowing, shallowing gulf forces the same water to pile higher.",{"itemId":932,"prompt":933,"options":934,"correct":261,"why":943},"tides.discover-q-safety","You are on a sandbank a long way from shore and the tide has turned. What is the right thing to do?",[935,937,939,941],{"id":255,"label":936},"Wait and see how fast the water rises",{"id":258,"label":938},"Walk along the sandbank looking for a shallow crossing",{"id":261,"label":940},"Start back towards the shore at once and call for help",{"id":264,"label":942},"Swim across the channel while it is still narrow","Head for the shore immediately and get help early. The channel behind you fills first and fastest, and a tidal current is far stronger than it looks.",{"id":945,"type":946,"title":947,"points":948},"cheat-sheet-discover","summary","Cheat sheet: tides in ten lines",[949,950,951,952,953,954,955,956,957,958,959],"**Tide** = the whole sea rising and falling, usually twice a day. Waves are wind; tsunamis are earthquakes; tides are the Moon.","**High water** is the top of the tide, **low water** the bottom, and the **tidal range** is the difference between them.","The Moon's pull is tiny, but water can flow, and the nudges add up over a whole ocean for six hours at a time.","There are **two bulges**: the Moon pulls the near ocean more than the solid Earth, and the solid Earth more than the far ocean. Difference, not flinging.","The bulges stay lined up with the Moon while **Earth turns through them**, so most coasts get two highs and two lows a day.","A **lunar day** is 24 h 50 min, so high tides are 12 h 25 min apart and everything is about 50 minutes later each day.","**Spring tides** (biggest) come at new and full moon; **neap tides** (smallest) at the quarter moons. Spring means *springing up*, not the season.","The Sun's tide is about 46% as strong as the Moon's, which is why alignment matters so much.","Tidal range depends enormously on **place**: about 1 m at Kochi, 4.4 m at Mumbai, 10 m at Bhavnagar, 16 m in the Bay of Fundy. Funnels and shallow gulfs make big tides.","Coastal life and coastal work run on the tide: barnacles, crabs, mangroves, fishing boats, ports, salt pans — and a **tide table** tells everyone when.","Tides are safe to enjoy and dangerous to ignore. On a flat shore the water comes back faster than you can walk.",{"id":961,"type":962,"sourceIds":963},"sources-discover","sources",[964,965,966,967,968,969],"tides-noaa-tides-tutorial","tides-nasa-moon-tides","tides-ncert-water-class7","tides-wikipedia-tide","tides-wikipedia-bay-of-fundy","tides-survey-of-india-tidal",[964,965,966,967,968,969],"needs_review",{"generatedBy":973,"notes":974},"claude-code","Draft generated locally; pending owner review.","db7eb800951359ff070a2764708df6098ddb367204f271ee66b0161c72ca7f66",{"logic:practice":977,"component:tide-lab@1":978,"component:match-pairs@1":979,"component:sort-game@1":980,"source:tides-nasa-moon-tides":981,"source:tides-ncert-water-class7":982,"source:tides-noaa-tides-tutorial":983,"source:tides-survey-of-india-tidal":984,"source:tides-wikipedia-bay-of-fundy":985,"source:tides-wikipedia-tide":986},"3d6b0fe1b15255975a32b0fcd94e8019bc959ad45cbf12e136e86149549c6878","5fef8b331bba35d6df96a31b84dd1f98200bcd0242eeed843d22914391057b6c","2a8ee4ac87460b4e1175a4bb13c96b03d577db06dde95670eb7fcfe4ad787899","b164f45a2c8ca08f26c450768ff0231e113e9fe45381eddb34dc6d0548596c38","d71a4932c5e1df6475bdbf45fedc9a0a348ce79ce9d887d3112d5b033fe39998","f50ce71e2c1996d6835461829618cdcc9eb5d21b86a0aba9d75b36e03b20c886","82683db912324c1f40c9e2f4d4cc0312c637b4af2490f13ca404549ec5e414a0","2e99068442e3818ea49964d49796e478ce8718d4e7d9cb9138f2efc27ba39702","a11570fb567503cb357092d9ba5ad5cc3f29fa50d9094d46c5a4b2435e012a92","0c80de822f7941587e525d4a3ae84aabe44b7c2a5d1851e5885d69db0da1e874",{"state":988,"reviewer":989,"selfReview":990,"reviewedAt":991,"method":992},"approved","The library owner",true,"2026-09-20T10:18:37.581Z","owner_bulk","preview-7e1cbbcc4f",1789899599036]