What is tides? What are the affects of the sun and moon on tides? How do the moon and the sun cause tides?
If you live near the seashore or have ever visited it, you have surely noticed how the ocean rises and falls regularly. And if you watched carefully, you probably saw the rising and falling each take place about every 12 hours or twice a day. These regular up-and-down movements of the ocean are called tides. Scientists have discovered that tides are caused by the moon and the sun.
For years, astronomers wondered what held the moon in its orbit about the earth and what made the earth and the other planets stay in their orbits around the sun. They knew that because of a property of matter called inertia, any moving body travels in a straight line unless a force makes it move in another direction. So they realized that some unknown force must be making the moon and the planets travel in an almost circular path. But until about 300 years ago, no one knew what this force might be. Then Sir Isaac Newton, the great British scientist, discovered that the earth’s gravity holds the moon in its orbit. He also showed that ali the heavenly bodies attract each other.
From Newtons studies and from many experiments, scientists now know that matter everywhere attracts other matter. This property of matter is called gravitation, or gravitational attraction. Newton wrote down the facts that he had discovered in a very important scientific law known as the Law of Universal Gravitation. The first part of the law is stated in this way: Any two objects in the universe attract each other. This means that every body in the world has gravitational attraction for every other body.
What we call gravity is really just the gravitational attraction of the earth. Every bit of matter on the earth pulls on every other bit. The earth as a whole attracts all the materials on the earth and without the moon above the earth. In the same way, the sun, the planets, and their satellites attract each other. This gravitational attraction holds the planets in their orbits around the sun and also keeps the satellites in their orbits around the planets.
Our moon has a gravitational attraction of its own, and it pulls on the earth. It pulls both the land and the water, but the water moves more easily than the land. So the water is pulled up a little on the side of the earth nearest the moon. As the earth rotates, it carries the land toward the pulled-up water. If you are on the seashore, you see the water rise. We say that the tide is rising or coming in. When it reaches the highest level, it is high tide. The earth keeps on rotating, and soon the part that was nearest the moon has turned away from it. As a result, the water falls. We say that the tide is falling or going out. When it reaches the lowest level, it is low tide.
Strangely enough, there is another high tide on the side of the earth farthest from the moon. The moon seems to pull harder on the land on this side than it pulls on the water. So the water bulges out on the side farthest from the moon. When the earth turns halfway around, there is high tide again. This is followed by another low tide. At any point on the shore of the ocean, there are two high tides every 24 hours and 50 minutes. There are two low tides during this same time.
The suns gravitational attraction for the earth also has an effect on the tides. But this effect is less than half of the moon s because the sun is so much farther away from the earth. Twice during each revolution of the moon about the earth, the sim works with the moon to cause unusually high tides. And twice during the same revolution, it works against the moon to cause unusually low tides.
Tides are the sum of the effects caused by the gravitational attraction of the Moon and the Sun.
However, it is the Moon that matters because it is closer to Earth. There is a much greater difference in the gravitational field of the Moon between the surface near the Earth, the center of the Earth and the far surface of the Earth than for the Sun.
This means that the contribution of the Moon to the tides of the Earth is approximately double that of the Sun. The reason why the Moon has a greater effect on the tides of the Earth is not the magnitude of its gravitational force on the Earth. Earth, but because it is closer to us than the Sun. And because it is much closer, the Moon constantly attracts the waters of the Earth.
The gravitational attraction between the Earth and the Sun is more than 177 times that between Earth and the Moon, but since the Sun is 390 times farther from the Earth than the Moon, the Moon governs the movement of the tides.
There is a big difference, 6 percent, in its gravitational pull on water on the near side and the opposite side, while for the Sun the difference is much smaller (even though the gravitational force of the Sun is greater).
It is the difference from one side to the other that is important to create tides, not the absolute force of gravitational attraction. The impact of the sun on the tides is 44 percent of that of the Moon, that is, a little less than half.
When the Moon is in line with one side of the Earth, it pulls the water, causing a high tide. Because the Earth rotates on its axis, the Moon completes an orbit in our sky every 24 hours and 50 minutes. Therefore, we observe two tidal peaks, as well as two tidal valleys, approximately every 12 hours.
Since the Moon moves around the Earth, it is not always in the same place at the same time every day. Then, each day, the times for high and low tides change by 50 minutes.
When the Moon, Earth and Sun are perfectly aligned, the sum of the gravitational attraction of the Sun and the Moon causes the maximum tide or extreme tides.
When the Moon is between Earth and the Sun, we observe a New Moon. When the Moon is on the opposite side of the Earth from the Sun, a full Moon is seen. In both cases, the tides are 20 percent lower and higher, respectively, than normal tides.
The Guinness Book of World Records declared that the Burntcoat Head of Bay of Fundy, in Canada, has the highest average range of piers with 47.5 feet (14.5 meters) and an extreme range of 53.5 feet (16.3 meters).
The tides change our favorite waves and spots, but they also alter the way we navigate them. Over time, experience and observation, you will notice that the tides may be more important than the classic wave variables of height and period.
Many times, if not always, the ideal conditions for a surf break are a sensible combination of tides, waves and wind. That’s why planning your surfing session around the tides is often very stressful.
In many breaks, and with the correct combination of waves and wind, surfing is only possible at high and low tides. In other swell peaks, the conditions of average tide will guarantee the best waves.
These variables can be considered, together with the ocean floor and the coastal geography, to establish standard patterns for the days of navigation above the average.
But the bottom line is that surfers must adapt to tide times to get the best waves in a relatively short window of time.
Discover the tide “Rule of the Twelfths” in surfing. For more information, see tide schedules, charts and tables for your region.