40 Fun Facts About Tides

Which Canadian bay, squeezed between New Brunswick and Nova Scotia, is famed for the highest tides in the world?

The Bay of Fundy narrows like a funnel, so its water level can swing by about 16 metres, while the tidal range in the open ocean is only about one metre.

Which wave, usually set off by an undersea earthquake, is wrongly called a tidal wave although tides play no part?

A tsunami is water shoved aside by a quake, landslide or eruption, so scientists dropped the old label. The Japanese name means 'harbour wave'.

The word 'tide' comes from an Old English word, also found in 'Yuletide', that originally meant what?

Time is what the Old English word 'tid' meant, and the sea sense probably grew out of the phrase for the hour of high water.

Which 11th-century king of England, in a famous legend, set his throne on the shore and ordered the tide to stop?

Canute the Great got wet feet in the tale, first written down in the 12th century, and then told his flattering courtiers that a ruler's power is empty beside God's.

The Sun outweighs the Moon about 27 million to one, so why does the Moon raise bigger tides on Earth?

It is far closer, and tide-raising force fades with the cube of distance. The Sun sits about 390 times farther away, so its pull on the oceans is roughly half the Moon's.

Which abbey-crowned island off Normandy was long reached by pilgrims on foot only when the tide was out?

Mont-Saint-Michel sits in a bay where the water level swings by roughly 14 metres, and the returning sea stranded or drowned attackers who tried to cross the sands.

A 'red tide' that kills fish and taints shellfish is not a tide at all; what actually causes it?

A bloom of algae, usually microscopic, multiplies until it stains the sea, and the toxins some kinds release can even make the air nearby hard to breathe.

Tidal squeezing heats the moon Io into the most volcanically active world known; which planet does the squeezing?

Jupiter flexes Io's interior again and again as the little moon circles it, and the friction feeds more than 400 active volcanoes.

Which Italian city endures 'acqua alta', when exceptional tides in the northern Adriatic flood its streets?

Venice floods when a high tide coincides with the sirocco, a wind that blows up the Adriatic and stops the water draining back out of the lagoon.

Which scientist was the first to explain tides as the gravitational pull of the Moon and Sun, in a 1687 book?

Isaac Newton set out the idea in the Principia, and his theory of gravitation was the first to show why most coasts see two high waters a day and not one.

Spring tides, which have nothing to do with the season, arrive around which two phases of the lunar month?

New and full moon put the Sun, Earth and Moon nearly in a row, so the two pulls add together. The name comes from the water 'springing forth', which it does twice every lunar month.

Which name is given to the weak tides, a week after each spring peak, when high and low water differ least?

A neap tide happens when the Sun and Moon pull at right angles, so the Sun partly cancels the Moon's effect and the sea rises and falls less than usual.

Compared with an average tide, what does a spring tide bring when the Sun and Moon pull together?

Higher highs, lower lows: the two pulls add up and stretch the tidal range at both ends, and the currents between high and low water run faster as well.

Which term describes the tidal pattern on most coasts, with two high waters and two low waters each lunar day?

Semidiurnal means 'half-daily', and it is the usual pattern on the East Coast of the United States, where both daily peaks reach about the same height.

Which body of water off North America has just one high tide and one low tide a day in many places?

The Gulf of Mexico shows that coasts do not all share one rhythm: the continents block the bulges of water as Earth turns, and here a single daily cycle wins out.

On a typical coast with two tides a day, about how long passes between one high tide and the next?

12 hours 25 minutes is half a lunar day. The Moon moves along its orbit while Earth spins, so Earth needs an extra 50 minutes each day to catch up with it.

Which word, also used for anything in decline, names the falling tide as the sea drains away from the shore?

Ebb is the outgoing half of the cycle and flood the incoming half, while the weak currents in the pause between the two are known as slack water.

What name is given to rocky basins of seawater left when the sea retreats, home to anemones and sea stars?

Tide pools are harsh homes: their residents must cope with constantly changing water levels, temperature, saltiness and oxygen.

So-called king tides peak when a spring tide coincides with which point in the Moon's orbit, its closest to Earth?

Perigee lines up with a spring tide several times a year, and the extra-high water can cause minor 'nuisance flooding' in low-lying areas.

Which word, from the Greek for 'to yoke together', names the lining-up of Sun, Earth and Moon behind spring tides?

Syzygy comes round about twice a month, and when the three bodies line up closely enough it also produces a solar or lunar eclipse.

Which shore habitat, home to barnacles and limpets, lies underwater at high tide and open to the air at low tide?

