Astronomers

• Thales


  One of the earliest Greek astronomers
• Thales was born around 624 BCE
• Thales


  • He was a merchant and businessman
  • He probably traveled widely and spent time in Egypt
  • He was the first of the Greek philosophers to move away from supernatural explanations of the natural world
• Thales' theory


  • Earth floats on water
  • All different parts of the Earth, from mountains to trees were made of different forms or states of water
• Thales' theory was not correct, but it pointed out that he had developed a theory that didn't depend on mythology
• Aristotle


  • A star pupil in Plato's Academy in Athens
  • The first person to develop a system to explain how the Universe worked
• Aristotle's theories regarding all aspects of science were generally regarded as being absolutely correct for almost two millennia
• Aristotle's concepts regarding the universe and the earth's position in it

  • Spheres that rotated around the earth carried stars, planets, and the sun
  • The earth was a spherical ball in the centre of the spheres and was therefore the centre of the universe
  • Stars were located on the inside of the largest sphere that rotated once a day around the earth
  • Planets were located on smaller, transparent spheres that rotated at different speeds and on a different axis
  • The universe is perfect. The circle is a perfect geometric shape. The planets and sun and all heavenly objects must move in circular paths around the earth
• Celestial sphere


  The farthest sphere from earth where the stars are fixed on the inside
• We still use the celestial sphere today for navigation purposes
• Ether


  The substance that filled the space between the planets, sun, and stars according to Aristotle
• Aristarchus


  • A brilliant mathematician who spent a great deal of time studying the universe
  • He was able to declare that the sun was a huge ball of fire and that the sun, planets, and stars did not revolve around the earth, but that the earth and planets revolved around the sun
• Aristarchus' theory was not accepted because it didn't seem to fit observations
• Hipparchus


  • Compiled an extensive star chart based on what he could determine with just his eyes
  • Included a classification of stars according to their brightness
  • Calculated the distance to the moon using parallax
  • Discovered the existence of precession
  • Developed a system of epicycles to help explain the movement of planets in a geocentric system
• Ptolemy


  • Generally regarded as the best astronomer in antiquity
  • Presented his picture of the universe in a book called the Almagest
  • Chose to side with Aristotle and describe a universe that was earth-centred (geocentric)
• Retrograde motion


  The apparent backwards motion of planets as seen from the earth
• Ptolemy established a complex theory of epicycles to explain the retrograde motion of planets
• Hipparchus' observations led him to explain the motion of planets as being outside of the centre of the earth, which Ptolemy would lean on
• After the fall of Rome, the great library at Alexandria in Egypt was destroyed and most of the works there were burned
• Ptolemy's work was highly regarded in the Arabic world and was gathered into a single document called the Almagest
• Ptolemy's incorrect model was studied and accepted as being correct for many centuries
• Several factors contributed to the acceptance of the geocentric model, including the lack of a scientific method, the reputation of Aristotle, the influence of the church, and the lack of a telescope
• Copernicus


  • Studied at Krakow University and in Italy
  • Studied to be a priest but was given a post that allowed him to pursue his interest in astronomy
  • Decided to try to develop a simpler model for the motion of planets than Ptolemy's
• Copernicus remembered there were a few Greek astronomers, such as Aristarchus, who had proposed a sun-centred model
• Galileo was the founder of the modern scientific method
• In an age still subjected to mysticism and the occult, Galileo demonstrated that there were rational explanations of the natural world
• Galileo was the first to state and use the scientific method effectively
• Galileo would show, at every opportunity he could, that observation and measurement overcame many wrong ideas held as being true for so many centuries
• Galileo's serious interest in astronomy began with the appearance of a new star in the sky in 1604
• Galileo immediately recognized the telescope's potential for studying the heavens and by 1609 he had built his own
• Galileo saw the moon wasn't a perfect sphere, as all objects in space were thought to be. Instead, the moon was filled with craters, valleys, and mountains
• Galileo discovered that what looked like a mist was really millions of stars forming the band of light called Milky Way
• Galileo saw Jupiter with three and then, a few weeks later, four distinct moons. The moons traveled around Jupiter and not Earth, as they should have in the geocentric model
• Venus showed distinct phases, much the same as the moon does over a period of one month. The only explanation for these phases was the sun shining on Venus and the light being reflected back to earth as Venus traveled around the sun and not around earth
• Galileo could not stop describing the Ptolemaic or geocentric system as wrong and the Copernican or heliocentric system as correct
• Galileo was called into church court where he was accused of heresy (making statements contrary to church doctrine)
• Galileo's defense of his propositions was logical and based on scientific measurement
• Galileo's observations of the moon revealed it to be less than perfect, contrary to Aristotle's statement that objects in space were perfect
• Galileo's discovered that moons traveling around Jupiter was proof that not all objects traveled around earth
• The phases of Venus, much like the phases of the moon, indicated Venus traveled around the sun