The tide falling at Costa da Caparica
The tide falling at Costa da Caparica

Why tides rise and fall and Thor’s drinking quest

Having grown up on the coast of the south of Portugal, the beach, the sun, and the sea have always been a big part of my life and of the country’s culture and identity.

Portugal boasts some of the best and most beautiful beaches in the world and its long coastline, which is bathed by the Atlantic Ocean, played a huge role in putting the country at the forefront of overseas exploration during the Age of Discovery.

This era of maritime exploration and colonisation were the beginnings of the globalisation that today allows millions of tourists to flock to Portugal’s golden beaches each year.

These beaches feature everything from immense stretches of sand as far as the eye can see, to little coves, cliffs and caves that sometimes are only accessible during low tides. But why do we have high and low tides throughout the day?

We now know that it is mainly the gravitational pull of the moon paired with the rotation of the Earth that causes tides to rise and fall around the world. However, tides throughout history have always been a mysterious phenomenon to ancient cultures. Even in semi-recent history, a 17th century German astronomer named Kepler believed that the Earth was a living breathing animal, that tides were due to its respirations, and us humans were like insects living on its back.

The gravitational pull of the Moon
The gravitational pull of the Moon

Descartes was eventually the first philosopher who advanced the theory that tides were due to the influence of the Moon and Newton then worked out the reasoning why.

Everything in the universe that has mass also has gravity, including us. The greater the mass of a planet or body, the stronger its gravitational pull. The gravitational force of the Moon pulls the oceans towards it, which causes them to bulge outwards. The side of the Earth facing the Moon experiences the Moon’s pull the strongest, and this causes the seas to rise, creating high tides.

However, a bulge of water is also created on the opposite end of the Earth. This happens because gravity is measured from the centre of mass of an object and weakens with distance. Therefore, the side of the Earth facing away from the Moon feels a weaker pull towards the Moon than the Earth’s centre. So, the centre of the Earth experiences the Moon’s pull more than the oceans, which, in turn, also creates a bulge of water or a high tide.

Since the Earth is constantly rotating, we experience two high tides and two low tides every lunar day (24 hours and 50 minutes). As the Earth rotates and our area moves closer to the Moon’s influence, water slowly rises up the shore and then slowly falls back again. As the Earth keeps rotating, we then experience another high tide when we are on the side of the Earth opposite the moon. Therefore, if you are worried about where to place your towel, it will take the sea six hours and 12.5 minutes to go from its lowest point to its highest point.

The Giant Skrymir and Thor (c. 1891) by Louis Huard

Like stated before, everything in the universe that has mass has gravity. Accordingly, the Sun also exerts a gravitational force on the Earth even though the Moon’s pull is stronger as it is much closer to us. Yet, when all three align, the Sun’s tidal force works together with the Moon’s tidal force creating the highest high tide – called a Spring Tide.

Likewise, the opposite happens when the Sun and the Moon are at a 90-degree-angle. When this happens, the tidal forces act against each other creating the lowest low tide – called Neap Tides. Both Spring Tides and Neap Tides occur twice a month.

It wasn’t just the German astronomer who came up with wild theories about the Earth. The first ships that were built in and sailed from Lagos in the Algarve kickstarted the Age of Discovery and carried the sailors and explorers into the fearful void.

Tales of sea monsters and the unknown were plentiful and although more than 80% of the ocean has still not been mapped, explored or even seen by humans, we now know today how tides work and when they occur. We can use them to our advantage whether we are sailing, fishing, surfing, or even just lazing on the beach with a cold beer waiting for the guy with the cooler full of doughnuts to pass by.

With that last note, I will leave you with one of my favourite ancient tales relating to the rise and fall of the oceans. In Norse Mythology, there is a tale where the god Thor is challenged to a drinking contest by the King of the Giants, Skrymir.

Thor enthusiastically accepts the challenge and is brought a huge horn filled to the brim with mead. The Giant King tells Thor that whoever can finish it in one drink is considered a great drinker, whoever finishes it in two is just about worthy, but no one has ever been so weak as to need to finish it in three.

Thor drank mightily but, as he felt his breath running out, the mead in the horn had barely lowered. The Giant King mocked Thor as he gave it a second attempt. He drank and drank until his breath failed him once more, and although the level had gone down, the better part of the horn still remained full. His third attempt was even more impressive than the last two, but, in the end, he found himself defeated.

The King of the Giants later on admits to Thor that the end of the horn was in fact connected to the ocean and Thor had nearly succeeded in drinking it all. He took such huge gulps that, as the legend goes, the tides are Thor’s valiant attempt to drink the ocean.

By Jay Costa Owen

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Jay works for a private charter airline, and is also a UX designer and aspiring author who enjoys learning about history and other cultures