February 25, 2019
Tides have an impact on just about everything on our planet. Animals plan migrations around the tides. Species reproduce according to the tides. Entire cultures make living around the tides' schedule. Even with the importance that they have on our planet, tides still are not fully understood.
Although tides can be predicted to a degree according to the moon, a precise prediction becomes far more complicated. The moon isn't the only force acting on the tides. The sun also has an effect despite the great distance. These forces are made even more complex by the their orbits. They revolve around each other in elliptical patterns, rarely in the same position twice. This makes tides a mysterious force to this day.
Environments, predators, and prey evolved to depend on each other so heavily that it's hard to tell which influenced the evolution of the other. Without the environment, like the tide, there would be no opportunity for the animals to exist.
Animals follow the cycles of their environment. They use their surrounding as cues. The cues can trigger small cycle which occur with every tide. For example, the mudshrimp burrow when the tide is low and come to eat and mate when the tide is hight.
It can also drive cycles on a grander scale. Like the migration of the sandpipers. These tiny birds embark on journey of thousands of miles every year while synchronizing their midpoint feeding time with the few days that the mudshrimp are at their peak abundance.
This type of interleaved relationship shows how fragile the balance in nature is. If the tides were to change, the shrimp might not come out in time. That would be catastrophic for the sandpipers. These birds are at their weakest before they get to feed on the mudshrimp. Without the mudshrimp, the migration could be catastrophic to the sandpipers.
Humanity relied on the moon as early as 600,000 BC. It was important to many aspects of culture. Even civilizations that did not live by the sea, and see the moon's direct effect on the tide, placed a great importance the effect the moon had on the world around them.
The moon was seen a symbol of fertility because of how closely it stayed with female cycle. This also made the moon great for time keeping. While the sun rise and sun set marked the start and end of the day, the moon's cycle were used to represent months.
These cycle became so important in societies that while the moon was growing, activities were conducted that had to do with growth, such as planting seeds and procreation. While the moon was shrinking, activities were conducted that had to do with the removal of things, such as healing and harvest crops.
The full moon was celebrate as a sign of wholeness, while the "dark" moon was seen as dead. Societies prayed and even went as far as offering sacrifices in an attempt to bring the moon back to life sooner.
Knowing the long cultural interleave with the moon , it's hard to believe how distant we become from the moon. The next time you are outside at night, take the time to notice where the moon is and what it's doing. Rekindle some of the connection that your ancestors felt to the moon.
Past societies viewed tied differently across the world. Some saw tides of 6 inches, like the Greeks, while others saw tides of over 20 feed, like in India. As cultures started to mix, The only thing that seemed to tie all the tides together was the moon.
The societies with small tides credited only the moon for influencing them. They thought that there were the daily cycle tied to the rising of the moon, and the spring cycles during the full and waning moon.
Other cultures knew better. With larger tides, they noticed the changes in the tide cycle throughout the year. Many of these societies had extensive knowledge of the cycle of the tide, but they did not understand why the cycles occurred. Like many early cultures, they attributed this to mythical forces.
It took Newton to suggest gravity to finally bring understanding to the forces effecting the tide. However, his discovery did not make the problem easier. The moon could no longer be the only thing acting on oceans, the sun had a major part as well. Due to the elliptical of the moon around the earth and the earth around the sun, the problem became increasingly hard to figure out.
People quickly realize that the 2 or 4 cycles they previously though existed was a gross underestimate. With careful calculation and observation, cycles were found ranging from only hours to thousands of year. In total, over 400 lunar cycles were discovered and documented.
Most tides are slow moving. Even the largest and fastest once take round 6 hours to come in. However, when the conditions are just right, there are exceptions to the rule. These exceptions are called Bores and occur on rivers with just the right shape.
Many rivers get minor waves caused by the tide at their opening towards the ocean, but there are rivers that get hit with giant waves. These rivers have a cone shape opening towards the ocean that causes the wave to accelerate down river. The most famous one is the Qiantang River Bore.
It is a massive wave that come in at over 25 feet during the spring tides. It relentlessly rips through the river for almost 40 miles. Before people understood the cycle of the tide, this bore destroyed entire villages and killed many people.
Even so, like people always do, the residents of this region adapted with the river. They developed tide table, or wave tables in this case, that predated any western tide table by almost a century. Using these tables, they could build houses on one side of the river and move to other when the tide changed direction with the seasons.
It's amazing how far and quickly necessity can drive progress. Figuring out a complex topic like the tides because of hardship is the ultimate testament to human perseverance.
Newton was against his own theory of gravity. He couldn't accept an invisible force acting on everything. However, his math kept coming back to a force being there. He could explain how this force worked. He just couldn't explain what it was, and to this day we still can't fully.
The final test of his theory was explaining the tide. He did so by coming up with a simplified model of the earth, involving only the moon, no sun, and two islands: one island at the top and one directly facing the moon.
As the moon applies it's gravitational force to the ocean closest to it, a centrifugal force, due to rotation, is pulling the ocean on the opposite side away. This gives the ocean an elliptical shape. With the moon position closest to the island on the right, it exposes more of the island at the top and less of the island facing the moon. Thus showing how the moon causes a high tide at one location and a low tide at another.
