Aristotle (384-322 BC) declared Thales of Miletus (c.624-c.546 BC) to be the first philosopher because he sought to replace mythological interpretations of the Universe with rational explanations. Thales believed that all things originate from and return to water, and that Earth is a flat disc floating on an infinite ocean from which Thales apparently deduced that earthquakes occur when Earth is rocked by waves. The story that he predicted within a year the solar eclipse that occurred during the Battle of the Halys River (28 May 585 BC) between the Medes and the Lydians is ancient, but its basis is doubtful. His emergence nevertheless marks the start of the period when oriental myth and science began to be transformed into Greek geometry, astronomy and cosmology.
Anaximander of Miletus (c.610-c.546 BC) held that the abstract apeiron (indefinite, infinite, boundless, unlimited) as the origin of the Universe. He conceived the world as being subject to a rule of law that imposes a balance upon warring opposites. The world process thus begins when opposites are separated out to generate the hot and the cold, the dry and the wet. Earth, sea and sky take shape and huge wheels of fire are formed to produce the Sun, Moon and stars. He believed Earth to be a cylinder with a height one-third of its diameter, at rest at the centre of the infinite, equally balanced in every direction. He revolutionised astronomy by treating the paths of the Sun and Moon as great circles passing beneath Earth. He is credited with introducing the gnomon into Greece and with being the first to draw a map of Earth. He also suggested an evolutionary theory that had the first human beings generated from a sort of embryo floating in the sea.
Anaximenes of Miletus (c.585-c.528 BC) also composed a world system based on an infinite or unlimited principle, which he identified as air. The cosmos consists of air, which when rarefied becomes fire, and when condensed becomes progressively wind, cloud, water, earth and stone. From these lesser divinities all other things are formed. He believed that Earth was at the centre of things and that the Sun and Moon had been formed out of fire. Anaximenes’ astronomy is retrograde from Anaximander’s, but his theory of condensation and rarefaction had a lasting influence.
Pythagoras (c.570-c.495 BC) was born in Samos, but later (c.531 BC) he founded a religious community in Croton, a Greek colony in southern Italy. The brotherhood seems to have formed a ruling oligarchy for after twenty years a conspiracy under Cylon, a nobleman from Croton itself, led to Pythagoras’ retirement to Metapontum, where he later died.
None of Pythagoras’ work has survived, but much has been attributed to him and his followers. It is unclear whether Pythagoras himself or a later follower discovered the theorem that bears his name. However, it does seem likely that Pythagoras himself discovered the numerical ratios determining the principal intervals of the music scale, which led to his hypothesis that ‘all things are number’.
The only reliable pre-Platonic account of Pythagorean thought is that of Philolaus (c.470-c.385 BC), who is credited with originating the theory that together with the fixed stars, Sun, Moon and the five (visible) planets, Earth (which thereby becomes a planet) orbits a hypothetical central fire.
Heraclitus of Ephesus (c.535-c.475 BC) conceived the world to be a ceaseless conflict of pairs of opposites (plenty and hunger, life and death, etc.) regulated by a natural law (logos), which he equated with transcendent wisdom and with his primary cosmic constituent, fire.
Parmenides of Elea (c.515-c.440 BC), unlike Heraclitus who held everything is in a state of flux, denied all motion and change and concluded what exists must be single, indivisible and unchanging and that appearances to the contrary are delusions of the senses.
Zeno of Elea (c.490-430 BC) defended Parmenides’ arguments and reduced opposing hypotheses of others to absurdity by deducing contradictory consequences from them. He attacked belief in motion in this way; a famous example of his method is that of Achilles (hare) and the tortoise. He also included a number of paradoxes showing the difficulties with the assertion that a line can be infinitely subdivided. His intention, however, could have been merely to question the understanding of such phenomena.
Anaxagoras of Clazomenae (c.510-c.428 BC), the first philosopher to reside in Athens, sought a pluralist solution to circumvent Parmenides’ arguments. He denied an original unity and postulated a plurality of eternal, different substances that filled the whole of space. He introduced a motive force, the action of mind, unmixed with any other substance but capable of ordering and controlling them.
He held that matter was infinitely divisible, i.e. that however far it is divided it always contains distinct portions of every other substance in the same ratio. In cosmology he believed the world was flat and the Sun was a hot and glowing stone. He was prosecuted for the latter as it was an affront to the divinity of the heavens. He escaped to Lampsacus in northern Troad where he founded a school and was buried with high honours and esteem.
Empedocles of Acragas (c.492-c.432 BC) postulated four ‘roots’ or elements – earth, air, fire and water – of which, variously blended and compounded, all material substances are composed. The four elements move under the influence of love (which brings together unlike particles of the elements) and strife (which joins like to like). The influence of love and strife alternates. When love is supreme, the world is a homogeneous whole, when strife conquers, the elements are completely separated. The history of the cosmos is therefore cyclical. Empedocles’ four-element theory dominated natural science for two millennia.
