Geoscience Reference
In-Depth Information
Chapter 3
Shape of the Earth
Financed by the French government as a practical measure to improve knowledge
about France, the work on the Paris Meridian became married by the Academy to
an intellectual rather than practical scientific investigation - a test of theories of
gravity. The applied “big science” project became a pure “big science” project. The
project was a test to set two teams of scientists on adventures beyond the boundaries
of France, north into the icy wastes of Lapland and south to the equator in present-
day Ecuador, its aim to measure the shape of the Earth.
Since antiquity, educated people knew that the Earth was approximately spherical;
the controversy as to whether Christopher Columbus would fall off the edge of the
world if he sailed from Spain westwards was founded in ignorance. The real doubts
centered on whether he would survive the dangers (weather, sailing hazards,
sea-monsters, etc.) and whether he would be able to achieve the objective, namely
an alternative route to the East Indies. As history tells us, he thought that he had
indeed achieved his objective when he found what he called the West Indies, which
we recognize now as part of the Americas.
The well-rehearsed arguments about the sphericity of the Earth included the fact
that the shadow of the Earth cast on the Moon during a lunar eclipse was always
circular and therefore must be the shadow of a spherical body ( Fig. 15 ); it was well
known, too, that a lookout at the top of a ship's mast would see the last glimpse of
land long after his fellow sailors on deck had lost sight of it ( Fig. 16 ).
The size of the Earth was also known to a good approximation, as well as its
shape. In the third century BCE, Eratosthenes (c. 276-c.195 BCE), the librarian of
Alexandria, determined the size of the Earth; he had heard that at Syene in Upper
Egypt (present day Aswan) the Sun was directly overhead at noon on the day of the
summer solstice. The Sun's rays reached the bottom of a deep well, and he deter-
mined the length of the shadow of a vertical post at Alexandria on the same day and
found that the angle of the Sun was 1/50 of a circle to the south of the zenith.
He believed that the distance between the two cities was 5,000 stadia, and it is sug-
gested he determined this by driving a carriage between the two cities and counting
the revolutions of the wheels. Thus, the circumference of the Earth was 50 times
this, or 250,000 stadia. The equivalent of this length of stadia is not well known but
is believed to be about 45,000 kilometers, remarkably close to the modern value of
40,000 km for the circumference of the Earth.
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