The Moon’s Effect on Ocean Tides
By: Priscila Ferreira Da Silva, Robyn Bray, Shelby Kaczur, Bryant Mills-Sackney, Katelyn Finlay
The Moon is one of the most recognizable features in the sky. It has been mesmerized and celebrated by human beings from different cultures throughout history. The moon has awakened the curiosity and interest of entire civilizations both due to its influence on our planet and for the mysteries that could be stored on its surface. Although the connection between the Earth and the moon has not always been fully understood, science has been able to provide a scientific explanation of this mysterious connectivity.
Tides are one particular aspect of the Earth-moon connection that science has been able to describe thoroughly, and such explanation had led to a more encompassing understanding that the moon is indispensable to the existence of life on Earth. The moon acts as a stabilizer of the Earth’s rotation, as if the moon was absent, the Earth would tilt as high as 85 degrees. The severity in the tilt of the Earth would cause the sun to shift from hovering over the equator to hovering over the North and South poles. The moon helps to center the Earth while it is spinning on its axial tilt, which is the reason we have a stable climate which allows for life to flourish, waters to flow and tides to occur.1
The moon’s gravity pulls the Earth toward itself, causing sea levels rise and fall, thus creating tides. Due to its proximity to the moon, water on the near side of the Earth is pulled more strongly towards the moon. In contrast, water on the far side of the Earth experiences the weakest gravitational pull from the moon. This effect causes water to bulge on both the near and far sides of the Earth, creating high tides in that region. On the other hand, water being drawn away from the oceans leads to low tides in that area. Therefore, as the Earth spins on its axis once a day, water at lower latitudes goes through two cycles of high and low tides. Since there is always a thinner layer of water at high latitudes near the north and south poles, these areas experience continual low tide, while the north and south poles experience low tide as a result of the water being mostly covered with ice over the surface of the Earth.
Below, we will present to you, information proving the reasons why the moon is the greatest influence on Earth’s tides. In reality it is not the tides that are important but rather the tidal force from the moon. The tidal force causes the tides to occur, it also causes Earth to maintain stability about its axis on rotation. We will demonstrate historical studies that have proposed explanations on tides, explain the mechanisms and effects of tidal cycles through Newton’s law of universal gravitation, as well as provide information on the prediction of tides and indicate evidence on the significance of tides to life on Earth.
What are Tides?
Tides are waves that move through the ocean causing the increases or decreases in water volumes along the coast lines. These volume changes are responses to changing forces due to the sun and the moon and the Earth’s continual rotation. The highest part of a wave is called a crest, and when waves reach a particular height it is called high tide. Thus low tide corresponds to the lowest part of the wave called the trough. The difference between high tide and low tide is thus called the tidal range.2
Figure 2: Relative Positions of the Earth and the Moon During High and Low Tide. Link: https://en.wikibooks.org/wiki/Adventist_Youth_Honors_Answer_Book/Nature/Stars_-_Advanced_(General_Conference)
Historical Background on the Search for an Explanation of Tides
In 330 B.C, a Greek astronomer named Pytheas notice that different moons in the night sky seemed to have an effect on the oceans tides.3 While sailing from the Mediterranean Sea to the British Isle where the astronomer noticed that the tides and the movement of the ocean was somehow controlled by the moon. Pytheas wrote about his discoveries, but his originals works are now lost but are quoted by other authors of this time, such as Aristotle. Pytheas also noticed that there were two tides per lunar day, and that the amplitude of the waves depended on the phases of the moon. This was the start of tidal research. Many other Greek astronomers followed in Pytheas’ footsteps but no one was able to make sense of the tidal shifting for quite a long time… however there were various attempts to understand the tides, some good and others not so good. In the 13th century, a Persian Physician and Astronomer named Zakariya al-Qazwini wrote a book describing how the tides were formed because the sun heated the water causing the liquid to expand and the moon cooled the water down causing the liquid to shrink. Even the great Galileo (1564-1642) could not figure out the science behind tides. He believed that tides were produced because of the Earth’s rotation on its axis and the Earth’s rotation around the sun. These motions would set the ocean waters oscillating which would produce tides. The gravitational force of the moon on tides was not completely understood until Isaac Newton created the theory of gravity in the 17th century.
“I deduced that the forces which keep the planets in their orbs must be reciprocally as the squares of their distances from the centers about which they revolve, and thereby compared the force requisite to keep the moon in her orb with the force of gravity at the surface of the Earth and found them to answer pretty nearly.”
— Isaac Newton, 16664
So How do Tides Work?
It was in 1687, that Sir Isaac Newton explained to the world that tides occur because of water’s gravitational attraction to the sun and the moon. Newton’s law of universal gravitation states “the gravitational attraction that occurs between two objects is directly proportional to the product of their masses.” 5 This means the closer the objects are to one another the greater attraction these objects will incur. This is the root reason why the moon has the larger effect on tides, due to the fact that the moon is the closest object which is gravitationally attracted to the Earth and the ocean. The ocean does experiences gravitational attraction to the sun, but because the sun is farther away from the Earth its gravitational pull is less than the moon’s.
