(Unknown - Unknown)
Eratosthenes
Mathematician, astronomer, geographer, philosopher and Greek poet
He was born in Cyrene (Shahhat, Libya today). He was a disciple of the Greek poet Callimachus of Cyrene and the Grammarian Lysanius. About 240 BC, he was director of the library of Alexandria and friend of Archimedes. His enemies called him the 'Beta', because according to them, it was the second best in everything. Those who knew him and they appreciated, however, they called it the "alpha", because they knew that it was the first in almost everything. In any case, and recognized their knowledge of exception. Their land circumference estimates were based on the observation that made in Siena, his hometown; at noon on the summer solstice, the Sun's rays fell perpendicularly upon the Earth and therefore not projected any shadow (Siena was located very near the Tropic of Cancer). In Alexandria he noticed that on the same date and time the shadows had an angle of approximately 7 ° to the vertical. To know the distance between Siena and Alexandria, it could find through trigonometric calculations, the distance to the Sun and the circumference of the Earth. Eratosthenes is also known for his measurement of the obliquity of the ecliptic, indicating him as half of the Meridian arc between the two tropics and that valued at 11/83 full angle (corresponds to 23 ° 51'18 "). To make this measurement, Eratosthenes took into account that the day of the summer solstice, the Sun is at the point most northern of the journey apparent through the constellations, and that, at noon, falls perpendicularly on all points of the tropics and, therefore, if choosing a town proper on the tropics, objects placed vertically cast no shadow. Siene, the modern Aswan was famous for this effect. Determining the latitude of Siene could calculate the latitude of the Tropic, i.e., the obliquity of the ecliptic. He created a catalogue of 675 fixed stars. He came to stay blind. He died in Alexandria by voluntary starvation.