This basic model grows more complex with the tilt of the earth, the motion of the moon itself, and the addition of the sun. But a good starting point is the simple model. This ability to break down a problem to the important pieces that explain the essence without unnecessary complexity, made Newton's theories stick.
Newtons simplified explanation of the tides clearly showed how the gravitational forces act upon the tide. However, it quickly came under scrutiny because it did not explain what actually occurring on Earth.
The reality of the tides is made complicated by a number of factors. The tide is constantly dragged across bottom of the ocean by the moon and the sun. Large landmasses get in the way of a continuous tide and rebound the wave, causing interference. The force of the wave squeeze the tide through narrow straights and channels, drastically altering the shape and timing. All these circumstances combine to create radically different tidal conditions across the coasts of the world.
In some place, like much of Europe, the tide follow the Newtonian laws perfectly, occurring 2 times a day and coming in 50 minutes later each day. In other place, there is a single tide in a day or 4 tides instead of the typical two. These complexities and differences in tides across the world present a challenge for scientists even today.
The tides don't follow the moon perfectly. They can vary by plus or minus 7 days. The big deviation from expected behavior is due to friction which has many more effects.
The moon causes a bulge so big it would been an ocean over 6,000 miles deep to travel freely. But the ocean is only 3,000 miles on average, so the waves drag almost the entire time. The drag they experience in the middle of the ocean is nothing compared to when they hit the continental plates. These shallow areas around the continents cause the tide to rise up from 2 feet in the open ocean to over 60 feet in some coasts. This effect is also responsible for the large waves we see at the beach.
The friction force experienced by the tides is powerful enough to slow down the rotation of the earth. This is why we need to account for a leap second every few year. The react to the tides from the ocean floor even effect the moon. Slowly the moon is pushing itself back and lessening its effect on the earth's oceans.
The remote Alaskan communities could not survive without the tide. Over 50% of their food comes from the miles of mud flats exposed by some of the largest tides in the world. These tides are so dramatic that even the delivery of supplies to these settlements must be carefully timed with tides. A miscalculation by the barges could ground them with a hole in the deck from a shifting boulders. Even when the coast is frozen, the tides serve a rich bounty.
When the winter is cold enough to produce thick ice, the tide create caverns underneath the ice. These caverns expose a rich landscape filled with oysters and other sea creatures stranded by the ocean. The locals have learned to find and use these to get additional protein during the scarce winter months.
Natives also use these beatiful ice domes as an excape from the cols during a long trip. Because the caverns are covered by ocean the whole time except during low tide, their temperatures stays close to that of the ocean, around 40 degrees. This is a strong contrast to the negative temperatures just above. The tides creates a bountiful and peaceful place under the ice for those that know how to find it. However, care must be taken since the tides can rush in quickly if left unchecked.
People have utilized tides for energy since the Egyptians in 200 AD. They had simple systems that used the tides to irrigate fields by pumping water down a network of canals spanning large fields.
Then came the use of water wheels to grind grain. The first itteration of these wheels relied on the flow of water passing by. This was a slow process that only assisted in the grinding process, but could not take it over fully. Later on, these systems expanded to capture the high tide behind a gated damn and release it during low tide. These systems not only powered the milling completely, but could run the entire interval between a low tide and high tide: 6 to 8 hours.
Today, we are looking towards the tides to help with the energy crisis. Scientists are still looking for the best way to provide power but they have come up with a number of promising solutions. They have use large damns, similar to the systems used to grind wheat, with multiple turbines, but found them to have a terrible ecological impact. A system finding popularity today place windmills or turbines, shaped like a lawn mower blade, under the ocean to directly gather energy from the incoming tide. This solution is able to generate hundreds of megawatts under the right condition, providing a sustainable alternative to fossil fuels.
The tides offer a predictable energy source that would compliment the unpredictability of solar and wind energy. By combining all the renewable options, we could rid ourselves of our dependence of fossil fuels.
The water levels around the world are rising, and global warming isn't the only cause. Along with the rest of the tides cycle, the most dramatic cycles is the 100,000 year cycle of the ice age. During this time, the poles expand to cover much of Europe, Russia, and South Africa in mile thick ice. The amount of water trapped in the ice is enough to drop the sea level by 40 feet, exposing the continental shelves. Like every dramatic action, it has an opposite reaction.
The last major ice age was around 30,000 years ago. That puts us in the cycle of warming and melting of the icecaps. The oceans are rising and taking back the land they once gave us. On top of the natural warming, the effect of the green house gasses we produce is amplifying an hastening this natural cycle.
The rising water level are a huge problem form many small island nations. Some nations are loosing their island already. Others, like Maldivians, are planning to buy land to still have a country. The rising water levels could wipe out entire cultures.
Not even the bigger land masses are free of danger. London, Venice, and the Netherlands already faced a major flood event caused by a combination apogee tide and severe storm system. They were force to build new tide wall systems to sustain the next fifty years, but what then?
Copyright © Artem Chernyak 2020