Name: Chris M.
Status: N/A
Age: 30s
Location: N/A
Country: N/A
Date: 2000-2001
Question:
In today's Chicago Tribune (6-6-01), Tom Skilling, "chief
meteorologist at WGN-TV," answers a reader's question: "Are there tides
on Lake Michigan?" He replies "...they are miniscule, ranging from about
0.5 inches to 1.5 inches." I have always thought that only the oceans
experienced tides to any degree. That is how it is explained in our
physics text by Paul Hewitt as well. Tides are caused by the differences
in the pull from the moon on the near and far sides of the earth -
essentially "stretching" the oceans and creating the tidal bulges. I
thought that for all practical purposes, since there is really no
difference in the distances between the moon and any point of Lake
Michigan at any point of time, that the lake would not exhibit any tidal
change. Who's right? My students want to know as well!! : )
Replies:
The weather man is correct. A tide is caused by the difference in the
gravitational attraction on the near and far side of the earth facing the
moon (and also the Sun, but that is a smaller effect). What matters is the
mass on the respective sides toward and away from the moon and the
"stretch-ability" of that mass. So yes Lake Michigan has a tide, but it is
small compared to the oceans because the mass of water in the lake is so
much smaller than in any ocean.
Land masses also have a VERY SMALL but detectable tidal effect. High
precision gravitometers and distance measuring instruments are able to
detect this "land tide".
Vince Calder
In response to another question this week, I found the website
http://home2.planetinternet.be/ballaux/
which gives a detailed treatment of moon/earth/sun interactions and tidal
effects.
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