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Berkeley ASTRON 10 - 3. Ptolemy, Copernicus

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How the Planets Move: Ptolemy, Copernicus, 9 September 2013!Ancients knew of seven objects in space called "wanderers" aka "planetes" 1.which did not move in lockstep with the other 3,000 stars.!the sun and the moon: "day light disks"!A.The disks move west to east against the background of stars (from east a.to west compared to the horizon).!The sun moves in a way that is pretty much circular. !1.The ancients noticed noticed early on that the motion of the moon is 2.not very circular. !With modern technology the moon's motion is predictable, but A.requires a lot of advanced computing. !Mercury, Venus, Mars, Jupiter and Saturn: points of light which moved against B.the background of stars!Mercury and Venus !a.Always stayed close to the sun: !1.Mercury was ~20 degrees from the sun!A. Venus was ~ 40 degrees from the sun!B.We never see mercury or venus at midnight because the sun is too C.far below horizon.!Their brightness sometimes changed: !2.They were brightest at an intermediate point between right up A.against the sun and farthest from the sun.!They spent most of their time going west to east against the 3.background of stars. !However, they occasionally slowed down, stopped and then spent a A.little time going from east to west. After they crossed the sun, they slowed down again, stopped, and resumed usual west to east motion. !Mars, Jupiter, and Saturn: !b.Also spent time going west to east until they approached being 1.opposite the sun. !They also stopped, went backwards for a while (where they were A.at their brightest), stopped again, and then resumed regular motion. !But they did it differently than Mercury and Venus, and had a.different brightnesses.!These backward movements of the planets against the stars were 2.called retrograde loops.!They came in different varieties: closed loops, and s shaped open A.loops!When a planet makes a loop, the loop is always farther from the B.ecliptic than the rest of that planet's path. !unless the loop is s shaped and then it crosses the ecliptic.!a.No one knew why any of this happened.!C.They could be at any distance from the sun, for all those astronomers a.knew.!Suggested explanations in history: !2.(325 BC) Aristotle!A.There were spheres around the earth which housed the sun, moon and a.the planets and finally the firmament of the stars.!This was the first proposed solution from one of the first scientists!b.Explains why the planets move around, but does not explain retrograde c.motion at all. !(140 AD) Ptolemy !B.In addition to major orbits of objects (called deferents), objects ride a a.minor orbit (epicycle). !All epicycles took a year, but the deferents were different.!1.Explained retrograde motion.!2.This was a useful idea, important major attempt.!3.Problems:!b.Awkward, required a bunch of circles!1.The published numbers on how big each circle must be, and where 2.the planets should be did not work the farther astronomers extrapolated them into the future.!As time went on, the less accurate they became.!A.Astronomers attempted to fix this by tweaking the absolute sizes of 3.the orbits, but the model fell apart a second time in a decade.!They continued to do this for a long time, because they did not A.have the appropriate frame of mind: They believed the motions of objects in the sky could be represented as if they were driven by mechanical gears. They did not think there were physical gears–they just thought the motions of planets should be regular. !Despite these problems, there was no rival to this system until the 4.end of the 1400s!All they got right was that the moon revolves around the earth. !c.This is obvious because the moon moves faster through the sky than 1.anything!The moon's phases are how much of the moon we see is lit: !2.People used to care about this more because they did not have A.indoor entertainment, and therefore actually looked at the moon. !New moon: When the moon is in the same direction as the the B.sun, we can't see it, and it is called a new moon. !Crescent moon: As the moon becomes more lit, it becomes a C.crescent moon!First quarter moon: When half of the moon is visible, it is a quarter D.moon (because it is 1/4 of the way through the moon's cycle)!Full moon: When the moon is opposite the sun, we see all of it, E.and it is called a full moon. !Third quarter moon: As the visible moon decreases by half, it F.becomes a third quarter moon (because it is 3/4 of the way through the moon's cycle)!When the moon is no longer visible, it is again a new moon. !G.(~1500 AD) Copernicus: !C.Background:!a.worked in Poland and spoke German (both countries claim him)!1.devout catholic !2.born in the mid 1400s in Poland!3.learned medicine, law, and theology at a Polish university !4.continued his studies in renaissance Italy, where he pursued 5.astronomy (his true passion)!took all of the available astronomy, and could see clearly that A.Ptolemy's model and calculations were not reflected in the sky (they were wrong)!Thought that a model where the sun was at the center of the universe b.would make more sense. !Being on a moving platform(the earth) would show that retrograde 1.motion was just an illusion: !The earth simply passes the other planets, which appear to move A.backwards. !The motion of the earth makes the sun appear to move in the sky.!B.This was a major advance. !C.He was also an official of the catholic church, so knew to keep this 2.idea to himself, as it was heresy. !The church's doctrine was geocentric (they believed the earth was A.the center of the universe). !There was currently an inquisition in all the christian countries, and B.even though poland's inquisition was mild compared to the spanish inquisition, he did not want to go against the church. !Copernicus discussed this with his contemporaries (who C.expressed interest), but did not publish a book until he was close to death. !Though Copernicus' numbers were at least 3 times more accurate than c.Ptolemy's, the numbers still don't match the planets perfectly. !The Roman church used this as evidence that Copernicus was wrong, 1.even though Aristotle's prediction was worse. !When that didn't work, they argued that Copernicus meant his A.formulas to be used as a tool to predict the motion of the planets, rather that a model of the universe. !The church enforced this using the inquisition. !B.The reason his numbers weren't right was because he assumed (like 2.all astronomers of his time) that all planets have perfect circular orbits. !The


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Berkeley ASTRON 10 - 3. Ptolemy, Copernicus

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