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Visitors to the ocean will be familiar with tides: the sea comes in (flowing or filling), the sea goes out (ebbing or emptying). This came as a great surprise to the Roman general Julius Caesar, who camped his troops on an Atlantic beach and was flooded out. Caesar was used to the Mediterranean Sea, where there are no tides to speak of.

So what causes the tides? The Moon's gravity is the simple answer. The Moon pulls on the Earth and on the ocean. Because the ocean on the side of the Earth facing the Moon is closer to the Moon than the centre of the Earth is, the Moon's gravity acts more strongly on it than on the Earth, so it is pulled upwards and a 'bulge' of ocean faces the Moon. Since the Earth turns on its axis once a day, the bulge moves around the Earth, causing the level of the ocean to rise and fall, forming the tides.

Why are there two tides in a day?

Of course, things are always more complicated than they seem. In most places , the tide comes in twice a day, not once. We are all taught that the Moon revolves around the Earth, but this is not strictly true. In fact, the Earth and Moon both revolve around a common point, called the barycentre. This point is inside the Earth but not at the centre. Being 4,700 km from the centre of the Earth, it is closer to the surface than the centre.

Because the Earth revolves around the barycentre, the ocean gets thrown outwards by centrifugal force1. This outward bulge is opposite the Moon. So this explains the second tide every day. There is a big bulge where the water is pulled towards the Moon and a smaller bulge where the water is flung outwards by the Earth's revolution about the centre of the Earth/Moon system.

How often does it happen?

In the simplified picture presented above, the tide comes in twice a day, so we would expect this to happen every 12 hours. But because the Moon is going around the Earth once every month, the Moon has moved on a little when the Earth has turned one full revolution. The Earth has to turn an extra bit to catch up. As a result, the Moon is in the same position in the sky every 24 hours 50 minutes. The tide comes in 50 minutes later each day.

Spring Tides and Neap Tides

Of course, the Sun has an affect on all this too. Even though it's a long way off, it is absolutely enormous, so its gravity is very powerful. In fact, at the Earth's surface, the force of the Sun's gravity is nearly 200 times that of the Moon's gravity. But the Sun's gravity is more even: it is nearly the same at the side of the Earth facing the Sun as it is at the side facing away from the Sun. This means that the Sun does not have as big an effect on the oceans. Its effect is about 45% as big as the Moon's.

The Sun causes two tidal bulges in the same way as the Moon but they are not as big and they come every 24 hours exactly, rather than every 24 hours 50 minutes. The effects of these are added to the normal Moon tide. They change the Moon tide in a few ways:

  1. At times of full Moon and new Moon, when the Earth, Moon and Sun are all in a line, the tides are greatest. The high tide is highest and the low tide is lowest. These are called Spring Tides. The name is connected to the sort of spring where water wells up out of the ground; it is nothing to do with the season of Spring.
  2. At time of half Moon, when the Moon, Earth and Sun form a right angle, the tides are least noticeable, with high tides not very high and low tides not very low. These are called Neap Tides. (This name has nothing to do with guinea pigs).
  3. In the neap tide at 1st quarter, when the Moon appears half full in the evening, the tide arrives up to two hours earlier than predicted by the twice every 24h 50m rule.
  4. In the neap tide in the last quarter, when the Moon appears half full in the morning, the tide arrives up to two hours late.

Local Tidal Peculiarities

If the Earth consisted of a perfect sphere covered in water to a uniform depth, the tides would be regular and boring. Because the water is contained in the oceans by the continents, the actual tides are far more interesting. The water must flow into bays and seas from the ocean and then drain out later the same day. Since this involves movement of large volumes of water which can only move at a finite speed, the tides are delayed. The further from the ocean, the later the tide will be.

In some places, resonance patterns are set up between incoming and outgoing tides. This can result in places known as amphidromes where there is no variation at all in water levels, even though the sea all around is moving up and down. There is one such amphidrome off the East coast of Wexford, Ireland. Another is found in the Persian Gulf, next to Saudi Arabia.

The Mediterranean Sea, as mentioned before, does not experience any significant tides. The entrance to the Sea at Gibraltar is too narrow to allow large movements of water. Nevertheless, there is a small tide, where water flows from East Mediterranean to West and back again.

The biggest tides in the world are experience in the Bay of Fundy, Nova Scotia, Canada. Here a particular combination of bay shape and depth exaggerates the action of the tides, causing a difference between high and low tide of as much as 15 metres.

A similar effect takes place in Europe in France's Mont St Michel Bay, between Normandy and Brittany. At the end of this bay stands the island of Mont St Michel. The tide goes out as far as 9 kilometres, leaving the island high and dry. The incoming tide is reputed to travel faster than a galloping horse. This is exaggeration, but it certainly comes in faster than a person can run. At the beach resort of Le Val André on the same bay, the lifeguards patrol the beach in boats, shouting through megaphones that the tide is coming in. It is a wonderful sight to see thousands of people being forced to flee. The sleeping sunbather can find himself half a kilometre from land in 2 metres depth of water in the space of only 15 minutes.

Predicting the Tides

It is obviously of great interest to navigators to be able to predict the tides. In shallow harbours, boats can only enter and leave the harbour at high tide, so it is essential to know when that happens. This problem was studied in great detail in the 19th Century, culminating in Lord Kelvin's machine for calculating tides. This was an impressive collection of pulleys, wires and dials, which when fed the correct data could accurately predict the tide in any port. The machine can be seen in the Science Museum in London, England.

Nowadays, special computer programs perform the calculations. Tide tables are published in the sailors' almanacs. Novelty clocks which predict the tides tend to be accurate only within a couple of hours - they don't take into account the fact that the tides are early or late at neap tide.

Tidal Waves and Whirlpools

The term 'tidal wave' is often incorrectly used for the giant wave known as a tsunami. These waves are created by earthquakes under the sea. They can cause devastation in coastal regions. They are most common in the Pacific Ocean. They are in fact nothing to do with the tides.

There is in fact such a thing as a genuine tidal wave. If the incoming tide is channeled by the sides of a bay of a particular shape, a large wave can form which can travel up an estuary for surprising distances. One good example of this is in the Severn river estuary in England/Wales. Here the incoming tide causes a wave called the 'Severn Bore', which can travel up the river for up to 30 km.

In Saltstraumen, Norway, the incoming tide is so vigorous entering a wide inlet with a narrow entrance that a giant whirlpool is formed. On a good day, this can emit groaning noises. Many people think this is the origin of the Maelstrom myth, where a giant whirlpool swallows up passing ships. On many old maps the Maelstrom was marked in the sea off Norway, close to Saltstraumen.

Are humans affected by the tides?

No. It is true that the moon's gravity is stronger when we are closer to the moon, but we have no way of feeling that because our entire body is immersed in the gravity field. All that is available for us to detect is the tiny difference in gravity between our head and our toes, the so-called tidal force. This force is tiny, less than one millionth the size of the actual Gravity caused by the moon.
A bird flying overhead would exert more gravitational force. So stories of people bleeding more at the full moon are nothing to do with tides.

1This is a convenient fiction.

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