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Monday October 15th 2012.docx

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Bozidar Mitrovic

Monday October 15 2012h Astronomy Lecture: Aristarchus: He used the observations to propose heliocentric (sun-centered) model. Objection: No stellar parallax (i.e apparent shift in the position of stars caused by change in the position of the observer) was observed. But the stars are so far away that the parallax is very small – less than 1 second arc (or arcsecond = 1/3600 degree) Erathosteves (276-196 B.C): - determined (around 240 B.C) the radius (i.e circumference of the earth using observations and geometry - on summer solstice at noon you could see the bottom of the well - well in Syene - at the same time in Alexandria there was a monument (was casting a shadow) - the angle must be A (underground) - d/circumference of the earth = A/360 degrees - d is measured and a is measured 2πr r=radius of the earth Aristotelian universe: - geocentric (earth-centered)- all heavenly bodies (Sun, Moon, Stars) move in uniform circular motion around the earth - but there was a problem of fitting the observed motion of planets into that scheme : generally move eastward towards the stars (so-called prograde motion) but every now and then they reverse direction (Planetai in Greek means wanderer) - to account for this the Greeks invented the epicycles - Hesperus (evening) - Phosphorus - Hesperus is Phosphorus th Wednesday October 17 2012 Astronomy in the renaissance: Nicolaus Copernicus (1473-1543): - He reintroduced and advocated the heliocentric (Sun-centered) model – he claimed all planets moved along circular orbits centered at the Sun - Then one can use observation to determine:  Orbital period of planets (i.e their “years”  Distances of planets from the sun in terms of earth – sun distance Tycho Barhe(1546-1601): - the greatest naked-eye observer in the history of astronomy - he used mural quadrat to measure the angles in the sky with high accuracy - repeated measurement many times and calculated the average values to reduce the effect of random error - his data was accurate to ½ of one minute of arc (1/60 of a degree) - over 20 years he collected detailed sets of data on planetary orbits Johannes Kepler (1571-1630): - he got to work on Barhe’s data - after eight years of work he developed three laws of planetary motion (which are called Kepler’s Laws) Friday October 19 , 2012 First Kepler’s law: - planets move along elliptical orbits with sun at one of the two foci of the eclipse - e=0 for a perfect circle - look up semimatior axis - planet mercury has the highest eccentricity (e=0.21 or 21%) - all other planets have eccentricity of less than 10% (for the earths orbit the eccentricity is about 1.7%) - area closest to the sun is perihelion - the area furthest from the sun is aphelion Second Kepler’s Law: - the orbital speed of a planet varies so that a line joining the planet with the sun sweeps over equal areas in equal times - this implies that the orb
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