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Home , Parallel postulate, Ptolemy

Saving the Heavens

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Logic at its Best

• Where and agreed was over the role of reason and precise logical thinking. • Plato: From abstraction to new abstraction. • Aristotle: From empirical generalizations to unknown truths.

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Mathematical Reasoning

• Plato’s Academy excelled in training . • Aristotle’s Lyceum excelled in working out logical systems. • They came together in a great mathematical system.

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1 The Structure of Civilization • Ancient Greek civilization is divided into two major periods, marked by the death of .

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Hellenic Period

• From the time of to the death of Alexander is the Hellenic Period, 800-323 BCE. – When the written Greek language evolved. – When the major literary and philosophical works were written. – When the Greek colonies grew strong and were eventually pulled together into an empire by Alexander the Great.

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Hellenistic Period

• From the death of Alexander to the annexation of the Greek peninsula into the , and then on with diminishing influence until the fall of Rome. • 323 BCE to 27 BCE, but really continuing its influence until the 5th century CE.

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2 in the Hellenistic Age

• The great philosophical works were written in the Hellenic Age. • The most important scientific works from came from the Hellenistic Age.

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Alexandria,

• Alexander the Great conquered Egypt, where a city near the mouth of the Nile was founded in his honour. • Ptolemy Soter, Alexander’s general in Egypt, established a great center of learning and research in : The Museum.

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The Museum

• The Museum – temple to the – became the greatest research centre of ancient times, attracting scholars from all over the ancient world. • Its centerpiece was the Library, the greatest collection of written works in antiquity, about 600,000 papyrus rolls.

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3

• Euclid headed up mathematical studies at the Museum. • Little else is known about his life. He may have studied at Plato’s Academy.

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Euclid’s Elements

• Euclid is now remembered for only one work, called The Elements. • 13 “books” or volumes. • Contains almost every known mathematical , with logical proofs.

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The Influence of the Elements

• Euclid’s Elements is the second most widely published book in the world, after the Bible. • It was the definitive and basic textbook of mathematics used in schools up to the early 20th century.

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• What makes Euclid’s Elements distinctive is that it starts with stated assumptions and derives all results from them, systematically. • The style of argument is Aristotelian logic. • The subject matter is Platonic forms.

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Axioms, 2

• The axioms, or assumptions, are divided into three types: – Definitions – Postulates – Common notions • All are assumed true.

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Definitions

• The definitions simply clarify what is meant by technical terms. E.g., –1. A point is that which has no part. –2. A is breadthless length. – 10. When a straight line set up on a straight line makes the adjacent equal to one another, each of the equal angles is right, and the straight line standing on the other is called a to that on which it stands. … – 15. A circle is a plane figure contained by one line such that all the straight lines falling upon it from one point among those lying within the figure are equal to one another.

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5 Postulates

• There are 5 postulates. • The first 3 are “construction” postulates, saying that he will assume that he can produce (Platonic) figures that meet his ideal definitions: – 1. To draw a straight line from any point to any point. – 2. To produce a finite straight line continuously in a straight line. – 3. To describe a circle with any centre and distance.

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Postulate 4

• 4. That all right angles are equal to one another. • Note that the equality of right angles was not rigorously implied by the definition. – 10. When a straight line set up on a straight line makes the adjacent angles equal to one another, each of the equal angles is right…. – There could be other right angles not equal to these. The postulate rules that out.

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The Controversial Postulate 5

d a e c b fg

• 5. That, if a straight line falling on two straight lines make the interior angles on the same side less than two right angles, the two straight lines, if produced indefinitely, meet on that side on which are the angles less than the two right angles.

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6 The “” Postulate

d a e c b fg

• One of Euclid’s definitions was that lines are parallel if they never meet. • Postulate 5, usually called the , gives a criterion for lines not being parallel.

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The Common Notions

• Finally, Euclid adds 5 “common notions” for completeness. These are really essentially logical principles rather than specifically mathematical ideas: – 1. Things which are equal to the same thing are also equal to one another. – 2. If equals be added to equals, the wholes are equal. – 3. If equals be subtracted from equals, the remainders are equal. – 4. Things which coincide with one another are equal to one another. – 5. The whole is greater than the part.

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An Axiomatic System

• After all this preamble, Euclid is finally ready to prove some mathematical propositions. • The virtue of this approach is that the assumptions are all laid out ahead. Nothing that follows makes further assumptions.

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7 Axiomatic Systems

• The assumptions are clear and can be referred to. • The deductive arguments are also clear and can be examined for logical flaws. • The truth of any proposition then depends entirely on the assumptions and on the logical steps. • And, the system builds. Once some propositions are established, they can be used to establish others. – Aristotle’s methodology applied to mathematics.

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Building Knowledge with an Axiomatic System • Generally agreed upon premises ("obviously" true) • Tight logical implication • Proofs by: – 1. Construction – 2. Exhaustion –3. Reductio ad absurdum (reduction to absurdity) • -- assume a premise to be true • -- deduce an absurd result

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Example: Proposition IX.20

• There is no limit to the of prime • Proved by – 1. Constructing a new number. – 2. Considering the consequences whether it is prime or not (). – 3. Showing that there is a contraction if there is not another prime number. (reduction ad absurdum).

