UNIT 2. WAVES AND BEACHES Tide Formation A combination of forces acts on the oceans to produce tides. These forces include the gravitational pull of the moon, the gravitational pull of the sun, the centrifugal force of the earth as it rotates, the relativepositions ofthe moon and the sun, and the angle ofthe orbit of the moon around the earth. Let's consider how each of these forces acts separately and then see how they work together to cause the daily variations in the tide. Gravitational Pull of the Moon Gravity is the force of attraction between two bodies. It is directly proportional ir masses and inversely proportional to the distance between them. Gravity draws the moon and the earth towardeach other. If there were no resistance to this force, the moon and the earth wouldcollide. However, as the moon and the earth revolve around a central point, a balancing centrifugal force is created thatkeeps them apart. You can see an exampleofcentrifugalforce when a large person and a child hold hands and spin around together. The centrifugal force tends to pull them apart. Water on the earth in the region directly beneath the moon is pulled by gravitational force toward the moon. On the opposite side of the earth, centrifugal force pulls water away from the earth, opposing the gravitational force ofthe moon. Water in the ocean bulges as shown in Fig. 7-4. (The bulge is greatly exaggeratedfor illustration. The actual bulge is only a few meters high in most cases.) earth omoon moon's gravitational force earth's centrifugal force Fig. 7-4. Tides are caused by gravitational and centrifugal forces acting on the ocean. 116
A person night day north pole full moon high tide"" 12 midnight -------'... sun B c D moon t.) moon 4[) low tide 6:13 am -------'... sun high tide 12:25 pm ---... sun E moon t) moon moves () ----. sun high tide 12:50 am -:... sun Fig. 7-5. Tides at various times of the day (north polar view) 117
UNIT 2. WAVES AND BEACHES Imagine yourself in a spaceship looking down at the north pole. See Fig. 7-5. Suppose you could see a person standing on the.. shore of the ocean near the equator. At 12 midnight the person would see a high tide, caused by gravitational force of the moon. See (A) in Fig. 7-5. As the earth rotates during the day, the person would gradually move into a region of low tide at about 6:12 a.m. See (B) in Fig. 7-5. At 12:25 p.m. therewouldbe anotherhigh tide, caused by centrifugal force. See (C) in Fig. 7-5. At about 6:37 p.m. there would be another low tide as in (D) in Fig. 7-5. Note also that the moon moves slightly (about 12 ) in its orbit around the earth during the 24-hr period. Thereforethe earthmustrotate slightly longer (about 50 min more) before the moon would be again directly overhead of the person. This explains why the moon "rises" about 50 min later each night. The times of the tides are also 50 min later the next day. Although a solar day is 24 hr, a lunar day is 24 hr and 50 min. QUESTIONS 9. What actually causes the tidal changes during a single day-the movement of the water, the movement of the moon, or the movement ofthe earth? Explain. 10. How much later would the high tide appearat the end ofthe secondday? The end of the third day? 11. In Fig. 7-5, a high tide occurs at 12:25 p.m., not at 12 noon. Why? Why does a low tide occur at 6:13 a.m.? At what time would the evening low tide occur? 12. Select several days from Table 7-1. Calculate the time interval between a high tide and the next low tide eachday. Are the time intervals exactly 6 hr and 12 min? Explain. Gravitational Pull of the Sun The sun exerts a second gravitational force on the earth, producing its own tide. Opposing the sun's tidal force is the centrifugal force ofthe earth, which produces a bulge ofwater on the side opposite the sun. As the earth rotates, the tide level changes due sun's gravitation from high to low and back again,justlike the moon. Butthese changes occur during a solar day, or 24 hr. Although the sun's massis much greaterthan the moon's, the sun is much farther from the earth, so its tidal force is abouthalfthatofthe moon. Interactions of the Moon and Sun As we have seen, the moon moves a little farther each day (about 12") in its orbital journeyaround the earth. Therefore the tides caused by the moon's gravity occur 50 min laterthan the tidescausedby the sun's gravity. It