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Activity #1: Graphing San Diego Tides
Ecology Chapter
Teacher Sheet
Objectives:
To create a graph of monthly tides in San Diego, and label the phases of the moon on the graph.
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Time:
This activity requires one 55-minute class period if the graph is produced on a computer, or 2 to 3 class periods if the graph is produced by hand.
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Background:
One of the most important abiotic factors affecting the Tijuana Estuary each day is the tide. This Ecology section of the guide to the Tijuana Estuary gives a very good description of the causes of tides, which can be very complicated. In general, tides are caused by the gravita- tional pull of the sun and moon on the earth, and by the centrifugal forces due to the earth's spin on its axis.
When the sun and the moon are on the same side of the earth, which happens during the new moon, their combined gravitational pull causes high tides to be higher and low tides to be lower, during any given period of time. These tides are known as spring tides (which have nothing to do with the spring season). A second, lower spring tide occurs each month when the sun and the moon are on opposite sides of the earth from each other, which occurs dur- ing the full moon. During the first quarter and third quarter phases of the moon, the gravita-
California Content Standard Investigation and Experimentation 1i
tional pull of the sun and the moon are in opposition to each other, causing lower high tides and higher low tides. These tides are called neap tides.
In general, most areas of the earth's coastlines experience two high tides and two low tides each day. These changing water levels have a dramatic effect on coastal and estuarine organisms. For instance, sedentary organisms that can't move around are exposed during low tides and submerged during high tides. These organisms must find means of adapting to these changing environmental conditions. San Diego has mixed tides, which means there are two low tides and two high tides each day, but they are uneven. One low tide is lower than the other and one high tide is higher than the other each day. This activity will give stu- dents a visual representation of the tides in San Diego during a one-month period. Students will be able to see the highest tides and lowest tides for the month, and will mark the spring tides, neap tides, and phases of the moon on their graph.
Materials:
Each student will need a copy of the Tide Chart (attached), and either a computer or graph paper. If computers are available for student use, they will need to have a software applica- tion such as Excel or any other application that can produce graphs. The activity can also be completed by hand on graph paper. These instructions are for graphs produced by hand. Each student will need three half-sheets of graph paper, cut vertically.
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Procedure:
- Students will need to pay close attention to the instructions to prepare their graph paper correctly. It may be helpful to make a graph paper transparency that you can use to aid stu- dents in setting up their graphs.
- If your students are preparing the graphs by hand and you have access to a computer, make a transparency of a computer-generated graph to show to the students. You can also include a paper copy of this graph on a test and have the students fill in the phases of the moon, and the spring tides and neap tides.
- Students frequently become confused about the blank spaces on the tide chart, and aren't sure where the next tide should be. Remind them that tides occur about every 6 hours, so they should calculate the approximate time the next tide should occur, determine if the next tide should be a high or low tide, and look in the appropriate columns for a tide that occurs close to the approximate time.
Answers to student questions: Note: You can have the students answer these questions on a separate piece of paper or on the back of their graphs.
- Using your graph, describe San Diego tides. San Diego has mixed tides. One high tide each day is higher than the other high tide, and one low tide each day is lower than the other.
Ecology Chapter
Student Sheet
Activity #1: Graphing San Diego Tides
PURPOSE:
You will be constructing a graph of San Diego tides for one month, determining and labeling the dates for the four phases of the moon, and labeling the spring tides and neap tides for the month.
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INTRODUCTION :
Read the Tides section of the guide to the Tijuana Estuary in the Ecology chapter. This will give you a very good idea about the causes of tides. In general, there are two main forces that cause tides - the gravitational pull of the sun and the moon on the earth, and the centrifu- gal force due to the earth's rotation on its axis.
When the sun and the moon are on the same side of the earth, which happens during the new moon, their combined gravitational pull causes high tides to be higher and low tides to be lower. These tides are known as spring tides (which have nothing to do with the season spring). A second, lower spring tide occurs each month when the sun and the moon on oppo- site sides of the earth from each other, which occurs during the full moon. During the first quarter and third quarter phases of the moon, the gravitational pull of the sun and the moon are in opposition to each other, causing lower high tides and higher low tides. These tides are called neap tides.
In general, most areas of the earth's coastlines experience two high tides and two low tides each day. These changing water levels have a dramatic effect on coastal and estuarine organisms. For instance, sedentary organisms that can't move around are exposed during low tides and submerged during high tides. These organisms must find means of adapting to these changing conditions.
