Cosmology: The study of the Universe
Anaximander (c611-c547BC)
Born in Miletus (Turkey)
First known attempt to model universe (c555BC)
He suggested the Earth was a cylinder and the Sun, Moon and stars were all located on concentric cylinders rotating around the Earth.
He thought the stars were rings of fire.
This model was very radical for its time because, up until that point, all heavenly bodies were thought to be living gods.
He suggested the Earth was a cylinder and the Sun, Moon and stars were all located on concentric cylinders rotating around the Earth.
He thought the stars were rings of fire.
This model was very radical for its time because, up until that point, all heavenly bodies were thought to be living gods.
Pythagoras (c569-c480BC)
Born in Samos.
Died around the year of the Battle of Thermopylae (“The 300 Spartans”)
Known for “Pythagoreanism,” a religious, political and philosophical movement.
Also known for the extensive mathematical investigations that became Number Theory.
Such studies established the scientific foundation of mathematics.
Also know for geometric studies.
Led to the Hypotenuse (i.e., “Pythagorean”) Theorem.
As a result, they discovered irrational numbers, which greatly upset them.
First to employ the concept of Reductionism.
The idea that all complex phenomena can be reduced to simple ones.
Both Newton and Einstein used this concept.
First to mathematically analyze musical sounds.
He discovered the octave scale.
Considered a radical and many of his mathematical ideas were ridiculed by his contemporaries.
First person to recognize that Venus was both a “morning star” and an “evening star.”
He thought the heavenly motions were perfect.
Thus, spherical orbits.
Thus, spherical Earth.
The Universe was a series of concentric spheres containing the 7 moving objects (five planets plus the Sun and the Moon.
He object had perfect circular orbits with perfectly uniform speeds.
The friction between these spheres produced the “music of the spheres” which either:
Only the gifted could hear.
Nobody could hear since we’ve been hearing it since our birth.
His legacy: Heavenly bodies are spherical.
Lunar terminator implies sphericity.
Others by analogy, though no real reason.
The idea the Earth is round never disappeared from Greek thought.
Philolaus (c450BC)
A student of Pythagoras.
First person to suggest the Earth moved.
Because it was “base” to presume the Earth was the center of all things.
All heavenly bodies revolved around a central fire.
The Earth, the seven “wandering stars,” and the fixed stars (‘celestial sphere”) made 9 spheres.
He regarded the celestial sphere as motionless with its apparent rotation caused by the rotation of the Earth.
He believed, however, that 10 was the most perfect number.
Proposed a 10th sphere of the planet “Antichon” (or “counter-Earth”).
Antichon was fixed directly between the Earth and the central fire; thus, always hiding the central fire.
But the Earth rotated as Antichon revolved so as to keep Greece always turned away from both Antichon and the central fire.
While an imaginative concept, it cannot be viewed as the forerunner to heliocentric theory because it was based on fancy and superstition.
Empedocles (c490-430BC)
Born in Agrigentum (now Agrigento) Sicily.
Refused to accept the crown when Agrigentum offered it to him after he helped overthrow the ruling oligarchy.
Instead, he helped to establish a democracy.
A student of Pythagoras.
He thought two hemispheres revolved around the Earth.
One composed of fire (day).
One composed of air with little fire (night).
He didn’t think the sun was made of fire.
He thought the stars were made of fire.
He thought the moon was made of air that reflected light.
He thought two hemispheres revolved around the Earth.
One composed of fire (day).
One composed of air with little fire (night).
Anaxarogas (c500-428BC)
Born in Clazomenae (Turkey).
His doctrine of “nous” (“reason”) was adopted by Aristotle.
His doctrine of atoms paved the way for the atomic theory of the philosopher Democritus.
He may have taught Socrates.
He believed all heavenly bodies must be made of the same substance as Earth after studying meteorites.
He believed the Sun was a red-hot stone.
For these beliefs, Athens arrested him for heresy and sent him to prison.
