History of Astronomy

1. The Greeks are generally acknowledged as the first people to elevate astronomy from the level of prediction to that of explanation and understanding.
2. Aristotle (384 - 322 BC) was one of the most influential Greek philosophers. He made important contributions to logic, criticism, rhetoric, physics, biology, psychology, mathematics, metaphysics, ethics, and politics  He believed (incorrectly) in separate laws for Earth and Heavens.
3. After Alexander the Great (356-323 BC) conquered Egypt in 332 BC, he founded the city of Alexandria, which became a leading center of Greek civilization. Much of the significant work of later Greek astronomers was carried out in or near Alexandria.
4. Ptolemy, also Claudius Ptolemaeus, (ca A.D. 150) formulated a geocentric model of the universe that was widely accepted until it was superseded by the heliocentric system of Copernicus, some 1500 years later.
5. Nicolaus Copernicus (1473-1543, Poland) developed the most coherent model at the time for the heliocentric cosmology.
6. Tycho Brahe (1546-1601) collected the best astronomical data before the age of the telescope.
7. Johannes Kepler (1571-1630) carried out the analysis of Tycho's data and developed a few empirical laws which described the behavior of the planets in their orbits.
8. Galileo Galilei (1564-1642), using the newly-invented telescope, made critical observations which demonstrated once and for all the correctness of the heliocentric model.
9. Isaac Newton (1643-1727), discovered the law of gravity and formulated the laws of motion to explain why the planets move as they do. In this, he was encouraged by Edmond Halley, who brought his work to a wider audience.
10. Harlow Shapley (1885-1972) was the first to realize that the Milky Way Galaxy was much larger than previously believed and extended the Copernican principle: Not only is the Earth not the center of our Solar System, but the Sun is also not unique, central, or special in any way. He participated in the "Great Debate" of 1920 with Heber D. Curtis on these and other issues. Up to that point, most people believe the Sun to be center of the Universe.
11. Edwin Hubble (1884-1953) extended the Copernican principle even further in the 1920's by demonstrating that even our Galaxy is not special in any way but rather one of myriad other galaxies in a much, much larger cosmos.

Geocentric vs Heliocentric

In the geocentrism model, the Sun and the other six visible objects in the sky revolve around the Earth.	In the heliocentric (Sun-centered) model, the Earth is just one out of many planets, all of which orbit the Sun in elliptical orbits.

• Both models are pretty good at explaining some basic, naked-eye observations:
  • days and nights
  • Moon phases
  • seasons
  • planets appear to "wander" across the night sky, relative to the fixed, background stars
• The order of the planets is similar in both models, with those objects moving slower placed farther.
  •  In the Geocentric model, the order is Earth (stationary and at the center), Moon, Mercury, Venus, Sun, Mars, Jupiter, and Saturn. As stars appeared to move much slower than the planets, they were placed in the outermost sphere, furthest away from Earth.
  • In the Heliocentric model, the order is Sun (stationary and at the center), Mercury, Venus, Earth (and Moon), Mars, Jupiter, and Saturn.
• The Coperican model introduced several innovative ideas:
• The Earth is one of several planets revolving around a stationary Sun in different orbits.
• The Earth has three motions: daily rotation, annual revolution, and annual tilting of its axis.
• Retrograde motion of the planets is explained by the Earth's motion.
• The distance from the Earth to the Sun is small compared to the distance from the Sun to the stars.
• But the greatest puzzle, back in the time of Copernicus, was the problem of retrograde motion, the periodic backward motion of the outer planets, as observed from Earth, particularly noticeable for Mars.
  • While retrograde motion was explainable within the geocentric model by imagining deferents and epicycles, the heliocentric model offered a much simpler explanation.
  • In the Copernican model, when two planets are near each other in their respective orbits (e.g., Mars relative to Earth), the more distant planet would appear to move backwards since the inner planet moves faster. No correction with epicycles and deferents would be required in this model.
• The Copernican model predicted stellar parallax as a result of Earth's orbital movement, which does occur but was not detectable at the time.
• The Copernican model was closer to reality but still far from perfect. It retained several of the Ptolemaic (incorrect) elements, including:
  • circular orbits of the planets (the orbits are actually elliptical)
  • uniform speeds (speeds actually vary in orbit)
  • epicycles, to correct for the inacurracies due circular orbits and uniform speed
• The ultimate collapse of the geocentric model would come with Galileo's observations with the telescope, particularly those related to the phases of Venus.
  • These simulations show the phases of Venus in the heliocentric vs Ptolamaic models.
  • If the geocentric model were correct, the only phase for Venus we could ever see would be the crescent.
  • In reality, Venus exhibits the quarter and gibbous phases, in addition to the crescent phase.
  • We can't see the full phase because it would be behind a very bright Sun.