Important to the development of astronomy in Greece was the transfer of knowledge from Mesopotamia. It is not known precisely when this happened but Babylonian influences were present in the work of Meton of Athens, who is dated by his observation of the summer solstice on 27 June 432 BC.
Heraclides of Pontus (c.387-c.312 BC) was born at Heraclea on the Black Sea. He joined the Academy and although he was apparently a pupil of Plato (427-347 BC), he also studied with Aristotle and with Speusippus (c.409-c.339 BC), Plato’s successor as head of the Academy. Defeated by Xenocrates of Chalcedon (c.395-c.314 BC) for the headship after Speusippus’ death, Heraclides returned to Heraclea. Only fragments of his writings survive but they suggest that he had wide interests. In astronomy he is credited with being the first to suggest that the apparent rotation of the sky is caused by the axial rotation of Earth, and with an attempt to explain the apparent movement of Venus in which some scholars have seen a theory that Venus and Mars revolve around the Sun.
Aristotle (384-322 BC), son of Nicomachus, a court physician to Amyntas III of Macedon (r.c.393-370 BC), was born at Stagira and may have spent part of his boyhood at the court of Pella. He entered the Academy at seventeen and at Plato’s death in 347 BC he accepted an invitation from a former fellow student Hermias (r.c.350-c.345 BC), tyrant of Assos and Atarneus in Mysia, and went to Assos. On Hymeas’ downfall and death, Aristotle went to Mytilene on Lesbos. In 343/2 BC he was invited by Philip II of Macedon (46; r.359-336 BC) to Pella as tutor to his son, the future Alexander III the Great (32; r.336-323 BC). He returned to Athens in 335 BC and began teaching at the Lyceum. When Alexander died there was a burst of anti-Macedonian feeling. Deprived of the protection of his former pupil, Aristotle left Athens and took refuge at Chalcis on the island of Euboea, where he later died.
Aristotle’s works cover every branch of philosophy and science known in his day. In his On the Heavens he defends the doctrine that Earth is a sphere, one of his arguments being that when there is an eclipse of the Moon the shadow cast by Earth on the Moon is always curved. He informs us that mathematicians had estimated the circumference of Earth to be 400,000 stadia. If one stade ≈200 yd (Pliny) then Aristotle’s figure is about twice that of the modern measurement.
Aristarchus of Samos (c.310-c.230 BC) taught at Alexandria. In his On the Size and Distances of the Sun and Moon he attempts to determine their distances and dimensions. During a lunar eclipse he saw that the breadth of Earth’s shadow on the Moon is about the width of two Moons (actually the ratio is nearer to 2.67). The angle of Earth’s shadow equates to the Sun’s angular diameter (≈0.5º), so the Moon’s radius is about one quarter (≈1/3.67≈0.27) of Earth’s radius (=R) and is ≈0.27R/tan0.25º ≈62R distant; and Earth’s shadow (umbra) extends ≈R/tan0.25º ≈230R.
When the Moon is half full it is ‘at quadrature’, i.e. the angle Earth-Moon-Sun is a right angle. Measuring the angle Moon-Earth-Sun thus gives the shape of the triangle joining the three bodies and hence the ratio of any two sides. This angle is difficult to determine accurately. Aristarchus gives it to be 3º short of a right angle: the true difference is ≈1/20th of this.
Although his conclusion that the Sun is nineteen times further off than the Moon (the modern ratio is ≈390:1) and therefore nineteen times larger than the Moon (the modern ratio is ≈400:1) has a large error, this was an important first step towards an appreciation of the size of the Universe. Using Eratosthenes’ calculation of Earth’s circumference, Aristarchus obtained a distance to the Moon in stades, but his estimate of the Sun’s distance was of course widely inaccurate.
Aristarchus was the first to suggest the heliocentric hypothesis placing the Sun at the centre of Earth’s orbit and the stars on a sphere at a great distance from the Sun. Hellenistic astronomers rejected this proposal because they thought that some parallactic displacement of the stars would be apparent.
Eratosthenes of Cyrene (c.275-c.194 BC) spent several years at Athens before accepting the invitation of Ptolemy III Euergetes-I (c.59; r.246-221 BC) to become the royal tutor and succeed Apollonius of Rhodes (c.295-c.215 BC) as head of the Alexandrian Library. Eratosthenes’ fame rests on his attempt to calculate the circumference of Earth.
He knew that at Syene (=Aswan) on the day of the summer solstice, a shaft of sunlight penetrated to the bottom of a well, i.e. a vertical stick there would cast no shadow. On a midsummer’s day in Alexandria he measured the angle of the shadow cast by a vertical stick at noon and found it to be 1/50th of a circle. Believing Alexandria to be 5000 stades due north of Syene, Eratosthenes measured the circumference of Earth as 50×5000 stades, or 250,000 stades. Although the modern equivalent of a stade is debated, Eratosthenes’ measurement fits well with our modern value of 40,075 km (24,901 miles).
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