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An illustration of how Eratosthenes actually
calculated the circumference
of the Earth. Click on the image for
larger view.
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Throughout history, the shape of the Earth has
been debated by scientists and philosophers. By
500 B.C. most scholars thought the Earth was completely
spherical. The Greek philosopher Aristotle (384-322
B.C.) is credited as the first person to try and
calculate the size of the Earth by determining
its circumference (the length around the equator)
He estimated this distance to be 400,000 stades
(a stadia is a Greek measurement equaling about 600 feet). With one
mile equal to 5,280 feet, Aristotle calculated the distance around the
Earth to be about 45,500 miles (Smith, 1988).
Around 250 B.C., another Greek philosopher,
Eratosthenes, measured the circumference of the
Earth using the following equation:
(360° ÷ θ) x (s)
In this calculation, (s) is the distance between two points that
lie north and south of each other on the surface
of the Earth. If you were to draw a line from each of these points to the center
of the Earth, the angle formed between them would be θ.
Obviously, Eratosthenes
could not go to the center of the Earth, so he got the angle measurement
using the rays of the sun. At noon on the longest
day of the year, the summer solstice, the sun shone directly into a deep well
at Syene (which is now Aswan, Egypt), casting no shadow.
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Eratosthenes
calculations were based on the
assumption that Syene lay on the
Tropic of Cancer and that Alexandria
lay directly north of Syene on
exactly the same line of longitude. Click
on the image for larger
view. |
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At the same time in
Alexandria, Egypt, he found that the sun cast a shadow equivalent to about
1/50th of a circle or 7.12°. Eratosthenes combined this measurement with
the distance between Syene and Alexandria, about 4,400
stades.
If we plug these numbers into the above equation,
we get: (360°÷ 7.12°)
which equals 50; and 50 x 4,400 equals 220,000
stades, or about 25,000 miles. The accepted measurement of the
Earth's circumference today is about 24,855 miles
(Smith, 1988). Given the simple tools and technology that Eratosthenes had
at his disposal over 2,000 years ago, his calculations were quite remarkable.
As
technology developed, scientists and surveyors began to use different techniques
to measure distance. In the 16th and 17th centuries, triangulation started
to be used widely. Triangulation is a method of determining the position
of a fixed point by measuring the angles to it
from two other fixed points that are a known distance
apart. Triangulation formed the basis for many
national surveys. By the end of the 19th century,
major triangulation networks covered the United
States, India, Great Britain, and large parts of Europe.
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The
concept of the shape of the Earth
has changed dramatically over time
as science and technology have
continued to advance. Click
on the image for larger
view. |
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At the end of the
16th century, the Royal Society in London and the L'Academie Royale des
Sciences in Paris were founded. Soon they became locked in a battle to
determine the shape of the Earth. The French argued
that the Earth was prolate, or shaped like an egg.
The English, using Sir Isaac Newton's universal theory
of gravity and the knowledge that the Earth spun
around its axis, thought that the Earth was oblate,
or flattened at the poles. To prove their idea, the
Academy in Paris staged two expeditions, one to Peru
(now Ecuador) at the equator, and the other to the
border of Sweden and Finland in the northern hemisphere.
Their objective was to measure the north-south curvature
of the Earth at each location's latitude and determine
whose concept of the Earth's shape was correct. The
Academy's efforts proved that Newton was right. The
Earth is flattened into the shape of an oblate sphere
(Smith, 1988).
During the last 100 years,
geodesy and its applications have advanced tremendously. The 20th century
brought space-based technology, making geodetic measurements extremely
precise. Today, NAVSTAR Global Positioning System satellites allow scientists
to measure changes in the Earth's surface to the centimeter.
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