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Lecture 3 Heliocentric Universe - The Sun How we know what...

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The SunHow we know what we know
Ancient Astronomy and Stellar Positioning
The Sun and the Moon moveacross the Sky in an Orderly,smooth path.The planets seem to “wander” -retrograde motion.Ancient Greeks
“Inferior” planets hadpaths tied to the Sunand appeared to haveretrograde motionwhen, we now know,they are between theEarth and the Sun.“Superior” planetswere not tied to theSun’s location and arein retrograde whenopposite the Sun.Ancient Greeks
The Perfect CircleAristotle (384-322 BC)created the firsttheoretical model of theSolar System with the(erroneous) assumptionof Geocentricity and theuse of the perfect form:the circle.Alright for the Sun andthe Moon but could notexplain the motion of theplanets.courtesy NASA
Ptolemy and theEpicycle140 AD : 80+ circlesemploying epicyclesThe circle is simple but intrying to conform to the“perfect form” the modelbecomes overlycomplicatedand not very effective in it’spredictive power.His work, published in the“Almagest” was the basis ofastronomical study for thenext 1000+ years!Georg Peurbach,Novae theoricae planetarum(Venice, 1534).
Heliocentric ViewFirst proposed by Aristarchusof Somos (310 - 230 BC)The Earth and other planetsrevolve around the Sun and theEarth is rotating on it’s own axisRefuted by Aristotle and hisfollowers: it doesn’t feel likewe’re moving; no wind is beingproduced; there is nostellarparallax(actually stars too faraway to see their relativemovement against backgroundstars as Earth moves in it’sorbit)
the “Dark” AgesDuring the so-called “Dark”ages in Europe, astronomers inthe Muslim World were makinggreat advances from the Greekknowledge base, particularly inmathematical techniques fortrigonometry. This advancedthe precision of celestial timingconsiderably.The Chinese had extensiveobservational records duringthis time, notable now for theirhistorical Supernovae data,then known as “guest stars”.
The CopernicanRevolutionEarth is not at the centre of EverythingThe Centre of Earth is not the Centre of theUniverse, but one of gravity about which theMoon rotatesAll planets revolve around the SunThe stars are much further away than the SunTheir motion across the Sky is a result of theEarth’s rotation about an axis.The motions of the Sun are also due to themotions of EarthThis view can explain retrograde motion
The Copernican Revolution: Heliocentric but still stuck to the idea of the “perfectcircle” and the use of epicycles. It could explain retrograde motion and the Seasons.

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Term
Winter
Professor
Staff
Tags
Planet, Astronomical unit

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