Ptolemy

• Ptolemaic diagram of the universe.Photos.com/Photos.com/Getty Images
  
  Ptolemy was an astronomer and mathematician who lived in the first century. He created a model of the universe that served the world for the next 1,400 years. In his system, the earth was a sphere at the center of the universe, with objects orbiting around it. He devised a system to predict the past and future locations of planets. In his book, "The Almagest," Ptolemy created a star catalog that named 48 constellations.
  
  

Hipparchus

• Approximately 300 years before Ptolemy was another Greek astronomer, Hipparchus. Hipparchus was the first to determine the motion of the sun and the moon, a method that survives to this day. Using this discovery and trigonometry, which he founded, Hipparchus was able to reliably predict solar eclipses. He also developed a calendar that listed the rising and setting of stars and correlated weather patterns. He is still considered one of the greatest astronomers in history, and his likeness was carved into the Astronomer's Monument in Griffith Observatory in California.
  
  

Aristarchus

• Going back 100 or so years before Hipparchus (310-230 B.C.), another Greek astronomer, Aristarchus, suggested that the earth orbits the sun and not the other way around. His brilliant observation would not take hold for another 1,800 years, mainly because Aristotle thought the earth was the center of the universe. Aristarchus also determined the sizes and distances of the sun and moon. He realized the sun was much larger than both the earth and the moon. Aristarchus also determined that the stars were much farther away than anything else in the night sky because of their fixed placement.
  
  

Pre-Greek Astronomy

• Both Aristarchus and Hipparchus used discoveries made by Babylonian astronomers to form their own observations of the solar system. In fact, the Babylonians devised the 12 astrological signs we still use today. They, and many other ancient civilizations, believed the cosmos could impact their everyday lives. The Babylonians developed a 12-month calendar with each month consisting of 29 or 30 days. These ancient civilizations used astronomy mostly to determine planting and harvest seasons and to offer metaphysical explanations of their lives.