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Teacher's Guide for Tides Discovery Activities

Questions Responses and comments

1-day graph (Graph 1)

How many high tides are there in a day?

 

2 high tides a day

 Note: some locations in the world have only one high and one low tide a day, because of the shape of the basin.

How many low tides in a day?

 2 low tides a day

What can you say about the heights of the two high tides? About the two low tides' heights?

  One high tide is higher than the other, and one low tide is lower than the other.
Can you suggest a reason for the differences?  Perhaps the higher high tide occurs when the moon is directly overhead and the lower high tide occurs when the moon is directly opposite.  Hard to say why the two low tides differ. 
What causes two high and two low tides a day?  Two high and two lows occur because there are two tidal bulges around the earth, one pointing at the moon and one pointing away from the moon.
10-day graph  (Graph 2)

Look at the heights of each high tide day by day.
Do you see a pattern in the heights of the high tides day by day?
Describe in words the pattern you see.

The heights of the high tides increase for about 5-6 days, then decrease. 
This is true for both the higher high tide and the lower high tide.
Look at the heights of each low tide day by day.
Is the pattern the same as for the high tides?		  The heights of the low tides also follow a pattern, but the the peaks and valleys do not occur on the same days as the peaks and valleys for the high tides. 
30-day graph  (Graph 3)

Look at the heights of each high tide day by day.
Is this the same pattern you saw in the 10-day graph?

  The patterns for high and low tides repeat.  Each pattern has its own repeat period, and the peaks and valleys are not necessarily correlated (not in phase, we would say).  For example, the highest high peaks on days 6, 18, 34, and 45; but the lowest low tides  occur on different days.  Overall, this is a very complicated pattern.  We should not expect students to explain all of this - indeed, scientists have difficulty explaining the pattern at each location.  In fact, the only reliable way to predict tides is to project from historical patterns.
Find the first day when there is the highest high tide. What day is that?  day 6
Now find the second day when there is the highest high tide.
How many days are there between these two highest high tides?		  day 18 (note: don't omit day 12, on which there is almost no high tide.)
12 days in between

60-day graph (Graph 4)

Look at the heights of each high tide day by day.
Is this the same pattern you saw in the 10-day graph and the 30-day graph?

  

The patterns do appear to repeat themselves.  See if the class can find variations.
In the 60-day graph you can clearly see the pattern made by the smaller high tide of each day.
How is this pattern the same as that made by the larger high tides?
How is this pattern different from that made by the larger high tides?		  

The pattern made by the smaller high tide of each day does not coincide with the pattern for the higher high tide.  It seems to be delayed by about 4 days.
Do the same for the lower of the low tides each day.  The pattern for the lower low tide is again offset from the pattern for the higher low tide.

Additional Exercises
1. Predict the height of the higher high tide on April 10, 2000. On April 30, 2000.

  
By extending the patterns, you can predict that the higher high tide on April 10 should be about 10:00 PM, and on April 30 should be about 1:00 PM.
2. Graphs for Feb. 1, 6, and 14 See Graph 5

3. Using the information in Graph 2 (10-day graph), find the time of highest high tide each day. How does the time of high tide change day by day?

 High tide is approximately 50 minutes later each day.  This is because the moon revolves around the earth, and it takes the earth about 50 minutes to catch up with the moon each day. (See science background.)
4. Identify as many patterns as you can in the 60-day graph, and try to find the cause for each pattern. By "pattern" we mean a cycle that repeats itself with a regular interval.  Different patterns in the tides have different repeat periods, because they arise from different causes. There are separate patterns for the low tides and the high tides, as well as a pattern for spring and neap tides.  All these are very complicated, and depend on the particular sea coast being observed.  Do not try to explain every pattern, but allow students to discover them and marvel at the complexity of nature.