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Home , Almagest, Spherical Earth

(624-546 BC)

• First in western civilization to explain natural phenomena using scientific philosophy (hypotheses involving other natural processes) rather than mythology • Story that he predicted solar in 585; unclear if true • Proposed model with disk-shaped floating in cosmic ; departure from strictly divine explanation (610-546 BC) • Student of Thales • Assumed existence of a cosmic order • Divine control of of reality • Earth is cylindrical & floats in the infinite without support • Celestial objects can move around and under Earth • , , are holes in invisible wheels through which we see of (600 BC)

• According to (440 BC), Phoenician sailors circumnavigated Africa during 3 year journey • Reported that Sun appeared in the northern sky, so Herodotus was skeptical that it really happened Evidence for

No account of how spherical shape of Earth first deduced, but following evidence began to be cited: • Ships sink below the rather than just shrinking • Different visible when traveling north and south • Curved shape of Earth’s shadow on Moon during

• Columbus set out to prove world is round? No, that’s a myth from a fictional biography of Columbus by Washington Irving (570-495 BC) • Said to be first to teach that Earth is spherical; universally accepted among by 400 BC • Believed in the “harmony of the spheres”: planets and stars moving according to mathematical equations corresponding to musical notes • Sun, Moon, stars, 5 planets attached to 8 transparent spheres centered on Earth • 7 muses were 7 “planets” singing together (5 planets, Sun, Moon) • When asked why humans exist: “to observe the heavens” (470-385 BC) • Successor of Pythagoras • He (or another Pythagorean) proposed that Earth & other celestial objects move around an unseen central fire (427-347 BC) • Student of Sacrates; teacher of Eudoxus and • Proponent of (Earth is stationary and center of ) • Model was mythical/mystical rather than mathematical • Stars and planets carried around spherical Earth on concentric transparent spheres in uniform circular motion • Described universe as “Spindle of Necessity” that is attended by the Sirens and turned by the 3 Fates • Posed question to students: how can uniform circular motion account for observed irregular motions of planets? Eudoxus (390-337 BC) • First description of most of the 48 classical , originating from Babylonians, Sumarians, and others • First attempt at mathematical model to explain non-uniform motion of planets (more myth-free than Plato’s model) • Geocentric model with 27 spheres: Sun (3), Moon (3), each (4), stars (1) Aristotle (384-322 BC) • Evidence cited by Aristotle and others for stationary Earth (geocentric model) • Doesn’t feel like Earth is moving • didn’t seem to change brightness (fixed distance) • Absence of among the stars = test of hypothesis • Increased number of spheres in geocentric model (55) to better match motions of planets Parallax is the apparent motion of a nearby object relative to a distant object due to the changing position of the observer.

Alexandria • Alexander the Great conquers Persia in 330’s BC • Greeks gain access to Babylonian astronomical records • Additional across longer time periods revealed errors in models of planetary motions; more modifications needed • and its great library established (310-230 BC) • First serious attempt at measuring relative distances of Sun and Moon, and hence relative sizes of Earth, Sun, Moon • Concluded that the Sun is larger than Earth, leading him to propose that the Sun is stationary rather than Earth (heliocentric model) • Proposed that stars are objects like the Sun, and that they don’t exhibit parallax because they are very far away (276-194 BC) • First measurement of Earth’s (190-120 BC) • Greatest observer in antiquity • Created the first catalog (850 stars) • Finalized the 48 classical constellations • Measured distance to Moon via its parallax • Discovered of the • Measured difference between sidereal/tropical years • Developed methods for predicting • Supported geocentric model because better reproduced motions of planets; also difficult to accept huge distances of stars needed to explain absence of parallax (100-170 AD) • Most influential astronomer in history • Only surviving ancient on astronomy () • Further refined the geocentric model of Hipparchus • Adopted epicyles in model to explain retrograde motion and improve predictions of planetary motion • His model predicted accurate positions of the planets and was widely accepted until the • Unclear if his model was intended as a representation of reality or only as a calculation tool Augustine (354-430) Here's what Augustine wrote in his Literal Commentary on Genesis: "Usually, even a non-Christian knows something about the earth, the heavens, and the other elements of this world, about the motion and orbit of the stars and even their size and relative positions, about the predictable eclipses of the sun and moon, the cycles of the years and the seasons, about the kinds of animals, shrubs, stones, and so forth, and this knowledge he holds as certain from reason and experience. Now it is a disgraceful and dangerous thing for an infidel to hear a Christian, presumably giving the meaning of Holy Scripture, talking nonsense on these topics; and we should take all means to prevent such an embarrassing situation, in which people reveal vast ignorance in a Christian and laugh that ignorance to scorn." Retrograde Motion Retrograde Motion Epicycles Islamic Astronomy • During 7th century, Muhammad and his followers conquered a region from North Africa to India • Greek and Indian astronomical texts translated to , and preserved during European decline in • Islamic astronomers studied, refined, questioned concepts from Ptolemy, but maintained geocentric nature Copernicus (1473-1543) • Puzzled by the fact that Ptolemy’s model predicted the Moon’s distance (and size in the sky) would vary by a factor of 2, which wasn’t observed • Proposed heliocentric model: • Earth & other planets orbit the Sun in perfect circles • Only the Moon orbits Earth • Daily motion of heavens due to Earth’s rotation • Yearly motion of Sun across the stars due to Earth’s orbit • Retrograde motion due to parallax • Still needed epicycles to reproduce motions of planets • Cited influence of Pythagorus and Islamic astronomers; unclear if familiar with heliocentric model of Aristarchus

According to Copernicus, retrograde motion is produced by parallax as Earth passes by a planet.

The heliocentric model naturally explains why some planets are always close to the Sun in the sky (smaller orbits than Earth) while other planets can appear either close to the Sun or on the opposite side of the sky (larger orbits than Earth). According to Copernicus, the motion of the Sun through the constellations during the year is a due to our changing point of view as Earth orbits the Sun. Opposition to Copernican Model • Opposition to the Copernican model developed only slowly over several decades • Biblical passages cited for opposition: – “The world is established. It can not be moved” – “He (the Lord) laid the foundations of earth, that it should not be moved forever” – Joshua’s prayers causing the Sun and Moon to stand still • Luther: ”So it goes now. Whoever wants to be clever must agree with nothing that other esteem. He must do something of his own. This is what that fellow does who wishes to turn the whole of astronomy upside down.” • Vatican eventually cited physical and mathematical reasons as well, some from Tycho Brahe Tycho Brahe (1546-1601) • Some of the most accurate non-telescopic observations • No measurable parallax for indicated it was in the realm of stars, but that was supposed to be unchanging • No measurable parallax for a comet, so not in atmosphere • No measurable parallax for stars, so stars >7000 times more distant than Sun if heliocentric model correct • Proposed hybrid of geocentric and heliocentric models