A Short History of Astronomy

Aristotle (384-322 BC) Aristotle relied on observations to prove his ideas. He proposed four observational proofs that the Earth was a sphere:

  • ships disappear slowly over the horizon
  • Earth's shadow over the moon during a lunar eclipse was curved
  • Different stars are visible in the north and south hemispheres
  • Elephants can be found in India (to the east) and Morocco (to the west), he theorized that these two places were equi-distant from Greece on a reasonably sized sphere
Aristotle did consider the possibility that the Earth was orbitting the sun, but rejected the idea because he thought that if the Earth did move, you would be able to see a change in the position of the stars. He observed no apparent shift in the positions of stars, which is now known as parallax. Parallax wasn't discovered until 1838 AD.

Ptolemy(~100-170 AD) Ptolemy wrote a history of previous observations called the "Almagest." He also performed many experiments on optics and discovered that starlight is refracted in the Earth's atmosphere. Unfortunately, Ptolemy also developed the Earth centered universe model. This model was adopted for close to 1500 years, and was incorrect. In his model the Earth is a round, stationary object centered in a "celestial sphere" where all the stars are located. In his model known as a geocentric model, everything revolves around the Earth. In order to explain the motion of the planets in the sky, Ptolemy reasoned that the planets move in smaller circles in their larger circle orbit around the Earth, these smaller circles were known as epicycles.

Copernicus (1473-1543) Copernicus was the first to challenge Ptolemy's geocentric universe theory. In his work, "De Revolutionibus Orbium Caelestium," published in 1543, Copernicus explained his new heliocentric, or sun centered, model. He believed that the motion of the earth around the sun explained the motion of the planets better than Ptolemy's epicycle theory. Copernicus also introduced the idea that day and night are the result of the rotation of the Earth on its axis and that Mercury and Venus are closer to the Sun than the Earth is. Copernicus still clung to the old idea that all heavenly bodies move in perfect circles, so his model was not any more accurate at predicting the observed motions of planets than Ptolemy's.

Tycho Brahe (1546-1601) Tycho Brahe believed deeply in the Earth centered model of the universe. So, for thirty years he labored to prove it with very careful observations. His observations were the greatest and most accurate done with the naked eye. His data was eventually used by Kepler to devise his laws of planetary motion.

Johannes Kepler (1571-1630) Using the careful observations of Tycho Brahe, Kepler devised three laws that govern planetary motion. His major contribution was the idea that planets and other satellites move in elliptical orbits, not perfect circles, with the orbitted body as one focus. Armed with this theory, Kepler was able to predict the future positions of planets with much greater accuracy.

Galileo Galilei (1564-1642) Galileo used the first telescope to observe the heavens in 1609. He discovered cratering on the moon, sunspots, four moons of Jupiter, and the phases of Venus. His discovery of the moons of Jupiter showed that not everything in the heavens revolved around the earth, which was the position believed by the Church at the time. Galileo most ardently believed in the truth of the heliocentric model. He did not keep his views secret and the church warned him not to publish his beliefs. Galileo ignored this warning and wrote, Dialogue Concerning the Two Chief World Systems. For the crime of defying the Church, Galileo spent the end of his life under house arrest.

Sir Issac Newton (1642-1727) Among his many accomplishments, Newton formulated the basic laws of motion that are the basis of mechanics. He published these laws in his work, Philosophiae Naturalis Principia Mathematica, perhaps the greatest scientific work in history. These laws explained why Kepler's laws of planetary motion are true. He also expressed gravitation as a universal mathematical law and created calculus. Newton's three revolutionary laws are:

  • an object at rest stays at rest, or an object in constant motion stays in constant motion, until it is acted upon by a force
  • the change in speed of an object is proportional to the force acting upon it
  • when an object 1 exerts a force on object 2, object 2 exerts a force on object 1 equal and opposite to the orginal force

For more history of astronomy:


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These pages designed by Erin McNally (CU'00)... Last modified 18Apr00