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[Phys-L] Measuring the Earth's Radius



Hi all-
The following will be an exercise for a local astronomy club.
The Ancient Greeks were seagoing people. Mariners are accustomed
to seeing approaching ships "climb" from the horizon - mashead first and
then coming into full view. They must have realized, therefore, over many
generations, that the world was a globe.
The obvious way to measure the radius (yes, I know about the
classical measurement using the sun that is described in the text) is to
measure the distance to a promontory, such as a lighthouse, when the top
first comes into view. Last night I tried a local variety of such a
measurement.
I live about 30 miles from Chicago. There is a suburban train
station at one edge of the village where passengers board on a raised
platform. The tracks go directly into Chicago from the Southwest. From
the platform, with my 10x50 binoculards mounted on a tripod, the top
approximately 2/3 of the Sears Tower (total height about 1500') was
clearly visible - startlingly so. Using 30 mile distance and 500' tower
height gives a nice number for the earth's radius.
One might worry about relative elevations. My village is about
125' above the elevation of Chicago, but Sears Tower is on a somewhat
elevated part of the City, so I don't think the "systematic error" is all
that great. I don't have a detailed picture of the tower, so my estimate
of the visible tower height is subject to large uncertainty.
Regards,
Jack

--
"Trust me. I have a lot of experience at this."
General Custer's unremembered message to his men,
just before leading them into the Little Big Horn Valley
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