Ancient Greek Astronomy and Cosmology Lecture Notes

Lesson 3: Ancient Greek Astronomy and Cosmology Introduction to Ancient Greek Astronomy: ● Scientific thought emerged in Ancient Greece owing to the region's fragmented geography and decentralized rule Geocentric & heliocentric models were conceived of, but the popularity of Plato & Aristotle led to ~2000 years of geocentric thought The Ionian Philosophers: Thales: - "Father of Science" - sought explanations for natural phenomena that didn't involve gods - gained support by demonstrating that certain acts of nature (eg. eclipses) can be predicted Anaximander: - set Earth "afloat" - developed 1st moving model of the Universe, with celestial bodies on spinning Wheels around Earth The Pythagoreans: Pythagoreanism: - the Universe is a cosmos (ie, a harmonious system that obeys knowable laws) - Earth is a sphere & all celestial motion is perfectly circular • Evidence that Earth is a sphere: 1. gradual, bottom-first disappearance of ships on the horizon 2. Earth's round shadow on the eclipsed moon 3. when traveling North/South, constellations shift more rapidly than if Earth were flat The Heliocentrism Philolaus: - set Earth in motion in a daily orbit around a "central fire" Herakleides: - with no evidence of central fire, set Earth into a daily spin - placed Mercury & Venus in solar orbit to explain their apparent Sun-centred motion and changing brightness Aristarchus: - used Earth's shadow on the eclipsed moon to determine: that the Earth is larger than the Moon - used the angle between the Sun & Moon to determine: that the Sun is further away than the Moon - led him to propose a heliocentric model of the Universe, with only the Moon in orbit around Earth Understanding the Motion of the Planets Features of apparent planetary motion (ie, the motion of planets we see across the sky): - both direct (forward) and retrograde (backward) motion changing apparent speeds changing brightness Plato and Eudoxus - (right-hand branch) Plato: - Theories of nature should be accepted on faith - All celestial bodies are perfect spheres with constant circular motion (CCM) Philosophers after Plato sought to "save the appearances" with a CCM model Eudoxus: - placed the planets on systems of invisible nested spheres ("crystal orbs"), each with CCM around Earth Aristotle: - Aristotle sought to match theory with observation, but available measuring techniques led him to wrong conclusions The Aristotelian model: - The universe is geocentric (Earth is fixed) - terrestrial realm (Earth) and celestial realm (heavens) have different laws of nature - terrestrial motion due to the natural tendencies of the four elements (earth. water. air and fire) - celestial bodies are perfect spheres composed of a pure and imperishable substance ("quintessence"/"ether") - celestial motion follows the Eudoxan CCM model with all daily motion is driven by a single outer orb - the motion of orbs driven by a 'Prime Mover' (a spiritual entity residing in the outermost orb) transient bodies in the sky (eg, comets) must be atmospheric Apollonius: Apollonius: - formulated the epicycle model (geocentric CCM model accepted for 1500+ years) - explains planets' retrograde motion, changing speeds and changing brightness Hipparchus: Hipparchus: - compiled one of the earliest comprehensive star catalogs (~1000 stars) - refined Apollonius' model to improve the accuracy of the Sun and Moon's motion By observing a difference in the solar year vs the sidereal year, Hipparchus discovered the precession of the equinoxes: - causes the Sun's coordinates on a given date to shift by ~0.01%/yr causes star coordinates on a given date to shift eastward (we are losing our Pole star!) used to determine the precise positions of the Sun & stars in the future or past (eg, Stonehenge, Hipparchus lost star catalog) Ptolemy: Ptolemy: - author of The Almagest ("The Great Treatise"), the definitive astronomy textbook for ~1500 years - improved the accuracy of the geocentric epicycle model by tying the motion of the planets to the Sun Ptolemaic model: - geocentric epicycle model with circular orbits - a planet's orbital speed is governed by its equant point (allows non-constant motion in space) - motion of inner and outer planets directed differently by the Sun (Explains differences in their paths across the sky) - planets (correctly) ordered by decreasing apparent speed (but sun & Earth incorrectly switched) no space between adjacent orbits led to the "Universe's" size of ~10,000 Earth diameters (~15x too small, but proved huge distances to stars)