Our view of the history of astronomy will now skip almost 1500 years to the next major advances in astronomy. Europe was beginning to emerge from a long period of instability in the Middle Ages. During the Middle Ages the Islamic civilization had flourished in the Arabic countries. They had preserved and translated the Greek writings and adopted the Greek ideals of logic and rational inquiry. Islamic astronomers were careful observers of the sky and created accurate star catalogs and tables of planet motions. Many of the names of the bright stars in our sky have Arabic names (e.g., Deneb, Alberio, Aldebaran, Rigel to name a few). For more about this, see the database of Islam and astronomy (will display in another window). However, advances in the explanations of the motions of the stars and planets were made by astronomers in Europe starting in the 16th century.
By the 16th century the following paradigm had developed: Man is God's special creation of the physical universe; the Earth is the center of a mathematically-planned universe and we are given the gift of reading this harmony. The Greek ideal of finding logical, systematic explanations to physical events was rediscovered and celebrated to trading with islamic nations. Along with this came an unbounded faith in the power of reason to solve physical problems. This time in history is called the Renaissance (french for ``rebirth'').
Scientists use a guiding principle called Occam's Razor to choose between two or more models that accurately explain the observations. This principle, named after the English philosopher, William of Occam, who stated this principle in the mid-1300's, says: the best model is the simplest one---the one requiring the fewest assumptions and modifications in order to fit the observations. Guided by Occam's Razor some scientists began to have serious doubts about Ptolemy's geocentric model in the early days of the Renaissance.
Copernicus was strongly influenced by neoplatonism (beliefs that combined elements of Christianity and Platonism) in developing a model to replace Ptolemy's. This led him to believe that the Sun is a material copy of God---God is the creative force sustaining life and the Sun gives us warmth and light. He adopted Aristarchus' heliocentric (Sun-centered) model because he felt that God should be at the center of the universe. Copernicus' model had the same accuracy as the revised Ptolemaic one but was more elegant.
Copernicus retained the Aristotelian notion that planets fulfill the goal of perfect (circular) motion. His model still used small epicycles to get the details of the retrograde loops correct, though they were only a minor feature. He used trigonometry to describe the distances of the planets from the Sun relative to the astronomical unit (average Earth-Sun distance), but he did not know the numerical value of the astronomical unit. He found that the planets farther from the Sun move slower. The different speeds of the planets around the Sun provided a very simple explanation for the observed retrograde motion.
Retrograde motion is the projected position of a planet on the background stars as the Earth overtakes it (or is passed by, in the case of the inner planets). The figure below illustrates this. Retrograde motion is just an optical illusion! You see the same sort of effect when you pass a slower-moving truck on the highway. As you pass the truck, it appears to move backward with respect to the background trees and mountains. If you continue observing the truck, you will eventually see that the truck is moving forward with respect to the background scenery. The relative geometry of you and the other object determines what you see projected against some background.
Copernicus thought his model was reality but other people used his model as a more convenient calculation device only. If the Earth were moving around the Sun, then the stars should appear to shift due to our looking at them from different vantage points in our orbit (a ``parallactic shift''). The parallax effect can be illustrated when you look at your thumb at arm's length with one eye and then the other---your thumb appears to shift position!
Now imagine that the Earth at opposite points in its orbit is your left and right eye and the nearby star is your thumb and you have the situation illustrated below.
However, no parallactic shift was observed in the stars. If there was actually a very small parallactic shift, then the stars would have to be very far away. Copernicus' contemporaries felt that God would not waste that much space! They argued that, therefore, there must be no parallactic shift at all---the Earth is not in motion. Astronomers now know that the stars are indeed very far away and telescopes must be used to detect the small parallactic shifts.
Try out the Solar System Models module of the University of Nebraska-Lincoln's Astronomy Education program for checking your understanding of the geocentric vs. heliocentric model explanation of the planet motions (link will appear in a new window).
Tycho calculated that if the Earth moved, then the stars are at least 700 times farther away from Saturn than Saturn is from Sun. Since Tycho felt that God would not waste that much space in a harmonious, elegant universe, he believed that the Earth was at the center of the universe. Astronomers now know that the nearest star is over 28,500 times farther away than Saturn is from the Sun!
Though Tycho's beliefs of the universe did not have that much of an effect on those who followed him, his exquisite observations came to play a key role in determining the true motion of the planets by Johannes Kepler. Tycho was one of the best observational astronomers who ever lived. Without using a telescope, Tycho was able to measure the positions of the planets to within a few arc minutes---a level of precision and accuracy that was at least ten times better than anyone had obtained before!
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last updated: January 8, 2013