Monday’s Virtual Astronomy Lecture

1. The Aristotelian Universe

2. The Contribution of Ptolemy to the Aristotelian Universe

Interpreting the Apparent Motion of the Stars, Sun. Moon and Planets

• Mankind does not long live without developing a model of the Universe around him. The model explains the physical relation between man’s habitat and the rest of nature. This model of the Universe provides mankind with a world-view which permeates and gives meaning to his every action, practical and spiritual.

• Primitive conceptions of the Universe displayed considerable variation, but all were shaped primarily by terrestrial events, the daily patterns of life.

Interpreting the Apparent Motion of the Stars, Sun. Moon and Planets

For example, the Ancient Egyptians developed a model of the Universe as an elongated platter. The platter’s long dimension paralleled the Nile; its flat bottom was the alluvial basin to which ancient Egyptian civilization was restricted, and its curved rippled rim represented the mountains bounding the terrestrial world. Above the platter was air supporting an inverted platter-dome which carried the stars. The platter was supported from underneath by water. The Sun was Ra, the principle Egyptian God, supplied with two boats, one for his daily journey through the air as a second for his nocturnal trip through the water under the platter.

Interpreting the Apparent Motion of the Stars, Sun. Moon and Planets

This conceptual model provided the ancient Egyptians with a mental picture of where they were. This in turn made them feel at home in the larger Universe. Their “platter” model of the Universe provided them with a sense of security and explained to them where they as a civilization “fit” in the larger framework. The model did not try to account for such details and the annual motion of the Sun or the wanderings of the planets. This failure of ancient models of the Universe to account for these physical characteristics was common.

Interpreting the Apparent Motion of the Stars, Sun. Moon and Planets

• Primitive conceptual models are only schematic sketches against which the play of nature takes place, very few details or explanations of astronomical phenomena are incorporated into these models.

• The requirement that a conceptual model of the Universe supply both a psychologically satisfying world-view and an explanation of observed phenomena like the daily change in the position of sunrise has vastly increased the power of these models.

Interpreting the Apparent Motion of the Stars, Sun. Moon and Planets

• Only in our Western civilization has the explanation of such astronomical details been considered a function of the conceptual model of the Universe.

• Only in pan-Hellenic Greek society has a model of the Universe also been required to explain physical details of astronomy.

Interpreting the Apparent Motion of the Stars, Sun. Moon and Planets

• We would reject a model like the ancient Egyptian platter as inadequate because it fails to account for observed phenomena.

• Aristotle had developed a geocentric model of the Universe that was able to explain the daily observations of astronomers and provide a setting in which mankind could interact with his God.

Interpreting the Apparent Motion of the Stars, Sun. Moon and Planets

• We have been studying the principle observations, all of them accessible to the naked-eye, upon which depend the two main scientific models of the West, the Aristotelian/Ptolemaic and the Copernican.

• Let us examine the key ideas of the Aristotelian Universe that dominated Western Europe from 300 B.C. to, perhaps, 1700 A.D.

Interpreting the Apparent Motion of the Stars, Sun. Moon and Planets

• In class, we will examine the innovation of Copernicus whose re-interpretation of the same observations led to a complete revision not only in astronomy, but also in the basic underlying beliefs of society.

• I have taken material in the notes that follow directly from

the on-line encyclopedia Wikipedia from the subjects Aristotle and Ptolemy. In addition I have included entire paragraphs from the reserve reading On the Copernican Revolution by Thomas S. Kuhn.

Who was Aristotle?

• The three most influential ancient Greek philosophers were Aristotle, Plato (a teacher of Aristotle) and Socrates (ca. 470 BC-399 BC), whose thinking deeply influenced Plato. Among them they transformed Presocratic Greek philosophy into the foundations of Western philosophy as we know it. Socrates did not leave any writings, possibly as a result of the reasons articulated against writing philosophy attributed to him in Plato's dialogue Phaedrus. His ideas are therefore known to us only indirectly, through Plato and a few other writers. The writings of Plato and Aristotle form the core of Ancient philosophy.

Who was Aristotle?

