The Tides

The tides are the regular rising and falling of the sea, happening at most coastlines around the world twice a day. They are caused mainly by the gravity of the Moon pulling on Earths oceans. The Sun also helps a little. Tides have shaped the lives of sailors, fishermen and coastal animals for thousands of years. They can be tiny in some places and huge in others (over 16 metres in the Bay of Fundy in Canada). At any moment, somewhere in the world the tide is going in, and somewhere else it is going out.

  • What causes themMostly the Moons gravitySun helps too
  • Number per day2 high, 2 lowIn most places
  • Time between high tidesAbout 12 h 25 minHalf a Moon day
  • Biggest tide16 m (Bay of Fundy)Tide range, not absolute height
  • Smallest tideAround 10 cmSome Mediterranean spots
  • Spring tidesAround full + new MoonHighest highs, lowest lows

How tides work

The Moons gravity pulls on the Earth and everything on it. Because the Moon is closer to the side of Earth facing it than to the far side, gravity pulls a tiny bit harder on the near side.

This stretches the Earth (and especially the easily moved oceans) slightly into a long, slim shape with two bulges:

  • One bulge on the side facing the Moon (pulled toward the Moon).
  • One bulge on the side facing away from the Moon (the Earth itself has been pulled slightly toward the Moon, leaving the ocean behind).

As Earth spins on its axis once every 24 hours, the bulges stay roughly pointed at and away from the Moon. Different places on Earth pass through the bulges as the planet rotates. Each time you spin into a bulge, the sea level rises (high tide). Each time you spin out, it falls (low tide). That gives most coasts two high and two low tides every day.

Why two high tides a day?

The two bulges (near side and far side) explain the two high tides. As you spin through them, you experience two high tides spaced about 12 hours and 25 minutes apart. (The extra 25 minutes is because the Moon itself moves slightly during the day, so the bulges shift forward.)

Fact Tides are not just in the ocean. The Moon also pulls on the solid Earth, stretching it by about 30 centimetres twice a day. We do not feel it because the stretching is gentle and spread over thousands of kilometres. The Moons gravity even pulls slightly on the atmosphere, causing atmospheric tides too small to notice on most days.

The Sun also helps

The Sun is much more massive than the Moon, but it is also much further away, so its tidal pull on Earth is only about 46 per cent as strong as the Moons. Depending on whether the Sun and Moon are pulling in the same direction or pulling at right angles, the tides can be especially strong or weak.

  • Spring tides: when the Sun, Moon and Earth line up (at full Moon and new Moon, twice a month), the Sun and Moon pull together and tides are especially high and low. Despite the name, they can happen at any time of year.
  • Neap tides: when the Sun and Moon pull at right angles to each other (at first quarter and third quarter Moon), they partly cancel and tides are smaller than usual.

Why tide heights vary so much

The shape of the coast and the seabed has a huge effect on how high the tides reach.

  • In narrow bays and inlets, the rising water gets funnelled into a small area, piling up much higher. The Bay of Fundy in Canada has tides of up to 16 metres for this reason.
  • The Severn estuary in the UK has tides up to 14 metres, the second-largest in the world.
  • In shallow seas, the tides slow and pile up. The North Sea has notable tides for the same reason.
  • In landlocked seas like the Mediterranean and the Black Sea, the tides are tiny (often under 30 cm) because the water has nowhere to flow in from.
  • On most open ocean coasts, tides are around 1 to 2 metres.
Did you know? Tides cause a slow but real loss of energy from Earths spin. This is gradually slowing Earths rotation down. The day is getting longer by about 1.7 milliseconds per century. At the time of the dinosaurs, a day was about 23 hours long. In about 50 billion years (long after the Sun has died), the Earths day will be 47 of our current days long. The Moon, meanwhile, is slowly moving away from Earth at about 3.8 cm per year, also because of tidal interactions.

Why tides matter

  • Shipping: many ports can only be entered or left around high tide. Sailors plan around tide tables published years in advance.
  • Fishing: many fish bite better at certain tide stages, so fishermen also plan around them.
  • Marine life: tidal pools are some of the richest habitats on Earth, with sea anemones, crabs, mussels, starfish and small fish all adapted to being alternately wet and dry.
  • Coastal towns: high spring tides combined with storms can cause flooding. Tide barriers like the Thames Barrier protect coastal cities.
  • Energy: tidal flows can be used to generate electricity. The MeyGen project in Scotland is one of the worlds biggest tidal-stream power stations.
Try this If you live near the coast, look up a tide table for your nearest beach (UK tide tables are free on the BBC weather website). Watch the tide rise and fall over a few hours. Notice how the time of high tide moves about 50 minutes later each day, matching the Moons movement across the sky. Build a chart of tide heights for a week and see how they change with the phases of the Moon: spring tides around full and new Moon, neap tides at half Moon.
Deeper dive: the tide that saved D-Day

One of the most famous decisions in military history depended on the tides. On 6 June 1944, the Allied D-Day invasion of German-occupied France went ahead despite stormy weather, partly because the next chance for the right combination of tide and moonlight was several weeks away.

The Allied planners needed three specific tidal conditions on the Normandy beaches:

  • A low tide at first light, so engineers could see and clear the German beach obstacles before the water covered them.
  • A rising tide during the main assault, so landing craft could come in close to shore and not get stranded on the sand.
  • A full Moon on the previous night, so airborne troops could see their drop zones in the darkness.

These conditions only lined up for a few days each month. The originally planned date of 5 June had to be postponed by one day because of bad weather. The next suitable window was nearly a month later. General Eisenhower made the difficult call to go ahead on 6 June, in a brief gap in the storms.

About 156,000 troops landed on the beaches of Normandy that day, the largest seaborne invasion in history. The careful timing with the tides played a part in the success: most of the German beach defences were exposed at low tide, and landing craft could approach safely on the incoming water. Within 11 months, the Allies had pushed across Europe and won the war.

For more, see what is gravity and gravity in space.