By Alex Scott
Astronomically high tides cause huge waves at the Portland Head Light in Cape Elizabeth, Maine.
It’s easy to take the movement of the water in oceans and connected bays moving up and down the beach throughout the day for granted; after all, it happens every day. But this slow and often invisible ebbing and flowing is anything but ordinary: it is the result of three enormous celestial bodies performing a dance through space and has captivated physicists and beachgoers alike for millennia.
Hopewell Rocks in the Bay of Fundy, New Brunswick, Canada, at low tide and high tide, where the tidal range can be up to 53 feet
Tidal movements were first observed and studied by the ancient Babylonian astronomer Seleucus in the 2nd century BCE, who theorized that they were linked to the Moon and its orbit. After his initial discovery, astronomers and philosophers and monks from England to Persia to China observed a pattern between the phases of the Moon, the time of day, and the height of the waterline. Although each had their own ideas about what caused this phenomenon, it was Isaac Newton who was the first to explain tides as a product of the gravitational forces of the Moon and Sun.
With this knowledge, scientists were able to form models and predict tides around the world, some of whose methods we still use today. We now know that as the Moon travels around the Earth every 28 days, it pulls the oceans towards itself. As the Earth spins on its axis daily, the places on its surface where the Moon’s pull is greatest changes, thereby creating high and low tides.
When the sun and moon are on the same side of the earth the solar and lunar tides combine to cause higher tides on both sides of the earth.
Because both the Sun and Moon exert gravity on the tides (the Moon’s pull is about twice as strong as the Sun’s), there are times when they work together and when they work against each other throughout the lunar cycle. When the Sun, Earth, and Moon are in a line and their gravity is combined together, the difference between low tide and high tide is largest. This is called a “spring tide” (from the word “jump,” not the season). When the Sun, Earth, and Moon form a 90° angle, the range is lowest and this is referred to as a “neap tide.”
Another factor in the height of the tides is the distance from the Moon to the Earth. Three or four times a year, when the Moon is closest to the Earth (at its perigee) and in a spring tide formation, the oceans experience a perigean spring tide, also known as a King Tide, which corresponds to the highest tidal ranges in the year. These extraordinarily high tides are useful for scientists trying to predict the effects of rising sea levels due to climate change.
The Great Lakes are a massive group of inland seas that contain over 20% of the world’s total volume of freshwater. Despite their enormous size, however, the Great Lakes have virtually no tides at all. Small tides have been observed in Lake Superior but they vary only by a few centimeters. The five lakes are instead influenced by gravity in a very different way: like rivers, the water in the lakes flows downstream all the way to the Atlantic Ocean, making all five lakes one giant interconnected body of water. Rainfall fills the lakes while the outflow of water from Lake Ontario into the Atlantic Ocean drains them, 2,212 miles from the western shore of Lake Superior to the Atlantic Ocean.
Because of climate changes, the Great Lakes have experienced more extreme weather, resulting in higher rainfall and higher water levels in recent years. The lake levels only fall as quickly as the water can flow from one lake to another, until they reach the ocean. Storm surges on the lakes can be as powerful as those on the ocean, and the Great Lakes are also affected by seiches, the movement of massive amounts of water back and forth due to winds and weather. Seiches are much more turbulent and unpredictable because they are not the result of regular cycles like lunar tides.
The rainfall, surges, and seiches have caused extensive erosion along the shores of the Great Lakes, with water swallowing up beaches and endangering local wildlife that depends on calm beaches to survive.
While beachcombers benefit from the ebb and flow of water along our beaches, with waves bringing in the glass, shells, fossils, and more that we look for, the future of our beaches is anything but clear. We may be the first to experience the effects of global climate change as we watch the changing waters and waves alter beaches around the world.
This article appeared in the Beachcombing Magazine January/February 2020 issue.