Greek Astronomy - Ptolemy


The issue of geocentrism vs. heliocentrism appeared settled by Ptolemy’s great work, the Almagast Here is a Greek astronomer (writing between 127 and 141 A.D.) who organized a series of highly complex and creative mathematics to explain the motion of the sun and other objects. The book is 13 volumes long and would fill 500 pages when translated into modern language, and the complicated epicycle motions of the planets.






An interesting phenomenon was his curiosity into reports of sailors seeing boats rise up out of the sea (the "hull-down" effect), and even whole islands doing so. While many presently hold to the actual historical existence of Atlantis, there may be another explanation for the rising and sinking of this fabled city. To any sailor who ventured far enough out to sea, any distant boat would have its hull down under the water until it neared, and sailors could see all of it rising from the sea. This "hull-down" effect took place in any direction on the ocean. While this effect was not recorded before 10 B.C., Ptolemy was one of the first to really try to explain it. Any distant island would look as if it were rising up as one approached it. The diagram demonstrates how this is possible. Sailors did not think about the oddity that the island was never completely visible. They could see the tip of the island and more of the base as they approached it, never seeing the entire island until relatively close. If the Earth were flat, the entire island should be visible from any distance. That the island became more visible in terms of total island, was evidence for the physical rising and falling of the piece of land. The second diagram demonstrates Aristotle's deduction. A curved earth surface would result in a line of sight for the view missing the base of the island below that same line of sight. Sailors may have been excited about Atlantis rising and falling mysteriously into the sea, but Ptolemy had a more practical idea of this phenomenon.

Ptolemy wrote his great book with the mindset that Earth occupied the center of the universe. His geocentric model is found in the drawing below. Each sphere out from the earth was occupied by a celestial object, moving in perfectly circular paths. Since the retrograde motion of the outer planets made such circular orbits impossible, Ptolemy carefully worked out a series of smaller circular motions within the larger circular orbit. He coined the small motions "epicycles." Clearly a 13 volume book with detailed mathematics was daunting to the average citizen, even if they had time to digest the work while philosophizing at the Agora. It was easier to accept the idea than to ponder it, and besides, basic survival required more attention than astronomical musings.

Ptolemy's geocentric model was satisfactory for the entire civilized world known to the Romans and Greeks due to four factors:

1) It fit well with the predominant religious views. Beyond mythology, Judaism and Christianity were quite popular. Both held to a Biblical view which places man as the center of God's creation, and in the center of His attention. A geocentric model would satisfy priest and rabbi.

2) It fit well with basic observation. Afterall, one only need look up at the passage of the sun and moon. Clearly they are moving and we are not. If we were moving or spinning, wouldn't we feel the motion and be dizzy?

3) It fit well with the basic philosophy that mankind holds ... we are the center of our universe. Every high school student believes this to be true, and it makes simple sense to see the universe act within the constructs of our worldview.

4) The mathematics were so detailed and complicated that none could understand it. Why argue with such beautiful work. Ptolemy was really smart and his Almagast very lengthy, so he must be right.

The geocentric model satisfied human observation, philosophy, and theology. It was not questioned publicly for almost 1200 years. Ptolemy himself is not responsible for the Dark Ages and the complete shutdown of science. But his views gave religious leaders no reason to accept any other notion. A drawing of the Universe, as Ptolemy saw it, is found below, and with the perfection seen in the orbital paths, it becomes apparent to any historical student of his day that this geocentric model fit the above-mentioned 4 criteria for widespread acceptance.

After Ptolemy died, the science of Astronomy came to a halt, the Roman Empire crumbled, and the Dark Ages ensued. The Church began a purge of all forms of knowledge which might appear contradictory to a literal interpretation of the Bible and thus books were burned in a frenzy. The vast Alexandria Library, with many of the important scientific discoveries was burned and all its books lost.

Peace and prosperity continued to reign until about the end of the second century A.D. Then a series of catastrophes dissolved the Roman Empire and all but destroyed civilization itself. Decadent political and social systems collapsed. The exhaustion of gold and silver coins, as the Spanish mines ran out of the precious minerals, paralyzed commerce. A horrible pestilence, brought back from Asia my the military, struck down the population at the rate of two thousand persons per day. Barbarian tribes pressed in, conquering and devastating southern Europe.
The centuries that followed were a sad time for science. Deprived of comfort in the world, people looked forward to solace in a better world hereafter, and they turned away from the science of the past. Christianity, the major force during the Middle Ages, brought with it a growing desire to reconcile science in general and concepts of the universe in particular with the literal statements of the Bible. Lactantius ridiculed the concept of a spherical earth. Most people came to believe that the earth was flat, square, and supported by four pillars, one at each corner. Under these conditions, contributions to science were rare. Any unusual celestial event became a sign of prophetic nature, portending evil or good for the earth. Superstition and fear were the motivating factors in astronomical thought for many centuries. This aura of superstition, mysticism, and magic obstructed the development of the sciences, especially astronomy and chemistry.
The pseudo science of alchemy, bearing a relationship to chemistry similar to that of astrology to astronomy, blossomed. There was widespread belief that with the use of magic, potions, and incantations one could turn base metal into silver or gold. Failure did not discourage the alchemists and magicians. From time to time they even made, by accident, some useful chemical discovery. In such an atmosphere, it is not surprising that astrology flourished. The idea that God, through the stars, could influence mankind was not unacceptable to the church fathers. And so, with full approval of the church, astrologers continued to cast horoscopes.
Observational astronomy began again early in the sixteenth century. Belief in a flat earth began to recede and in its place came an understanding of the universe as visualized by Ptolemy and Aristotle.

The Greeks set the tone for an explanation of the Natural World. The Romans were exercising control over the world with military strength and heavy taxation. The developing Roman Catholic Church was spreading, despite efforts of Nero and Diocletian to wipe out all followers of "the Way. It is my sincerest opinion that issues of Church and Science are inseparable during the next 1400 years, and in order to really grasp the importance of the work of Copernicus, you must first understand the effect of Church history, the rise and power of the Papacy, and the impact of historical monarchs, or Roman Emperors who struggled with or against each other for control of the people. Please move next to a look at this history by clicking on Martin Luther, or return to the Introduction for this History Unit, the Syllabus, or the Home page.

Do not forget to send in your responses to the questions from the first half of this history unit. They can be found in the Assignments folder under the heading Ancient Astronomy Quiz.

*** While the Greeks encouraged people to see the Earth as being spherical, there were still many people who insisted that it was flat. There is even a society today devoted to disproving the "Spherical Earth Hypothesis." If you do not wish to accept the opinion of the Greeks, or the photographs of astronauts and satellites, and desire instead to believe the Earth to be flat, then click on Flat Earth Society to see their website, information on membership, and reasons why their view is correct in their opinion :) ***

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