Eudoxus (408-355BC)
Born in Cnidus (Turkey)
He studied for a short time under Plato.
He made important contributions in the field of geometry.
Much of his work was later used by Euclid, whose “plane geometry” is still taught in high schools today. (including yours!)
Credited with discovering that the Solar year is 6 hours longer than 365 days.
Also attempted to explain the motion of the Sun, Moon and planets using a complicated system of rotating spheres.
The stars were on a fixed sphere that rotated separately.
This model achieved modest success in predicting these motions.
He claimed that the system did not necessarily represent what was “truly” happening, but was done to merely “save the phenomena” and predict such things as eclipses.
Aristotle (384-322BC)
Born in Stagira (Macedonia).
A student of Plato.
A teacher of Alexander the Great.
Known as the father of everything intellectual from science to politics to philosophy to reasoning.
Observation and sense experience is crucial to all things.
“ASTRONOMICAL experience supplies the principles of ASTRONOMICAL science.”
To cover a multitude of observations, develops the theory of natural motions and places of the four natural elements.
Mix Earth, air, and water in a jar and you find the Air rises while the Earth sinks.
Bodies (elements) move only to attain their proper places, so Earth, being at the bottom of all things, is in its proper place and need not move.
Heavenly bodies have no apparent change save for motion.
– They neither come into being or perish.
– They do not change in size or quality
– Whereas sub-lunary bodies move in a straight line, the local motion of the heavenly bodies appear circular rather than rectilinear.
Concludes that there must be two types of matter:
Corruptible (Terrestrial).
– It can be created and destroyed.
– It exists within Earth’s atmosphere.
– It moves in straight lines.
Incorruptible (Celestial).
– It can be neither created nor destroyed.
– It exists in the Heavens.
– It moves in circles (which are perfect).
Summarized by Aquinas:
– “Plato and all who preceded Aristotle held that all bodies are of the nature of the four elements” and consequently “that the matter of all bodies is the same. But the fact of incorruptibility of some bodies was ascribed by Plato, not to the condition of matter, but to the will of the artificer, God….This theory Aristotle disproves by the natural movement of bodies. For since he says that the heavenly bodies have a natural movement, different from that of the elements, it follows that they have a different nature from them. For movement in a circle, which is proper to the heavenly bodies, is not by contraries, whereas the movements of the elements are mutually opposite, one tending upwards, another downwards….And as generation and corruption are from the contraries, it follows that, whereas the elements are corruptible, the heavenly bodies are incorruptible.”
Four Arguments of Aristotle’s Physics:
The nature of moveable bodies.
The nature of the motor virtue.
The nature of the place in which the movement occurs.
The perfection of the circle.
ASTRONOMICAL Observations and Theories.
The faces of the moon.
Eclipses.
The spherical shape of the earth.
During a lunar eclipse, the earth’s shadow is always round.
Northbound travelers see previously unseen stars on the northern horizon, and likewise for southbound travelers; therefore, the earth has a curved surface.
Elephants were observed to the east in India and to the west in Morocco, so these two places must be close together!
The motion of the earth.
Pointed out that the apparent daily motion of the sky could be explained by either a heliocentric or geocentric hypothesis.
Rejected heliocentric theory due to a lack of an observable stellar parallax.
A stellar parallax is the apparent movement of nearby stars versus background stars, a phenomenon barely perceptible by even today’s most powerful telescopes.
Geocentric Theory.
An elaboration of Eudoxus.
Developed by adding another 22 spheres.
Unlike Eudoxus’s probable conviction that his spheres were just a mathematical device, Aristotle believed they were real, solid orbs of crystal.
Aristarchus of Samos (c310-c250BC)
First to propose the Earth revolved around the Sun.
He developed the corrected methodology for measuring relative distances of the Sun and the Moon from Earth.
His calculations were incorrect, though, because of lack of mathematical knowledge and inaccurate measurements.