San Diego has mixed tides, which means there are two low tides and two high tides each day, but they are uneven. One low tide is lower than the other and one high tide is higher.
- Some species in the estuary, such as barnacles, are sessile (they don't move). Barnacles cement themselves to rocks or other hard surfaces, and stay there their entire lives. Describe how barnacles would be affected by the tides in San Diego. What stresses would these ani- mals need to cope with due to high tides? Low tides?
- Other species, such as worms and some crabs, burrow into soft sediments such as sand and mud. How would these animals' lives be affected each day by the tides? Describe how predator/prey relationships would be affected by changing water levels each day.
- Some months have two full moons in them - the second full moon is called a "Blue Moon". Why do you think there are two full moons during these months?
- How long is one "lunar day" (the amount of time it takes the moon to revolve completely around the earth? Why do you think the length of a lunar day is different than the length of a solar day?
N 32° 40', W 117° 14'
- California, San Diego, Point Loma Tide Table August
- 1 F 12:37pm +4.4 11:59pm +5.2 6:16am -0.3 6:08pm +1. Date High Tide Ht. High Tide Ht. Low Tide Ht. Low Tide Ht.
- 2 Sa 1:17pm +4.6 6:51am +0.0 7:10pm +1.
- 3 Su 12:53am +4.6 2:02pm +4.8 7:29am +0.6 8:28pm +1.
- 4 M 2:03am +3.8 2:55pm +5.0 8:12am +1.1 10:01pm +1.
- 5 Tu 3:43am +3.3 3:57pm +5.3 9:07am +1.7 11:34pm +0.
- 6 W 5:44am +3.1 5:04pm +5.6 10:19am +2.
- 7 Th 7:17am +3.4 6:08pm +5.9 12:48am +0.0 11:40am +2.
- 8 F 8:18am +3.7 7:06pm +6.3 1:46am -0.5 12:51pm +2.
- 9 Sa 9:03am +4.0 7:57pm +6.6 2:35am -0.9 1:50pm +2.
- 10 Su 9:41am +4.2 8:44pm +6.7 3:18am -1.1 2:39pm +1.
- 11 M 10:16am +4.4 9:26pm +6.6 3:57am -1.1 3:24pm +1.
- 12 Tu 10:50am +4.5 10:06pm +6.4 4:33am -1.1 4:06pm +1.
- 13 W 11:21am +4.6 10:43pm +5.9 5:07am -0.8 4:47pm +1.
- 14 Th 11:52am +4.6 11:20pm +5.4 5:37am -0.3 5:28pm +1.
- 15 F 12:23pm +4.6 11:57pm +4.7 6:06am +0.0 6:11pm +1.
- 16 Sa 12:54pm +4.6 6:33am +0.6 6:59pm +1.
- 17 Su 12:38am +4.1 1:28pm +4.5 6:59am +1.2 7:59pm +1.
- 18 M 1:29am +3.4 2:08pm +4.5 7:24am +1.7 9:23pm +1.
- 19 Tu 2:55am +2.9 3:02pm +4.5 7:52am +2.3 11:10pm +1.
- 20 W 5:49am +2.7 4:14pm +4.5 8:41am +2.
- 21 Th 7:41am +3.1 5:26pm +4.7 12:33am +1.1 10:41am +2.
- 22 F 8:16am +3.4 6:24pm +5.0 1:24am +0.7 12:12pm +2.
- 23 Sa 8:40am +3.6 7:11pm +5.5 2:03am +0.2 1:07pm +2.
- 24 Su 9:03am +3.8 7:51pm +5.8 2:36am -0.1 1:50pm +2.
- 25 M 9:26am +4.1 8:29pm +6.1 3:07am -0.4 2:28pm +2.
- 26 Tu 9:51am +4.4 9:06pm +6.3 3:36am -0.6 3:05pm +1.
- 27 W 10:16am +4.6 9:44pm +6.3 4:06am -0.7 3:43pm +1.
- 28 Th 10:44am +4.8 10:23pm +6.0 4:35am -0.6 4:24pm +1.
- 29 F 11:14am +5.1 11:06pm +5.6 5:05am -0.3 5:09pm +0.
- 30 Sa 11:47am +5.3 11:54pm +4.9 5:37am +0.0 6:00pm +0.
- 31 Su 12:25pm +5.4 6:09am +0.6 6:59pm +0.
