Astronomy is the study of the stars, planets, and other objects that make up the universe. Astronomers observe the locations and motions of heavenly bodies. Beginning around 600 BC, Greek philosophers and scientists developed a number of important astronomical ideas.The early Greek astronomers knew many of the geometrical relationships of the heavenly bodies.
Pythagoras, who lived during the 500s BC , argued that the earth was round. He also tried to explain the nature and structure for the universe as a whole. He developed an early system of cosmology.
In about 370 BC, Euxodus of Cnidus had developed a mechanical system to explain the motion s of the planets. Euduxos taught that the planets, sun, the moon, and the stars revolved around the earth. In 300s BC, Aristotle incorporated this earth centered, or geocentric, theory into his philosophic system.
Heraclides of Pontus proposed that the seeming westward movement of the heavenly bodies is due to the eastward rotation of the earth on its axis. He also taught that Venus and Mercury revolved around the sun, not the earth. In about the 200s BC, a man named Aristarchus of Samos suggested that all the planets, even the earth, revolve around the sun. It is said that Heraclides and Aristarchus were way ahead of their time, and their theories were not really accepted.
Eratosthenes who was born in 276 BC, demonstrated the Earths circumference. Hipparchus, who lived around 140 BC, was a creative and talented astronomer. He divided the stars that he could see into classes of apparent brightness.He estimated the size and distance of the moon, found a way to predict eclipses, as well as calculating the length of the year to within 6 and a 1/2 minutes!
During about the 100s AD, Aristotle's and Hipparchus's theories were expanded upon by a man names Ptolemy. He was a Greek astronomer who live in Egypt. He published a book called the Almagest, in which he presented his ideas and summarized those of his predecessors. This work is our main source of knowledge of Greek astronomy.
Greek astronomers help form the foundation for modern astronomy. Hipparchus's system of magnitude of brightness is still used today, although it is modified slightly. Some versions of his ideas and his table of planetary motions were accepted for nearly 1,500 years. Some of ancient Greece astronomer's theories were very influential, and many still stand today.