History of Astronomy

Scientific Method

  1. Observations (collect data)

  2. Try to explain observations - HYPOTHESIS

  3. TEST the hypothesis (with experiments) and as a result:
    1. Accept
    2. Reject
    3. Modify

  4. Devise a better explanation in accordance with the tests - THEORY
Note that a theory must be testable, and verifiable by others. If it can't be subsequently verified, it will have to be rejected. You can never PROVE a theory. Theories simply stand until disproved, or until they are seen to have limits.

Aristotle (384-322 BC) performed one of the first known uses of the scientific method.

Size of the Earth

About 200 BC, Eratosthenes used Aristotle's ideas to calculate the size of the Earth.

On a certain day of the year, observers at Syene, Egypt saw the sun directly overhead. Observers at other locations (Alexandria, Egypt) saw the sun at an angle on that day. The angular displacement was measured.

Using simple geometry, he calculated the circumference and the radius of the Earth.

The result was within 1 percent accuracy of the figure known today.

Ancient Astronomy

Planetary Motion

The ancient Greeks were aware of 5 bodies in the sky that did not behave in a regular, predictable pattern. They were called "wanderers", planetes.

These planets are all on (or very close to) the ecliptic.

Planets rise in the east and set in the west (due to the Earth's rotation), but normally appear farther east each night. Occasionally, however, they seem to slow down and move backwards (westward) for a month or two. This reversal in direction is called RETROGRADE MOTION.

This motion is easily explained by a model in which the Earth and the planets orbit the sun at different distances and at different speeds (faster closest to the sun). However, it was a problem for the early astronomers who did not use a sun-centered model for the solar system.

Models of the Universe

Our modern view of astronomy in which the Earth rotates on its axis and revolves around the sun evolved during a time called the scientific revolution. This was in the 16th and 17th centuries (1500's and 1600's), and followed the Renaissance (1275-1475 AD). This period in time provides a transition between the medieval and the modern world.

In the Mid 16th Century (mid-1500's) there were 2 models to describe the structure of the universe:

  1. Geocentric (earth-centered) model
  2. Heliocentric (sun-centered) model
Both were good models because they:
  1. accounted for all observations of the movement of the sun and the moon, and the planets, and the stars
  2. were good predictors of future positions of celestial bodies; models were verifiable
  3. simplicity (Occam's Razor or the Principle of Parsimony) - as few assumptions or rules as possible; no contradictions.

Both were complex models.

  1. Geocentric Model

    Probably originated with the Greeks or Egyptians prior to 300 BC. Was summarized by Ptolemy (about 100 - 200 AD), a Greek philosopher who studied in Egypt.

    1. The Earth is still; motionless
    2. Earth at center of Universe
    3. Celestial bodies move in perfect circles at uniform speeds
    4. Stars were set in a rotating sphere that turned E to W once a day
    5. Planets, moon, sun also set in separate spheres that moved slower
    As observations become more detailed, the model became more complex (more spheres).

    Planets moved in circles (called epicycles) centered on the Earth.

    Quality of observations was fairly poor, and model worked fairly well. (With more precise observations, it did not hold up; many corrections had to be added.) It did not explain some things, such as changes in brightness of planets.

    The Ptolemaic model required 80 circles. Complex. A simple model would be better.

    Aristotle had a strong influence, and his teachings carried great weight. He promoted the geocentric model.

    The Ptolemaic "Earth-centered" model survived for almost 1300 years.

  2. Heliocentric Model

    Aristarchus of Samos, a Greek about 310-230 BC, had a heliocentric model. He proposed that all of the planets, including Earth revolved around the Sun, and that the Earth rotates on its axis once a day. His ideas did not gain widespread acceptance during his lifetime.

    This sun-centered model was later revived by Nicholaus Copernicus, a Polish cleric (1473-1543) around 1500. He was dissatisfied with the complexity of the geocentric model. He got in some trouble with the Church for his ideas.

    1. Sun is at the center of the universe, motionless; stars are motionless around the edge
    2. Planets all revolve around the sun (6 total including Earth)
    3. Moon revolves around Earth
    4. Earth rotates on axis causing apparent daily motion of the heavens
    5. Earth revolves around sun causing sun's annual movements
    6. Retrograde motion of planets is due to relative planetary motions
    7. Planetary orbits are perfect circles

    We know that some parts of this model have been modified - the planetary orbits are actually elliptical; the sun rotates on its axis, for example.

    Copernicus was the first to determine the relative distances of planets from the sun.

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This page created by Pamela J. W. Gore
pgore@gpc.edu
DeKalb College, Clarkston, GA
January 28, 1996
Last modified September 5, 1997