Tides: the sea's daily rise and fall

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.

Nobody 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.

That 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.

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.

The 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.

Here 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.

Sailors and fishers have precise words for all this, and they are worth learning because every tide table in the world uses them.

• 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.
• 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.
• The tidal range is the height difference between the two. It is just a subtraction: high water height minus low water height.
• 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.

Notice 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.

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.

But 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?

Two reasons.

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.

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.

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?

The answer is beautiful, and it is about difference, not strength.

Gravity gets weaker with distance. So the Moon does not pull equally on all of Earth:

• The near ocean is closest, so it is pulled hardest.
• The solid Earth in the middle is further away, so it is pulled a bit less.
• The far ocean is furthest of all, so it is pulled least.

Now 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.

Nothing 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.

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.

Why? Because the Moon does not stay still while Earth turns.

Earth 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.

Astronomers 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.

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.

So there are two tides being made at once, and what you get depends on whether they help each other or fight each other.

• 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.
• 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.

Because 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.

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.

And 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.

India has more than 7,500 kilometres of coastline, and the tide behaves completely differently along it.

Go 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.

The 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.

The 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.

For anyone who works on a coast, the tide is not scenery. It is the timetable that everything else has to fit around.

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.

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.

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.

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.

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.

The 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.

Reading 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.