OCEANS IN MOTION: Tides
Rise and Fall: Mr. Fisher took his class to the seashore to study. Part of the assignment was to find as
many different kinds of living things as possible. Billie Bright took off, determined to win the
prize for finding the most living things. He walked down the beach and around a rocky bank that
jutted out into the beach. He was finding lots of plants and animals. After a couple of hours Billie
decided to return to the starting spot and show Mr. Fisher all of the neat critters he had found.
Billie walked back along the beach. To his surprise, the rocky point was now covered with water
to a depth of about three feet. He was stuck! He couldn’t go forward because of the rocks. Billie’s
problem stems from the fact that when he walked past the rocks, the water level was at its low
point or low ebb. When Billie started to walk back, the water was rising or flooding. We call this
rise and fall of ocean water the tide.
Figure 1
1) Will Billie need to spend the rest of his day on the beach? Explain why or why not.
2) What caused the rapid movement of water onto the beach? How might Billie have predicted that the water was going to rise?
History: From ancient times, people who lived along the ocean shores have watched the rise and fall of the tides. The regular rhythm of the rising and falling was said to represent the breathing of the earth. People noticed that there were two high tides (the peak of the flooding stage) and two low tides
(the low point of the ebb) every day. By watching the tides for long periods of time and recording the heights, man was able to predict the tides. Billie would have benefited from the knowledge of these predictions.
What else did people notice about the rising and falling of the tides? From their records they soon discovered that were two especially high tides and two especially low tides every 29 days. Not only were the changes regular, they also followed the changes seen in the phases, or light and dark patterns, of the moon. People began to wonder, “Did the moon cause the tides?”
3) What is the tide?
4) People who lived along the coasts noticed several things about the tides. List 4 of their observations.
a. b. c. d.
What Causes the Tides? In the late 1600’s, Sir Isaac Newton produced a theory that would
eventually explain the tides. In what has since come to be known as the Law of Universal
Gravitation, Newton stated that everything (large or small) in the universe exerts a pull on
everything else. Two concepts from Physics are helpful in explaining tides. Newton’s Law of
Universal Graviation states that any two objects are attracted to each other by a force, gravity, that
is directly proportional to the product of their masses and inversely proportional to the square of
the distance between them. The greater the mass of an object, the greater its gravitational
attraction… the greater the distance from an object, the smaller the effect of its gravity. Newton’s
Law of Inertia states that a body at rest tends to stay at rest, and a body in motion tends to stay in
motion, in a straight line, unless acted upon by an outside force. These two phenomena – gravity
and intertia – work together to create the tides.
According to Newton’s Law of Inertia, the Moon, as it moves through space, has a tendency to continue its motion in a straight line and bypass Earth. Considering Newton’s Law of Universal Gravitation, Earth and the Moon are attracted to one another due to gravity. Since Earth has a much greater mass than the Moon, the effect of Earth’s gravity is stronger. It acts in conjunction with the
Moon’s inertia to keep the Moon in orbit around the Earth. Without the effect of Earth’s gravity, the
Moon would fly off in a straight line tangent to its orbit around the Earth. See Figure 2.
The Moon also exerts a gravitational attraction on Earth. Land masses and the ocean are both affected by the Moon’s gravity, but because water is more easily deformed, land masses are affected to a lesser degree and ocean water moves over the area of Earth facing the Moon. In this way, the Moon’s gravity causes the tidal bulge which is oriented toward the Moon. See figure 3.
As the Earth rotates about its axis, Earth like the Moon, has an inertial tendency to continue in a straight line through space. Everything on the planet, including its ocean, is also subject to the effect of inertia. While Earth’s gravitational attraction for the ocean keeps it from flying off the planet, it does move away from Earth somewhat because of its fluid nature. This is what creates the tidal bulge on the side of Earth opposite the Moon. The bulge is shown in an exaggerated way in Figure 4. In summary, the two tidal bulges on Earth result, for the most part, from two different factors.
The bulge on the side of Earth facing the Moon is due primarily to the effect of gravitational attraction of the Moon on Earth’s water. The bulge on the opposite side of Earth is due primarily to the inertial tendency of water to travel in a straight line away from Earth as the Earth/Moon system rotates.
Figure 5
Tidal bulge due Tidal bulge due to to inertia Moon’s gravity
The moon rises later each day. It takes a point on the earth about 52 additional minutes each day to catch up with and pass through the moon’s two tidal bulges. As a result, every 24 hours and 52 minutes there are two high tides and two low tides. The tides are about 52 minutes later each day.
5) What causes the two bulges seen in Figure 5?
6) Why do high tides come about 52 minutes later each day?
The Sun Also Plays a Role: Keep in mind that in the “real” world there are no simple tide bulges.
Although the sun is much farther away than the moon, its much greater mass allows it to play a role in the tides. The sun’s gravitational pull also tends to create two tidal bulges. One is under the sun and the other is on the opposite side of the world. These bulges are smaller than those caused by the moon and ordinarily do not have a significant effect. Instead, they increase or decrease the moon’s tide. Let’s look at the sun’s effect. The difference between successive high and low tides is called the tidal range. The tidal range changes as a result of the changing positions of the sun and the moon with respect to the earth. As the moon rotates around the earth about once a month, it is in line with the sun twice a month and it is at right angles at two other times during the month.
When the moon and sun are in line, the attractive forces are added together and the tide rises higher and falls lower than the average. The tidal range is at the maximum. These tides are called spring tides. The name refers to the springiness of the water and not to the time of year. Figure 2 shows the spring tides.
When the moon and the sun are at right angles with respect to the earth, the attractive forces of the sun and the moon work against each other. At this time, the tidal range is decreased. The tide rises and falls less than the average. We call these tides neap tides. Figure 3 shows neap tides. 7) If the high tide Monday is 9.7 feet and the low tide is 0.3 feet, what is the tidal range from
Monday?
8) What causes the spring tide?
9) What causes the neap tide?
“REAL” Life: From what we have seen, there should be two high tides and two low tides in every place about every place about every twenty-four hours. Yet if we look around, we find places with only one high and one low tide a day. We find other places that sometimes how two high and two low tides a day and at other times have one high and one low. We also see great variations in the height and duration of the tides. Why?
Scientists discovered that the distribution of the oceans and landmasses is irregular. This irregularity causes oceanic basins of the world to respond to the tide producing forces as individual closed basins. In spite of the fact that all oceans are connected, the oceans do not behave as a unified whole. Each ocean independently feels the effect of the moon and the sun. In each of the natural basins, the water sloshes back and forth at a speed determined by the length and depth of that basin. The Importance of Tides:
How are tides important to us? Billie Bright would have at least one answer for you. Ships and boats watch the tides to avoid running aground entering and leaving shallow harbors. Fishermen watch the tides. The incoming tides bring new food supplies and the fishing is best on an incoming tide People who gather shellfish watch for low tides. Tides are important in the lives of people. They are also important in the lives of all seashore animals and plants that live on the shoreline.
10) What causes variation we see in real life tides around the earth?
11) From your reading, what do you think the terms “tideland” and “tide pool” mean?