The intertidal zone, also called the foreshore, makes its residents survive both a soaking and a drying-out, usually twice a day.

Which salt-tolerant trees, propped on stilt roots, line tropical shores between the high and low water marks?

Mangroves cope with regular flooding by filtering out salt and by taking in air through roots that stick up above the waterlogged mud.

Which amphibious fish, able to breathe through its skin, hops about on open shores when the tide goes out?

The mudskipper stays busy out of water, feeding, courting and defending territory, and several species keep a pocket of air in the burrow for when the sea returns.

Which slender fish rides the waves onto Southern California beaches to lay eggs just after the highest tides?

The grunion's eggs stay buried in the sand for about a fortnight and hatch only when the next run of big tides sends waves to shake them loose.

Which effect explains why the Moon always shows the same face to Earth, spinning exactly once for every orbit?

Tidal locking works both ways for Pluto and its moon Charon: each keeps one face turned to the other, so Charon can be seen from only one side of Pluto.

Tidal friction is slowly pushing the Moon away from Earth; about how far does it drift each year?

3.8 centimetres a year is the figure from bouncing lasers off the Moon. The same tidal braking is lengthening Earth's day by roughly 2 milliseconds each century.

Saturn's main rings lie inside which boundary, where tidal forces would pull apart a moon bound only by gravity?

Inside the Roche limit, loose orbiting material spreads out into rings instead of clumping into a moon, which is why nearly all known planetary rings lie within one.

Which term, popularised by Stephen Hawking, describes tidal forces near a black hole stretching a body long and thin?

Spaghettification happens because gravity tugs far harder on the near end of a falling body than on the far end, the same imbalance that raises Earth's seas, taken to the extreme.

Which astronomer wrongly argued that tides are the seas sloshing as Earth moves, and dismissed the Moon's influence?

Galileo Galilei hoped the sloshing would prove that Earth moves, but his theory allowed only one high water a day, when most coasts plainly get two.

Which physicist, knighted for work on the transatlantic telegraph cable, designed the first tide-predicting machine?

Lord Kelvin's machines ran a cord over a row of pulleys, one for each rhythm in the tide, and could plot a year of predictions for a port in about four hours.

Which Roman general, unused to Atlantic tides, had his beached warships swamped on the coast of Britain in 55 BC?

Julius Caesar lost some ships outright and patched up the rest with timber from the wrecks before retreating across the Channel.

Which Chinese river has the world's largest tidal bore, a wave up to 9 metres high known as the Silver Dragon?

The Qiantang bore races upstream at up to 40 km/h, and the oldest known tide table, from 1056, was drawn up for this very river.

The pororoca, a tidal bore that sweeps far inland from the Atlantic, runs up which South American river?

The Amazon's pororoca draws surfers to an annual championship on a neighbouring river, where one record ride lasted 37 minutes, and its name may come from a Tupi phrase for 'great roar'.

Which Northumberland island, raided by Vikings in 793, is cut off twice a day when the tide covers its causeway?

Lindisfarne, also called Holy Island, displays tide tables at both ends of its causeway, which crosses sand and mudflats that vanish under the sea at high water.

Which English word for a violent whirlpool began as the name of a tidal current off Norway's Lofoten Islands?

Maelstrom comes from Dutch words for 'grinding stream', and Edgar Allan Poe's 1841 tale about the Norwegian whirlpool made it an everyday English word.

Which sailors' shortcut has the tide rising in hourly steps of 1, 2, 3, 3, 2 and 1 parts of its range?

The rule of twelfths reflects how the water moves fastest in mid-cycle: half of the whole rise or fall comes in the third and fourth hours alone.

Which name do oceanographers give a spot with almost no tidal range, around which the tide circles like a clock hand?

An amphidromic point takes its name from Greek words for 'around' and 'running', and the Coriolis effect helps set each sea's tidal wave circling one.

In which 1943 Pacific battle did an unexpectedly low tide strand US Marine landing craft on a coral reef?

At Tarawa the water over the reef stayed about 3 feet deep where planners had counted on 5, a miscalculation that contributed to heavy American casualties.

Which country opened the world's first tidal power station, a dam across the Rance estuary, in 1966?

France built it in Brittany, and at 240 megawatts it stayed the largest tidal plant on Earth for 45 years, until a South Korean station edged past it in 2011.

Which capital has been shielded since 1982 by steel gates that close against tidal surges from the North Sea?

London's Thames Barrier grew out of the serious North Sea flood of 1953, and its gates work by rotating, an idea their designer took from the taps on his gas cooker.