Activity: Tides and The Moon
I. Goals: Have you been to an ocean-side beach? If so, you
probably noticed that the level of the water slowly changes over the
course of an afternoon -- we may say "the tide is high" or "the tide
is low", "the tide is coming in" or "going out". The goal of this
activity is to help you understand how the moon influences the level
of the ocean around the world.
II. A Simple Model: In your reading, you learned how the
moon's gravity raises a "tidal bulge" in Earth's oceans. The picture
below shows a simplified view of Earth:
- the solid, rocky part of Earth is depicted by the brown
circle,
- the liquid ocean is depicted by the blue oval,
- the red bar is a "yardstick" to help you measure the depth of
the ocean,
- the stopwatch (now reading 00:00) to record the time,
- the moon is off to the right, and has caused Earth's ocean to
flow into a bulge.
We are viewing the Earth from above its north pole, so as time
passes, the Earth will rotate counter-clockwise, carrying the
yardstick along with it. The movie below shows the Earth spinning
under the tidal bulge.
Notice how the water level at the yardstick changes as the Earth
rotates. Watch the movie go through several cycles and think about
what you are seeing. Then answer the questions on your Answer
Sheet.
If you didn't get one in class,
click here to see the
Answer Sheet and print a paper
copy.
Please hand in the completed answer sheet (paper copy) at the next
class.
Here are "snapshots" of the movie, frame by frame,
which will enable you to measure the water depth more easily. The
numbers indicate the time on the stopwatch from the movie. Click on a
number to see the frame from that time. You may use all 24 frames to
make a complete graph, or as many frames as you need to see the
pattern develop clearly on the graph.
Here is the link to the NOAA
tide observations for Nantucket, Massachusetts.
If you are interested in tides at other sites around the U.S., you
can visit NOAA's Tides
Online page and select a site from the bar on the left. Try some
sites along the West Coast and Gulf Coast -- what's up with
that?!
Andrew
Layden, BGSU
Physics & Astronomy, September 2003.