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Star Gardens

Instructional Objectives
Background Information
Activity
Procedures
Evaluation
Extension for Lower Grades


Topic: Tides

Instructional Objectives:

Students will:

  1. Learn to use a tide table in plotting tidal curves.
  2. Create a tide calendar by plotting a month-long tidal curve on an ordinary pictorial calendar.

Background Information:

During the program, we learn that in the cold waters of the Pacific Northwest, tremendous tidal currents are largely responsible for the diversity and wealth of life in benthic (seafloor) communities. The tidal currents bear large volumes of nutrients, which feed these communities. But what are tides?

Tides, caused by the gravitational pull of the moon and sun on the surface of the ocean, are predictable changes in sea level that occur at regular intervals. Tides rise and fall roughly twice during each 24-hour period. They follow the phases of the moon, with the highest and lowest tides occurring during the full and new moons.

Because tides affect both the natural environment and human activities in so many ways, it is important to know when they will occur.

Activity: Plotting a Tidal Curve

Time Needed For Activity: One to two 45-minute periods

Target Grade Level: High School

Materials:

  • tide tables
  • calendars
  • large format graph paper
  • fine-line magic markers

Procedures:

  1. Choose a month that contains occurrences of both a full and new moon.
  2. Using magic markers, divide the graph paper into a duplicate of the above calendar page. Number the days and mark the phases of the moon on their correct dates.
  3. Use the vertical left margin of your graph calendar for Sea Level in Feet and evenly number each date box from bottom to top with the numbers 0, 3 and 6.
  4. Use the horizontal top margin for Times and evenly number each box from left to right with the hours 6 a.m., 12 noon, and 6 p.m.
  5. Refer to the tide tables, and check the level of both the low tides and the high tides in feet for each 24-hour period.
  6. In pencil, lightly mark points within the grid squares for each of the above.
  7. Next, draw evenly rising and descending curves to connect the points. When you are satisfied with the evenness of your tidal curves, trace over them with magic markers.

Evaluation:

The students will have successfully graphed a tidal curve running for an entire month, through all four phases of the moon, from First Quarter to Full to Last Quarter to New.

What is most noticeable about the behavior of the curve they have traced? How does curve behave in relation to the phases of the moon? When are the periods of greatest tidal fluctuation? When are the periods of least?

Discuss the tides in terms of what they provide for marine life. Consider the program content, particularly the subject of tidal currents and nutrient transport. Also, consider the wealth of life found at the seashore within the tidal zones themselves. Have members of your class ever visited a tide pool area? This is the richest zone of marine life, and yet it is located at the very edge of the sea, not in its depths. What does this indicate about the function of the tides in the evolution of life between the sea and the land? How might the tides have affected the migration of life onto land hundreds of millions of years ago? What are the chances that life would have migrated onto land if there were no tides?


Extension for Lower Grades:

Have the students construct a Tide Mobile. They will need three Styrofoam balls (such as those available in craft stores for making Christmas ornaments), a yardstick, string, wire coat hanger, pin, lump of clay and wire cutters. Begin by cutting the wire coat hanger's greatest length (across the base). Save the rest of the hanger. Stick the smallest Styrofoam ball on one end of the wire length, and the midsized ball on the other. These will represent the moon and the earth. Suspend the moon-earth structure by tying a string to the wire at the point of balance and tying the other end to the yardstick. Near the center of the yardstick, use a string with a pin at the end to suspend the large Styrofoam ball (representing the sun) and at the opposite end of the yardstick place the lump of clay as a counterbalance. Suspend the yardstick from the hooked end of the coat hanger. Suspend the entire mobile and make adjustments to balance the earth-moon-sun structure.

Spin the moon around the earth and then the earth around the sun to demonstrate the function of your mini-solar system. Have students change alignments so that the moon is in line with and then perpendicular to the earth-sun line. Explain how, when all three heavenly bodies are in line, the tides are most extreme. Ask them to consider why. When the moon is perpendicular, the tides are least extreme. Again, why? Illustrate alignments on the blackboard, shading the outer edges of the earth to show where the ocean bulges in relation to the phases of the moon.


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