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8 Proof of Proposition IX.20

• Example 1: {2,3,5} • Given a set of prime numbers, • Q=2x3x5+1 =31

{P1,P2,P3,...Pk} • Q is prime, so the original set was not complete. • 1. Let Q = P1P2P3...Pk + 1 (Multiply them all together and 31 is not 2, 3, or 5 add 1) • 2. Q is either a new prime or a composite • Example 2: {3,5,7} • Q=3x5x7+1 =106 • 3. If a new prime, the given set • Q is composite. of primes is not complete.

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Proof of Proposition IX.20

• 4. If a composite, Q must be • Q=106=2x53. divisible by a prime number. • Let G=2. • -- Due to Proposition VII.31, • G is a new prime previously proven. (not 3, 5, or 7). • -- Let that prime number be G. • If G was one of 3, • 5. G is either a new prime or one of 5, or 7, then it the original set, {P1,P2,P3,...Pk} would be divisible • 6. If G is one of the original set, it is into 3x5x7=105. divisible into P1P2P3...Pk If so, G is • But it is divisible also divisible into 1, (since G is into 106. divisible into Q) • Therefore it would • 7. This is an absurdity. be divisible into 1. • This is absurd.

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Proof of Proposition IX.20

• Follow the absurdity backwards. • Trace back to assumption (line 6), that G was one of the original set. That must be false. • The only remaining possibilities are that Q is a new prime, or G is a new prime. • In any case, there is a prime other than the original set. – Since the original set was of arbitrary size, there is always another prime, no matter how many are already accounted for.

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9 Euclid’s Elements at work

• Euclid’s Elements quickly became the standard text for teaching mathematics at the Museum at Alexandria. • Philosophical questions about the world could now be attacked with exact mathematical reasoning.

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Eratosthenes of Cyrene

• 276 - 194 BCE • Born in Cyrene, in (now in Lybia). • Studied at Plato’s Academy. • Appointed Librarian at the Museum in Alexandria.

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“Beta”

• was prolific. He worked in many fields. He was a: – Poet – Historian – – – Geographer • He was nicknamed “Beta.” – Not the best at anything, but the second best at many things.

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10 Eratosthenes’

• He coined the word “” and drew one of the first of the world (above).

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Using Euclid

• Eratosthenes made very clever use of a few scant observations, plus a theorem from Euclid to decide one of the great unanswered questions about the world.

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His

• Eratosthenes had heard that in the town of Syene (now ) in the south of Egypt, at noon on the summer (June 21 for us) the was directly overhead. – I.e. A perfectly upright pole (a ) cast no shadow. – Or, one could look directly down in a well and see one’s reflection.

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11 His data, 2

• Based on reports from on a heavily travelled trade route, Eratosthenes calculated that Alexandria was 5000 stadia north of Syene.

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His data, 3

• Eratosthenes then measured the formed by the sun’s rays and the upright pole (gnomon) at noon at the solstice in Alexandria. (Noon marked by when the shadow is shortest.) • The angle was 7°12’.

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Proposition I.29 from Euclid

b a a b

b a a b

A straight line falling on parallel straight lines makes the alternate angles equal to one another, the exterior angle equal to the interior and opposite angle, and the interior angles on the same side equal to two right angles.

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12 • Eratosthenes reasoned that by I.29, the angle produced by the sun’s rays falling on the gnomon at Alexandria is equal to the angle between Syene and Alexandria at the centre of the Earth.

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Calculating the size of the Earth

• The angle at the gnomon, α, was 7°12’, therefore the angle at the centre of the Earth, β, was is also 7°12’ which is 1/50 of a complete circle. • Therefore the circumference of the Earth had to be stadia 7°12’ x 50 = 360° = 250,000 stadia. 50 x 5000 = 250,000

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Eratosthenes’ working assumptions • 1. The Sun is very far away, so any coming from it can be regarded as traveling in parallel lines. • 2. The Earth is a perfect . • 3. A vertical shaft or a gnomon extended downwards will pass directly through the center of the Earth. • 4. Alexandria is directly north of Syene, or close enough for these purposes.

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13 A slight correction

• Later Eratosthenes made a somewhat finer observation and calculation and concluded that the circumference was 252,000 stadia. • So, how good was his estimate. – It depends….

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What, exactly, are stadia?

• Stadia are long measures of length in ancient times. • A stade (singular of stadia) is the length of a stadium. – And that was…?

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Stadium lengths

• In Greece the typical stadium was 185 metres. • In Egypt, where Eratosthenes was, the stade unit was 157.5 metres.

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14 Comparative figures Circumference Stade Length In Stadia In km 157.5 m 250,000 39,375 157.5 m 252,000 39,690 185 m 250,000 46,250 185 m 252,000 46,620

Compared to the modern figure for polar circumference of 39,942 km, Eratosthenes was off by at worst 17% and at best by under 1%.

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An astounding achievement

• Eratosthenes showed that relatively simple mathematics was sufficient to determine answers to many of the perplexing questions about nature.

