ASTRONOMER, GEOGRAPHER

Life
One of the greatest of the ancient Greek sages and the first great geographer of the ancient world, Eratosthenes is considered the founder of physical and mathematical geography. He studied in Athens with Ariston of Chios and Archesilaus, then moved to Alexandria, where he spent most of the rest of his life. Ptolemy III Euergetes appointed him director of the famous Library of Alexandria. It was Eratosothenes who coined the term "philologist". Archimedes, his senior by 11 years, held him in very high esteem, and dedicated two of his works to him: "On the Method of Mechanical Theorems" and "The Cattle Problem". He is cited by Stobaeus. One of the moon's craters has been named "Eratosthenes" in his honour.


Work
His principal works are:

"Proof of the sphericity of the earth": In this work Eratosthenes employed a method still used in modern astronomy, which was based on four assumptions:

A) At Syene (now Aswan, in Upper Egypt), which is in the Tropic of Cancer, the sun's rays fall vertically at noon at the summer solstice.

B) Syene and Alexandria "lie under the same meridian circle".

C) The distance between the two points (length of the arc) is estimated at about 500 stades (~ 900 km).

D) The rays of the sun are parallel when they reach the earth.

Eratosthenes accepted that the earth was a sphere, and knew that the difference in latitude between Alexandria and Syene was equal to the angle formed by the rays of the sun at its zenith at noon in Alexandria. He measured the length of a vertical rod (gnomon) and its shadow and calculated that the angle formed was 1/50th of the circumference of a circle. The length of the equator, therefore, could be calculated to be 250,000 stades (50 x 5000).

"Astronomy, or Placings among the stars": This book contains, besides, a detailed list of the stars, a very accurate calculation of the polar diameter of the earth, of the ecliptic, and of the distance between the earth and the sun.

"A new map of the world": Partially based on the map drawn by Dicaearchus of Messene.

"On geography": Treatise in three parts.

A) History of geography.

B) Mathematical and physical geography.

C) Preliminary data for the projection of the map and descriptive geography with economic and ethnographic material.

"On ancient comedy": Study.

"Chronology": Complete system of dating covering 1076 years, with every important scientific and historical event beginning with the Fall of Troy.

"The sieve": Method for finding successive odd numbers.

"The means": Instrument to help in resolving the problem of the duplication of the cube.