takes the moon about 29.5 days to complete its orbit around the earth. This period is called a lunar month. The moon and the sun cause predictable, periodic changes in tidal range during a lunar month (29.5 days). Therefore a lunar month is also called a tidal month. When the earth, moon, and sun are in a straight line, they exert maximum gravitational pull on the surface of the ocean. See Fig. 7-6. This is when the moon is a new moon orafull moon. Extra-high andextralow tides occur at this time. They are called springtidesbecausethey springorjumpup. When the sun and moon are at a right angle (90 ) to each other, the moon is in its first quarter or its third quarter. In this 118
Topic 7. Tides position the two tend to neutralize each other's tidal pull, and we get a reduced tide called a neap tide. See Fig. 7-7. There are two spring tides and two neap tides in a tidal month. In Figs. 7-6and 7-7 the ideal tide curves caused by the sun and the moon are shown as if they occur independently of each other. However, as you may remember from your study of waves, when two waves meet, they combine to form a single wave bigger than either of the two alone. This is also true of tide waves. ACTIVITY 2 Graph the combined effect of tidal changes caused by the sun and the moon during a spring tide and during a neap tide. PROCEDURE 1. Construct a tide graph showing the combined effectofthe sun and the moon when they are in line with the earth. a. Determine the tide height at 12 midnight caused by the moon and by the sun. Use (B) in Fig. 7-6. b. Add the two tide heights together. c. Plot the combined tide height in (C) in Fig. 7-6. d. Repeat steps (a) to (c) for each hour during a full day (24 hours). e. Connect the data points to produce the tidal curve for this day. 2. Construct a tide graph showing the combined effects of the sun and the moon when they are at right angles earth. Repeat the steps in Procedure 1 but use the information given in the graphs in (B) in Fig. 7-7. 3. The drawing in (A) in Fig. 7-6shows the tide at full moon. Make a similardrawing ofthe tide at new moon. What would such a tide look like? 4. Thedrawingin (A) in Fig. 7-7 shows the tide at the first quarter of the moon. Make a drawing of the tide when the moon is in its third quarter. Whatwould such a tide look like? QUESTIONS 13. Which of your graphs shows spnng tides? Which shows neap tides? 14. Approximatelyhow many days does the moon take to move from its position shown in (A) in Fig. 7-6 to its position shown in (A) in Fig. 7-7? Explain your reasomng. 15. Assume that the tide graph you drew in Procedure 1 occurred on March 1. On what date would the tides plottedin Procedure 2 occur? In Procedure 3? In Procedure 4? 16. Look at the combined tide curves in Fig. 7-6 and Fig. 7-7. At what time are the low tides? 17. Note that in Fig. 7-6 the high tides caused by the sun and the moon both occur 'it 12 midnight, but that in Fig. 7-7.ne high tide caused by the moon occurs at 6 a.m, Explain why this is so. 18. Using the tide graphs you produced, how could you predict what the tides would be on any given day of the month? Try it by making a I-month tide graph. 119
tide caused by the sun (solar tide) o"":li-n U moon's _ gravitational pull tide caused by the moon (lunar tide) sun's gravitational pull.. A. Tides caused by the sun and the moon at full moon -s: Cl '0; s: Ol "0 '';::: 12 midnight 6 a.m. 12 noon 6 p.m. B. Changes in the tide caused by the sun and the moon 12 midnight -..c: Cl 'iii s: Ol :2-12 midnight 6 a.m. 12 noon 6 p.m. 12 midnight C. Changes in the tide caused by the combined gravities of the sun and the moon Fig. 1-6. Changes in tide levels when the sun and moon are in line with the earth 120
moon moving in its orbit around the earth x \ \ I gravitational pull.. : '''''' of the sun \, I tide caused by the sun (solar tide) "\. ""''''00'' j pull '"tida caussd by tho moon (lunar tide) A. "'e, caused by the,::<..:h:.:() and the moon at first quarter -.c:.2'.c: :2- solar tide,,-/.......... 12 midnight 6 a.m. 12 noon 6 p.m. B. Changes in the tide caused by the sun and the moon 12 midnight... til c: ::> III... -e.e.2' s: :2-12 midnight 6 a.m. 12 noon 6 p.m. 12 midnight C. Changes in the tide caused by the combined gravities of the sun and the moon Fig. 7-7. Changes in tide levels when the sun and moon are at right angles earth > 121