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Hipparchus of

• of Rhodes • Became a famous astronomer in Alexandria. • Around 150 BCE developed a new tool for measuring relative distances of the from each other by the visual angle between them.

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15 The Table of Chords

• Hipparchus invented the table of chords, a list of the ratio of the size of the of a circle to its radius associated with the angle from the centre of the circle that spans the chord. • The equivalent of the function in .

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Precession of the equinoxes

• Hipparchus also calculated that there is a very slow shift in the heavens that makes the solar year not quite match the siderial (“”) year. – This is called precession of the equinoxes. He noted that the equinoxes come slightly earlier every year. – The entire cycle takes about 26,000 years to complete. • Hipparchus was able to discover this shift and to calculate its duration accurately, but the ancients had no understanding what might be its cause.

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The Problem of the , again

• 300 years after Hipparchus, another astronomer uses his calculating devices to create a complete system of the heavens, accounting for the weird motions of the planets. • Finally a system of geometric motions is devised to account for the positions of the planets in the sky mathematically.

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16 Claudius Ptolemy

• Lived about 150 CE, and worked in Alexandria at the Museum.

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Ptolemy’s Geography

• Like Eratosthenes, Ptolemy studied the Earth as well as the heavens. • One of his major works was his Geography, one of the first realistic atlases of the known world.

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The

• Ptolemy’s major work was his Mathematical Composition. • In later years it was referred as The Greatest (Composition), in Greek, Megiste. • When translated into it was called al Megiste. • When the work was translated into and later English, it was called The Almagest.

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17 The Almagest, 2

• The Almagest attempts to do for what Euclid did for mathematics: – Start with stated assumptions. – Use logic and established mathematical to demonstrate further results. – Make one coherent system • It even had 13 books, like Euclid.

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Euclid-like assumptions

1. The heavens move spherically. 2. The Earth is spherical. 3. Earth is in the middle of the heavens. 4. The Earth has the ratio of a point to the heavens. 5. The Earth is immobile.

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Plato versus Aristotle

• Euclid’s assumptions were about mathematical objects. – Matters of definition. – Platonic forms, idealized. • Ptolemy’s assumptions were about the physical world. – Matters of judgement and decision. – Empirical assessments and common sense.

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18 Ptolemy’s

• The basic framework of Ptolemy’s view of the is the ’ two-sphere model: – Earth in the center, with the four elements. – The at the outside, holding the and making a complete revolution once a day. • The seven wandering stars—planets—were deemed to be somewhere between the Earth and the celestial sphere.

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The Eudoxus-Aristotle system for the Planets • In the system of Eudoxus, extended by Aristotle, the planets were the visible dots embedded on nested rotating spherical shells, centered on the Earth.

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The Eudoxus-Aristotle system for the Planets, 2 • The motions of the visible were the result of combinations of circular motions of the spherical shells. – For Eudoxus, these may have just been geometric, i.e. abstract, paths. – For Aristotle the spherical shells were real physical objects, made of the fifth element.

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19 The Ptolemaic system

• Ptolemy’s system was purely geometric, like Eudoxus, with combinations of circular motions. – But they did not involve centered on the Earth. – Instead they used a device that had been invented by Hipparchus 300 years before: Epicycles and Deferents.

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Epicycles and Deferents

• Ptolemy’s system for each planet involves a large (imaginary) circle around the Earth, called the deferent, on which revolves a smaller circle, the epicycle. • The visible planet sits on the edge of the epicycle. • Both revolve in the same direction.

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Accounting for Retrograde

• The combined motions of the deferent and epicycle make the planet appear to turn and go backwards against the fixed stars.

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20 Saving the Appearances

• An explanation for the strange apparent motion of the planets as “acceptable” motions for perfect heavenly bodies. – The planets do not start and stop and change their minds. They just go round in circles, eternally.

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How did it fit the facts?

• The main problem with Eudoxus’ and Aristotle’s models was that they did not track that observed motions of the planets very well. • Ptolemy’s was much better at putting the planet in the place where it is actually seen.

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But only up to a point….

• Ptolemy’s basic model was better than anything before, but still planets deviated a lot from where his model said they should be. • First solution: – Vary the relative sizes of epicycle, deferent, and rates of motion.

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21 Second solution: The Eccentric

• Another tack: • Move the centre of the deferent away from the Earth. • The planet still goes around the epicycle and the epicycle goes around the deferent.

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Third Solution: The Point

• The most complex solution was to define another “centre” for the deferent. • The equant point was the same distance from the centre of the deferent as the Earth, but on the other side.

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Third Solution: The Equant Point, 2

• The epicycle maintained a constant distance from the physical centre of the deferent, while maintaining a constant angular motion around the equant point.

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22 Ptolemy’s system worked • Unlike other , Ptolemy actually could specify where in the sky a star or planet would appear throughout its cycle – within acceptable limits. • He “saved the appearances.” – He produced an abstract, mathematical account that explained the sensible phenomena by reference to Platonic forms.

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But did it make any sense?

• Ptolemy gave no reasons why the planets should turn about circles attached to circles in arbitrary positions in the sky. • Despite its bizarre account, Ptolemy’s model remained the standard cosmological view for 1400 years.

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