An unexplained phenomenon: Retrograde Motion
Ptolemy (c100AD-170)
Real name: Claudius Ptolemaeus reflects a Roman and Egyptian (Greek) heritage.
The last Greek astronomy of antiquity.
Wrote many works including:
Tetrabiblos – astrology.
Guide to Geography – mapmaking and maps, authoritative until the age of discovery.
The Mathematical Collection (aka. Almagest) – the motion of planets and the layout of the universe.
Almagest.
A geometric representation of planetary motions within the solar system which had considerable accuracy.
Conforms to the look of the world, which is why it is still the one used in practical courses in navigation.
Formulated to “save the appearances.”
An extension of the star catalog of Hipparchus supplemented with his own observations.
A detailed discussion of whether the earth is fixed in space with the conclusion that it must be stationary.
Explained the movements of the planets using the basic concept of uniform motion in a circle, together with mathematical devices proposed in the third century BC by Apollonius of Perga.
“Deferents” – large circles.
“Epicycles” – small circles.
A movable eccentric (a large movable circle).
Successfully accounted for retrograde motion of the planets. Ptolemy solved the problem by having a planet revolve in an epicyclic orbit about “C.” The center of the epicycle C in turn revolved in the deferent about the earth. When the planet is at the position x, it is moving in its epicyclic orbit in the same direction as the point C moves around the earth, and the planet appears to be moving eastward. When the planet is at y, however, its epicyclic motion is in the opposite direction to the motion of C. By choosing the right combination of speeds and distances, Ptolemy succeeded in having the planet move westward at the right speed at y and for the correct interval of time. However, because the planets, as does the earth, travel about the sun in elliptical orbits, their actual behavior cannot be represented accurately by so simple a scheme of uniform circular motions. Consequently, Ptolemy made the deferent an eccentric, centered not on the earth, but slightly away from the earth, “A”. Furthermore, he had the center of the epicycle, C, move at a uniform angular rate, not around A, or the Earth, but at point “B,” called the equant, on the opposite side of A from the earth. (Abell, pp. 28-29)
He made no claim that his cosmological model described reality.
Rather, it was intended that the scheme be a mathematical (geometric) representation to predict the positions of the planets at any time.
Nonetheless, his model represented a persuasive synthesis of all Greek astronomical knowledge.
It exerted a profound and lasting (with some modifications) influence on all subsequent generations of astronomers.
It went unchallenged for 1300 years.
Ptolemy’s rationalization.
Admits that it is quite “plausible” to suppose “the heavens immobile and the earth turning on the same axis from east to west very nearly on revolution a day….As far as the appearances of the stars are concerned, nothing would keep things from being in accordance with this simpler conjecture.”
But, the above supposition is “absolutely absurd” given:
The speed and direction of the motions of bodies within the earth’s own atmosphere; and
The gross non-conformity to Aristotelian physics.
Explains the irony of his own non-conformity to Aristotelian physics (the use of imperfect circular motion.
He defends his theory on the ground that ASTRONOMY, being mathematical rather than physical, could admit such “unrealistic” complications if they served the purposes of calculation and of “saving the appearances.” (i.e., the ends justify the means.)
From the thirteenth and last book of Almagest:
“Let no one, seeing the difficulty of our devices, find troublesome such hypothesis…. It is proper to try and fit as far as possible the simpler hypothesis to the movements of the heavens; and if this does not succeed, then any hypotheses possible. Once all the appearances are saved by the consequences of the hypotheses, why should it seem strange that such complications can come about in the movements of heavenly things?”
Rather than judge the simplicity of heavenly things by comparison with what seems to be simple in the explanation of earthly phenomena, “We should instead judge their simplicity from the unchangeableness of the natures in the heavens and their movements. For thus they would all appear simple, more than those things which seem so here with us.”
In other words, simplicity is relative to the particular sphere with which one is concerned.
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