Eratosthenes of Cyrene (276 BC-194 BC) is a famous Greek mathematician, astronomer, geographer, artist and historian. He was from the land of Cyrene, Greece which is now part of the present Libya.
He is known as the first person to measure the circumference of the earth as accurately as possible. With limited knowledge in astronomy and resources, Eratosthenes measured the circumference of the earth using the rays of the sun and the shadow it shed at Cyrene. He believed at summer solstice, the sun is directly shining at Egypt and measure the angle of elevation to be 1/50th of a circle or approximately 7 degrees. With the assumption that the Earth is spherical, he measured the distance of Alexandria to Syene and used it to measure the circumference of the earth by ratio and proportion.
He also measured that the earth is tilted in the axis at around 23.5 degrees. This discovery led to the formation of different seasons of the Earth. He also gave the idea of leap year by measuring that there are 365 days in a year and 366 on the fourth year. He also tried measuring the distance of Earth from the Sun and the diameter of the Sun.
Eratosthenes of Cyrene was more than just an astronomer, also being a mathematician and historian. Many of his discoveries crossed subjects, bringing math and other aspects. In astronomy, he is best known for being the first to accurately meadure the circumference of Earth. He did this by observing shadows and realizing that the sun's rays do not shine parrallel. A few other things that he did while he was alive that impacted Greece and the world greatly included the making of a star catalog that showed close to 700 stars along with creating an accurate calander that included leap years.