Eratosthenes (c. 276 BC – c. 195/194 BC) was a Greek mathematician, geographer, poet, astronomer and music theorist. He was a man of learning, becoming the head of the Library of Alexandria. Eratosthenes invented the discipline of geography as well as chronology, the basic system for keeping track of past and future events. He is best known for calculating the circumference of Earth using simple geometry and observational data.
Eratosthenes was born in Cyrene (now Shahhat), North Africa around 276 B.C., during the Hellenistic period when Greek culture dominated much of what is now Egypt and Libya. His father’s name was Lycomedes or perhaps Lycus; his mother’s name is unknown but may have been named Cleobulina or Cleobulus. Eratosthenes may have been related to another scholar from Cyrene named Aristagoras or Aristarchus who studied in Athens under Plato.
In about 240 B.C., Eratosthenes wrote a book called On the Circular Motion of Land and Water, which contained his theory that Earth was a round sphere rather than flat surface supported by columns or pillars as Aristotle believed. Eratosthenes calculated that Earth’s circumference was 250,000 stadia (about 46,000 miles) around 5 percent less than modern measurements show today! He also created an improved star chart that included all 1,027 stars visible from Alexandria at night with respect to latitude and longitude so it could be used by sailors or travelers on land or sea who needed directions at night.
