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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.

Vince Calder



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