• Aristotle was born in 384 BCE. at Stagirus, a Greek colony and seaport on the coast of Thrace. His father Nichomachus was court physician to King Amyntas of Macedonia. At age 17 his guardian, Proxenus, sent him to Athens, the intellectual center of the world, to complete his education. He joined the Academy and studied under Plato, attending his lectures for a period of twenty years. In the later years of his association with Plato and the Academy he began to lecture on his own account, especially on the subject of rhetoric.

Who was Aristotle?

• At the invitation of Philip of Macedonia he became the tutor of his 13 year old son Alexander (later world conqueror); he did this for the next five years. He then set up his own school at a place called the Lyceum. When teaching at the Lyceum, Aristotle had a habit of walking about as he discoursed. It was in connection with this that his followers became known in later years as the peripatetics, meaning "to walk about." For the next thirteen years he devoted his energies to his teaching and composing his philosophical treatises.

Who was Aristotle?

• He is said to have given two kinds of lectures: the more detailed discussions in the morning for an inner circle of advanced students, and the popular discourses in the evening for the general body of lovers of knowledge. In the first year of his residence at Chalcis he complained of a stomach illness and died in 322 BCE.

• Aristotle is known for being one of the few figures in history who studied almost every subject possible at the time. In science, Aristotle studied anatomy, astronomy, embryology, geography, geology, meteorology, physics, and zoology. In philosophy, Aristotle wrote on aesthetics, economics, ethics, government, metaphysics, politics, psychology, rhetoric and theology. He also dealt with education, foreign customs, literature and poetry. His combined works practically comprise an encyclopedia of Greek knowledge.

The Aristotelian UniverseThe Terrestrial World

• Four “elements” comprised all matter on the Earth in various proportions. Those “elements” were “earth”, “water”, “air” and “fire”

• For example a human body obviously had a solid component in its bones and muscle this was the “earth” component. In addition, there was a liquid component seen in blood and urine dubbed “water”. The heat from the body was from the “fire component” and we all know of a unsociable gaseous component of the body that is represented by the “air’ element.

The Aristotelian UniverseThe Terrestrial World

• All terrestrial matter was characterized by change through decay or death. Terrestrial matter was considered imperfect due to its changing nature

• The natural state of motion for terrestrial matter was a state of rest. A person only need look around and see that all objects are stationary (at rest) unless “forced” into motion by some unnatural cause.

• The Earth sat at the center of the Stellar Sphere immovable (“fixed and central”).

The Aristotelian UniverseThe Celestial World

• The stars, Sun, Moon and other planets were considered to be made of a fifth element not found in Earthly matter.

• All celestial matter was characterized by its unchanging nature. The stars and planets never changed their appearance or motion. They seemed eternal.

• Celestial matter was considered perfect due to its unchanging “incorruptible” nature.

The Aristotelian UniverseThe Celestial World

• The natural state of motion for celestial matter was constant motion, never ceasing, in a circular pattern. The apparent motion of the stars best typifies this motion.

• The stars sat on a very large stellar sphere which contained the whole Universe and was set into rotation (once every 23h56m 4.09s) by a “prime mover” (i.e. God).

• God lives on the Celestial Sphere and Heaven was a place on the celestial sphere.

The Aristotelian Universe

Terrestrial Matter• Temporary• Corruptible• Naturally at rest

• Physically imperfect

Celestial Matter• Eternal• Incorruptible• Naturally in constant

circular motion• Physically perfect

Aristotle made an additional conceptual leap by proposing that Celestial Matter was not just physically perfect, but morally perfect also, while Terrestrial Matter (including

people) were physically and morally imperfect.

The Aristotelian Universe

Aristotle explained mankind’s painful life and inevitable death as a direct consequence of being at the center of the Celestial sphere farthest removed from the eternal and perfect nature of God (The center of a sphere is the single most distant point in the interior from the surface). Man suffered because he was physically far or distant from God. I hope you can see how the story in Genesis of Adam and Eve’s expulsion from the garden of Eden symbolizes the physical distance of man from God first codified by Aristotle. Many other Aristotelian concepts were incorporated into the belief structure of the early Christian church.