So while the Earth is spinning about on its axis, the oceans waters are kept in a state of equilibrium around the planet with the water being acted upon by Earth’s gravity and centrifugal force.6 As previously stated the moon has a strong gravitational pull which causes an alteration in the oceans and thus the tides. When the moon appears its gravitational attraction actually pulls the water towards the moon. This causes a bulge of water that gains force from Earth’s rotation about its axis, which is known as high tide. A high tide occurs on the side of the Earth directly facing the moon at any given time because of the gravitational force.7 The opposite is shown with low tides when the sun is out. Low tide occurs on the sides at 90 degree angles to the moon because the ocean is stretched over the Earth’s surface.8
Figure 3:Forces That Influence the Tides
Further Information 9
Predicting the Tides
High and low tides occur daily, generally twice a day (diurnal). However, these periods do not happen at the same time each day because the Moon takes slightly longer than a full day to line up again exactly with the same point on the Earth.10 Thus, the timing of high tides is staggered throughout the course of a month, about 24 hours and 50 minutes later than the one before it.11
Thus, the tides can’t be perfectly predicted by astronomical calculations that track the Sun and Moon. For greatest accuracy, tide prediction tables always integrate data from actual observation, often over a period of many years.
Why are Tides Important?
Tidal waves are a very important part of our ecosystem, they cause smaller fish to gather together making them easy prey for larger aquatic animals. Aswell other organisms like coral reefs depend on tides to deliver food to them. For without the consistnent tidal movements stationary aquatic plants and unable to survive. Thus the importance of Tides to our natural environment can not be overstated enough.12
Tides are also a very reliable source of renewable energy, because of the predictability of tidal waves. Tidal energy is still in its infancy as a renewable resource but scientists are working hard to harness the renewable energy caused by tidal waves in a larger commercial fashion. Below are the top four uses today for Tidal Energy.
Tidal Electricity – Like other forms of energy, the main usage of tidal energy is in the generation of electricity. Tidal energy is being used by the Rance Tidal Power Station in Brittany, France to generate 240 MW of tidal electricity. There are other plants in operation in Canada, China and Korea as well. The power generated from tidal energy is reliable as tides are generally uniform and predictable in nature.
Grain Mills –Tidal energy can be used for the mechanical crushing of grains in grain mills. The movement of turbines due to tidal energy is used in the crush grains. However, with the advents of fossil fuels, this usage of tidal energy has become quite low.
Energy Storage – By creating hydroelectric dams or tidal barrages (A tidal barrage is a dam-like structure used to capture the energy from masses of water moving in and out of a bay or river due to tidal forces) tidal energy can also be used and stored as a form of energy.
Protection – Tidal barrages can prevent damage to the coast or even cities during a high storm season by providing an easy transport method.13
The moon is thought to be one of the key reasons that Earth is a habitable planet, because it helps to center the Earth while it is spinning on its axial tilt, which is the reason we have a stable climate which allows for life to flourish, waters to flow and tides to occur. The phenomenon of tides had baffled generations of scientists, astronomers like Pytheas predicted the moon was the cause of these dramatic water movements. However, it was not until 1678 when Sir Isaac Newton explained gravitational attraction that people started to understand how important our moon really is. For it is the moon’s gravity is what causes sea levels to rise and fall.
1 N.Taylor Redd, “Earth Stabalizing Moon Maybe Unique Within Universe” 29 July 2011, http://www.space.com/12464-earth-moon-unique-solar-system-universe.html (19 October 2016).
2 USA. gov., n.d., “What Causes Tides?” http://scijinks.jpl.nasa.gov/tides/ (3 October 2016).
3 Martin Ekman, “A Concise History of the Theories of Tides, Precession-Nutation and Polar Motion,” 13 May 1993, http://www.afhalifax.ca/magazine/wp-content/sciences/vignettes/supernova/nature/MAREES/HistoireMarees.pdf (19 October 2016).
4 Matt Williams, “Who Discovered Gravity?” 28 Apr 2016, http://www.universetoday.com/53898/who-discovered-gravity/ (3 October 2016).
5 NOAA National Ocean Service Education “Tides and Water Levels,” 06 Oct 2016, http://oceanservice.noaa.gov/education/kits/tides/tides02_variations.html (2 November 2016).
6 Deborah Byrd, “Tides, and the pull of the moon and sun,” 7 Apr 2016, http://Earthsky.org/Earth/tides-and-the-pull-of-the-moon-and-sun#two-tides (2 October 2016).
7 Andrea Alfano, “How The Moon Affects Tides,” 31 Aug 2015, http://www.techtimes.com/articles/80715/20150831/moon-affects-tides.html (3 October 2016).
8 Charles Choi, “Moon Facts: Fun Information About the Earth’s Moon,” 19 Nov 2014, http://www.space.com/55-Earths-moon-formation-composition-and-orbit.html (18 October 2016).
9 Josh Clark, “How Do Tides Work?- Brain Stuff,” 17 Mar 2014, https://www.youtube.com/watch?v=5ohDG7RqQ9I (15 November 2016).
10 Moon Connection, “Apogee and Perigee of the Moon,” 2016, http://www.moonconnection.com/apogee_perigee.phtml (2 October 2016).
11 NOAA National Ocean Service, “What are Tides?” 6 Feb 2006, http://oceanservice.noaa.gov/facts/tides.html (19 October 2016).
12 NOAA National Ocean Service Education, “Tides and Water Levels” 25 March, 2008, http://oceanservice.noaa.gov/education/kits/tides/tides09_monitor.html (25 November 2016).
13 Abhishek Shah,“Uses of Tidal Energy – Tidal Electricity the Biggest One” 15 March 2011, http://www.greenworldinvestor.com/2011/03/15/uses-of-tidal-energy-tidal-electricity-the-biggest-one/( 10 November 2016).