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). 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 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.
One such astronomer, Nicolaus
Copernicus (lived 1473--1543 C.E.), found
many deficiencies in the Ptolemaic model. He felt that any model of the planet
motions must account for the observations and have circular, uniform motion.
The Ptolemaic model did not do
that. Also, the Ptolemaic model was not elegant and, therefore, ``un-Godlike''.
During the years between Ptolemy and Copernicus, many small epicycles
had been added to the main epicycles to make Ptolemy's model agree with the
observations. By Copernicus' time, the numerous sub-epicycles and offsets had
made the Ptolemaic model very complicated. Surely, God would have made a
cleaner more elegant universe!
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'').
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.
Tycho Brahe (lived 1546--1601 C.E.) revived Heroclides' model that had the
all of the planets,
except the stationary Earth, revolving around the Sun. Because Brahe was not a
neoplatonist, he
believed that the Sun, Moon, and stars revolved around the Earth.
Tycho's model was mathematically equivalent to Copernicus' model but did not
violate Scripture and common sense.
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 precision and accuracy at least ten times better than obtained by any other naked eye observer.
| astronomical unit | epicycle | geocentric |
|---|---|---|
| heliocentric | instrumentalism | Occam's Razor |
| paradigm | Pythagorean paradigm | realism |
| retrograde motion |
Giordano Bruno (lived 1548--1600 C.E.) revived Democritus' (a contemporary of Socrates) view that the Sun was one of an infinite number of stars. This infinite sphere was consistent with the greatness of God. Bruno believed in a heliocentric universe. He believed that God gave each of us an inner source of power equal to all others, so there was no justification for domination and servitude. His model had definite political ramifications that threatened the Church's political authority.
 Select the image to go to the Galileo Project's homepage at Rice University |
One of Galileo's telescopes. Select the image to go to Joseph Dauben's homepage for The Art of Renaissance Science. |
Galileo Galilei (1564--1642 C.E.) was the first person we know of that used the telescope for astronomical observations (starting in 1609). The telescope was originally used as a naval tool to assess the strength of the opponent's fleet from a great distance. He found many new things when he looked through his telescope:
Galileo also made advances in understanding how ordinary objects move here on the Earth. He set up experiments to see how things move under different circumstances. He found that Aristotle's view of how things move was wrong. Galileo's observations contradicted the long-unchallenged physics of Aristotle, who taught that in order for something to keep moving at even a constant speed, a force must be continually applied. Aristotle also thought that something falling will fall at a constant speed and that heavier things will always fall more quickly than lighter things. Galileo discovered that an object's motion is changed only by having a force act on it. He also discovered that objects falling to the ground will accelerate as they fall and that all objects, regardless of size, would fall with the same acceleration in the absence of air drag.
Galileo is often considered the father of modern
science because his ideas were not derived from thought and reason alone.
He used the guidance of nature (experiments). This marked a revolutionary
change in science---observational experience became the key method for
discovering nature's rules. His arguments for the heliocentric model and the
critical role of objective observation of nature in science got him into
trouble with the Church.
The struggle between Galileo and the Church was
not a battle between science and religion but was part of a larger battle over
different conceptions of the proper routes to knowledge, God, and world view.
Galileo's intent was to improve the Church by giving a truer understanding of how God actually worked in the physical universe and by allowing greater access to God for more people. Galileo loved to debate and had the bad habit of ridiculing those he disagreed with. Some of those he ridiculed were powerful political figures in the Church. He wrote a book detailing the arguments for and against his model of the universe in a way that ridiculed the official view of the Church. It was written in Italian (the language of everyday discourse) rather than the scholarly Latin, so even non-scholars were exposed to his scathing arguments against the geocentric universe. He may have had more success in getting greater acceptance of his different views of God and research if his style was different but perhaps his ideas needed just such a champion at that time.
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last updated 25 January 1999
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