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BU ASTR 114 - Harmony of the Spheres and Starry Messengers

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ASTR 114 1st EditionLecture 4Harmony of the Spheres and Starry MessengersLecture: 4 Motions of the Clerical Sphere and Historical AstronomyPeople to Remember: Tyco Brahe, Gallieo, KeplerLibrary of Alexandria - Where the greeks kept all their data and recordings of space and the night sky.Greeks were the first to make models of nature with complete consistency. They tried to explain patterns in nature without resorting to the gods or myth. Eratosthenes - Tried to measure the size of earth for the Greek models, and came very close.All of the original models had earth at the center They said the heavens and things in the night sky must be perfect because all the thingsin the sky seem to move in perfectly circular/spherical motions. Retrograde MotionAs planets like mars move through the constellations there are times when they seem tomove backwards. Retrograde motion is caused when earth passes planets like mars in orbit. Earth is closer than mars to the sun, so mars lags behind in orbit. We are able to conceptualize this now because we place the sun at the center of the solar system. It really stumped the greeks because they had the earth at the center of the solar system. The earth centered model created by the Greeks was used for 1,200 years even though it was wrong.Thursday, February 5, 2015The Copernican RevolutionCoperniciciousThe first person to propose an idea that there was another way to plot out the universe rather than the way people had been plotting it out for centuries. He came up with the heliocentric model, he just swapped the sun with the earth. TychoMade the most accurate naked eye measurements ever made of planetary positions. He thought earth was at the center of the solar system, but recognized other planets went around the sun. Hired Kepler who used his obsenations to discover the truth about planetary motion.Johannes Kepler8- arch-minute - Very small discrepancy caused him to wage war on mars. Ellipse - Looks like on elongated circleEccentricity of an ellipse (Kepler’s First Law) - Used for mars and Jupiter.e = .5 distance between foci semimajor axis0.34 = .5 distance between foci12(0.34)(12) = .5(d) -> 8———-> 100 = 13003!2Thursday, February 5, 2015Keplers 2nd LawWhen planets were far away from the sun they went in slower orbit, when they got closer they went faster. As a planet moves around its orbit, it sweeps out equal areas in equal times. Keplers 3rd Law (most important)P^2 = a^3P = Orbital period in yearsa = Average distance from Sun in


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