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History of Astronomy

by: Margaux Stickney

History of Astronomy AST1002

Marketplace > Florida State University > Astronomy > AST1002 > History of Astronomy
Margaux Stickney

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Chapter 2 notes
Planets, Stars, and Galaxies
Professor Vladimir Dobrosavljevic
Class Notes
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This 3 page Class Notes was uploaded by Margaux Stickney on Wednesday September 14, 2016. The Class Notes belongs to AST1002 at Florida State University taught by Professor Vladimir Dobrosavljevic in Fall 2016. Since its upload, it has received 4 views. For similar materials see Planets, Stars, and Galaxies in Astronomy at Florida State University.


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Date Created: 09/14/16
9/12 • people thought the Earth was flat like a pancake • BUT we figured out the Earth is round bc things go UNDER the horizon line • also, at higher altitudes we can see farther into the horizon • Erastothenes (200 BC): Alexandrian astronomer and mathematician who measured the  Earth’s diameter • he used an obelisk in Alexandria and an obelisk in Syene and their shadows (which  were different at the same time of day), used trig to find the radius of the Earth (and  then the diameter) • Plato (400 BC): heavens are perfect  • Aristotle (350 BC): crystal spheres with stars, planets, and moons attached to them that are  rotating around the Earth at different rates • almost right, but he used a geocentric model • Ptolemy (140 AD): retrograde motion • put forward the theory of epicycles to explain the weird loop (went against the crystal  sphere theory):  • nobody knew what was in the center of the epicycle???? but if u questioned it u  would be burned at the stake • Copernicus (1500): heliocentric model • explained retrograde motion bc as the Earth overtakes Mars in orbit, it appears that  Mars is going backwards bc Earth is fast • Galileo (1600): used the telescope, observed the phases of venus • planets don’t twinkle, stars do (jimminy cricket didn't sing twinkle twinkle little planet) 9/14 • Kepler was a student of a prince Tycho De Brache • 20 years of planetary observation • Kepler’s Laws of Planetary Motion: • the orbit of the planets are ellipses, with the sun located at one of the focus of the  ellipse • ellipse: (x/a)^2 + (y/b)^2 = 1 • eccentricity is deviation from circle • most planets have a weak eccentricity • Mercury’s orbit is more eccentric • comets have a very large eccentricity, motion is faster as it goes around the  sun • The Law of Equal Areas: A line from the planet to the sun sweeps over equal areas  in equal intervals of time. Hence, the planet moves faster when closer to the Sun • “rollercoaster law” • a planet’s orbital period squared is proportional to its average distance from the sun  cubed 2 3 • P  = a • P = orbital period in years, a = distance from the sun in AU • 1 AU = 150,000,000 km = distance between the Earth and the sun 2 • Newton discovered the universal law of gravity: F  = G(m g /r ) 1 2 • Law of Inertia: a body continues at rest or in uniform motion in a straight line unless acted  upon by some external force • Second Law of Mechanical Motion: a body’s change of motion is proportional to the force  acting on it and is in the direction of the force • F = ma • Force = (mass)(acceleration) • Law of Action and Reaction: when a body exert a force on a second body, the second body  must exert an equal and opposite force back on the first body


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