Ptolemy’s Contribution to the Aristotelian Universe

Claudius Ptolemaeus (ca. 100 – ca. 178), known in English as Ptolemy, was an Ancient geographer, astronomer, and astrologer who probably lived and worked in Alexandria in Egypt. Ptolemy was the author of several scientific treatises, two of which have been of continuing importance to later Islamic and European science. One is the astronomical treatise that is now known as the Almagest (in Greek Η μεγάλη Σύνταξις, "The Great Treatise"). The other is the Geography, which is a thorough discussion of the geographic knowledge of the Greco-Roman world.

Ptolemy’s Contribution to the Aristotelian Universe

In the Almagest, one of the most influential books of classical antiquity, Ptolemy compiled the astronomical knowledge of the ancient Greek and Babylonian world; he relied mainly on the work of Hipparchus of three centuries earlier. It was preserved, like most of Classical Greek science, in Arabic manuscripts (hence its familiar name) and only made available in Latin translation (by Gerard of Cremona) in the 12th century. Ptolemy formulated a geocentric model (see Ptolemaic system) of the solar system which remained the generally accepted model in the Western and Arab worlds until it was superseded by the heliocentric solar system of Copernicus. The Almagest also contains a star catalogue, which is probably an updated version of a catalogue created by Hipparchus.

Ptolemy’s Contribution to the Aristotelian Universe

An Epitome of the Almagest (Epitome in Ptolemaei Almagestum) was written between 1460 and 1463 by the Austrian astronomer Georg Peurbach and his famous pupil Johannes Regiomontanus at the suggestion of Cardinal Bessarion. It gave Europeans the first sophisticated understanding of Ptolemy's astronomy, and was studied by every competent astronomer of the 16th century.Unlike earlier systems (such as 'the stars move because that is the will of the gods', or the model of concentric spheres), the Ptolemaic model explained all phenomena in the sky, while holding to Plato's dictum which states that all motions in the heavens can be explained with uniform, circular motion, and obeying Aristotelian physics.

Ptolemy’s Contribution to the Aristotelian Universe

According to the Ptolemaic model, the spherical Earth is at the center of the universe. All heavenly bodies are attached to crystal spheres which rotate around Earth. The Moon is on the innermost sphere, and touches the realm of Earth, thereby contaminating it, and causing the light and dark spots and the ability to go through phases. It is not perfect like the other heavenly bodies, which shine by their own light.

Ptolemy’s Contribution to the Aristotelian Universe

The planets are actually attached to 2 spheres: one sphere which is centered on Earth (the deferent), and another sphere (the epicycle) embedded within the deferent. The epicycle rotates within the deferent, causing the planet to move closer to and farther from Earth at different points in its orbit, and even to slow down, stop, and move backward (in retrograde motion). The epicycles of Venus and Mercury are always centered on a line between Earth and the Sun (Mercury being closer to Earth), which explains why they are always near it in the sky. The order of spheres from Earth outward is: Earth, Moon, Mercury, Venus, Sun, Mars, Jupiter, Saturn, Stars.

Ptolemy’s Contribution to the Aristotelian Universe

Unfortunately, the system still did not quite match observations. Sometimes the size of a planet's retrograde loop (most notably that of Mars) would be smaller, and sometimes larger. Ptolemy could not explain this even when he moved deferents off-center, for the change in loop size did not match with the change in speed. This prompted Ptolemy to come up with the idea of an equant. The equant was a point near the center of a planet's orbit which, if you were to stand there and watch, the center of the planet's epicycle would always appear to move at the same speed. Therefore, the planet actually moved at different speeds at different points in its orbit.

Ptolemy’s Contribution to the Aristotelian Universe

By using an equant, Ptolemy claimed to keep motion which was uniform and circular, but a lot of people didn't like it because they didn't think it was true to Plato's dictum of "uniform, circular motion." The resultant system which eventually became Catholic dogma was an unwieldy one, using two sets of epicycles, revolving on a deferent, offset by an equant which was different for each retrograde planet (then known to be only Mars, Jupiter, and Saturn), but it predicted the beginnings and ends of retrograde motion far more accurately than either earlier Platonic spheres or early (and falsely perfect) Copernican systems.