Eratosthenes geodesy cartography |

## Geodesy

Eratosthenes realized that the earth was a sphere. He knew that at noon on the summer solstice in Aswan a man looking down blocked the sun’s reflection on the water at the bottom of a deep well, whereas at the same time in Alexandria the shadow of a gnomon made an angle of one-fiftieth of a circle, which meant that the distance from Alexandria to Aswan was one-fiftieth of the earth’s circumference. Given the distance between Aswan and Alexandria, Eratosthenes calculated the circumference of the earth.

## Geography

Eratosthenes invented geography. On his map of the known world he named and located four hundred cities, he divided the world into five climate zones, and he placed parallels and meridians so that distances could be estimated easily.

## Measuring

Judging the accuracy of Eratosthenes’ calculation
is confused by uncertainty over his unit of distance.
A *stadion* is six hundred feet, but a foot
could be different lengths in different countries.
The first international standard meter,
one ten-millionth part of the quarter of a meridian,
which is the distance from the north pole to the equator,
quickly proved to be impractical.
Extending the calculation of *pi* to more and more digits
will always fall far short of infinity,
and the number of irrational numbers
is an even bigger infinity.

Geodesyis the study of the size and shape of the earth. In addition to estimating the circumference of the earth, there are reports that Eratosthenes estimated the length of the year, distances to the moon and sun, and the diameter of the sun.See also in

The book of science:Mercator projection—Gerardus MercatorTransit of Venus—Jeremiah HorrocksMetric system—John Wilkins (and the French Revolution)Planetary distances—Giovanni Domenico CassiniSurveying the earth—Jean PicardDistance to a star—Friedrich BesselReadings in wikipedia: