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Astronomy Notes Week 5

by: Erin Bleck

Astronomy Notes Week 5 EESC1150

Erin Bleck
Harvard University
GPA 3.91

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About this Document

These notes cover what was discussed during week 5 for Astronomy at Boston College taught by Professor Kuchar.
Dr. Thomas Kuchar
Class Notes
GeocentricModel, philosophy, PythagoreanModel, Prograde/RetrogradeMotion, PtolemaicModel
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This 4 page Class Notes was uploaded by Erin Bleck on Tuesday July 19, 2016. The Class Notes belongs to EESC1150 at Boston College taught by Dr. Thomas Kuchar in Summer 2016. Since its upload, it has received 8 views. For similar materials see Astronomy in Earth and Environmental Sciences at Boston College.

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Date Created: 07/19/16
Notes February 18, 2016 History - Modern science and its ancient Greek roots o Greeks were the first people known to make models of nature o They tried to explain the patterns in nature without resorting to myth or the supernatural o Greek geocentric model (c. 400BC)  Earth-centered universe o They tried to explain what was going on, but they didn’t have the exact concepts - Cosmogony o A theory about ones place in the universe  How the Solar System fits into the idea of the universe  Geocentric – theory that proposes Earth to be at the center of the universe  Generally accepted  Strong philosophical bend (man was the center of God’s creation)  Heliocentric – theory that proposes the Sun to be at the center of the universe - Pythagorean Universe o First formulation of what they thought the universe was and its motions  First recorded attempt at rational explanations of Heavens o 550 BC o Formed foundation for Greek astronomy - Philosophical Ideas o The heavens represented perfection (because the gods were perfection, so where they lived also had to be perfect) o The heavens are immutable  Unchangeable (if you’re perfect, you don’t need to change and there’s no need for improvement) o The circle is the perfect shape  So everything in the universe had to have this shape (the objects themselves and their motions)  All heavenly motions must be circular - Pythagorean Model o Stars reside on stellar sphere  Rotates about an axis in 23 hours and 56 minutes o Sun follows a path called the ecliptic, going around Earth once in one year o Accounts for most observations!  Stars rising and setting  Circumpolar stars  Height of the Sun changes throughout the year (so had to have its own sphere and it must be tilted with respect to the background stars on the stellar sphere)  Tilted at 23.5 degrees from equator (celestial equator = Earth’s equator)  The Sun rotated every 24 hours (took a little longer, so it made the Sun look like it was going across the background stars) o Planets  “Special” stars appeared to move across the sky without staying in their respective constellations  Wandering stars  Greek word for wanderer = planete 1. Mercury 2. Venus 3. Mars 4. Jupiter 5. Saturn  Earth was NOT considered a planet  Earth didn’t have the motion as these wandering stars did  Earth is not part of the heavens and is not perfect, so it is not a planet - Philosophical Ideas o The heavens represented perfection o The heavens are immutable o The circle is the perfect shape o All heavenly motions must be circular  From Plato’s “dictum” – all movements in the heavens can be explained by “uniform circular motion”  Everything goes at a constant speed and in the same direction in a circle  Followed idea of things won’t change and that circles are perfection - Geocentric Model o Everything revolves around the Earth on tilted “crystalline spheres” - So why did the ancient Greeks reject the real explanation for planetary motion? o Aristarchus (310-230 BC) thought it made better sense that the Earth goes around the Sun, contrary to the belief of the day  proposed the heliocentric idea (first to do so!)  Did not see observations that would back up this theory  Their inability to observe stellar parallax was a major factor o Stellar Parallax – looking at something from two different perspectives, the nearby object appears to shift against the background  Can use to measure distances  As Earth orbits the Sun the position of a nearby star appears to shift against the background of more distant stars  Rejected that the stars were extremely far away - The lack of observable parallax could mean one of two things: o Stars are so far away that stellar parallax is too small to notice with the naked eye  But the idea that the stars were extremely far away was rejected o Earth does not orbit the Sun; it is the center of the Universe o With rare exceptions (Aristarchus), the Greeks rejected the correct explanation because they did not think that stars could be that far away! - But there was a complication o Everything they thought about did not work with the planets o Planets were called “wandering stars” because they appeared to move slowly - Prograde and Retrograde Motion o Prograde Motion (normal motion) – the apparent West to East motion of objects (over many nights) as compared to the stationary background stars o Retrograde Motion – the apparent East to West motion of objects (over many nights) as compared to the stationary background stars o Moon and Sun: from one day to the next the motions of both the Moon and the Sun against the background stars are West-to-East (Prograde) o Planets: from one day to the next the motions of the planets can either be Prograde or Retrograde relative to the stars o HOWEVER: the daily/nightly motions are similar to the stars - Models of the universe MUST adequately describe this retrograde motion! - Ptolemy o Whatever we consider to be the actual truth, we should make it as simple as possible  Rule of parsimony o Greek - Ptolemaic model o The most sophisticated geocentric model (100-170AD) o Sufficiently accurate to remain in use for 1,500 years o Almagest – Arabic translation = “greatest compilation” o Ptolemy tried to create a geocentric model that would account for retrograde motion and preserve the assumption of uniform circular motion  He placed the planets in orbits (deferents) on orbits (epicycles) all around the Earth  made planets really go backwards! o Ptolemy’s Epicycles  Retrograde motion occurs on the inside of the circular motion of the planet’s deferent - As Ptolemy (and others) made more observations, they needed to add more epicycles to explain the data  model eventually became very complicated with later versions having nearly 100 epicycles to explain the observations o Did not follow his original rule of keeping explanations as simple as possible - Copernicus (1,500 years later) o Polish o Known for first concentrated Sun-centered model of the universe - Why heliocentric now? o Renaissance  Art  Literature  Travel/Exploration  Medicine o Rolled over into science  maybe we should stop hanging on to these outdated things and stop adding more epicycles and come up with a more elegant and simple solution - Copernicus (and later Tycho and Kepler) challenged the Earth-centered model o Proposed a Sun-centered model (published 1543)  Was not the first to propose this idea, but was the first to create this model o Used model to determine layout of solar system (planetary distances in AU) o But the model was no more accurate than the Ptolemaic Model in predicting planetary positions, because it still relied on using perfect circles o Said retrograde motion is an apparent motion caused when one planet moves from being behind another planet to being in front of the other planet  Everything moves in the same direction, just at different speeds (think about when you’re driving on the highway) - Copernicus was close in his estimates of distances between the Sun and other planets o One of major successes of his model - Copernican Model o Successes  Simpler explanation for retrograde motion  Gets accurate sizes for orbits (relative to the Earth’s) o Problems  Still clung to the idea everything moves in perfect circles  Still used epicycles to account for inaccuracies o Acceptance – the beginning of modern Astronomy  Because gave up on long-held beliefs and replaced by new observations and data collected and the simplest ways to fit those  Copernicus was cautious in discussing, published his ideas almost from his death bed  But mood was changing in Europe (Renaissance), more receptive to new, different ideas (including in science because of cultural exchange occurring)


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