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**History of Astronomy Renaissance Astronomy (1400 A.D. to 1640 A.D.)**

Lesson 1-4 History of Astronomy Renaissance Astronomy (1400 A.D. to 1640 A.D.)

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**Lesson overview Copernicus (heliocentric theory)**

Brahe (observations via instruments) Kepler (3 laws of motion) Galileo (telescopes to observe the solar system)

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Introduction Renaissance period—four people established the way we see our solar system: Nicolaus Copernicus—challenged earlier theories Tycho Brahe—made amazing observations Johannes Kepler—built on Brahe’s observations; created three basic laws Galileo Galilei—used a telescope to observe the Moon, Sun, and three planets His observations and Keplar’s math destroyed the earlier geocentric theories

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**Nicolaus Copernicus (1473-1543)**

Polish physician and lawyer, renaissance humanist and Catholic cleric Had trouble with Ptolemy’s geocentric (earth-centered) model Believed in Aristarchus’ heliocentric (sun-centered) model Asserted Earth rotates on its axis once per day

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**Heliocentric and retrograde motion?**

Ptolemy used epicycles to explain retrograde motion (but got very complicated) Heliocentric models explain retrograde motion as a natural result of two planets passing each other as they revolve counterclockwise around the Sun Animation: The retrograde motion of Mars according to the heliocentric model

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**More Copernicus Wrote “On Revolutions of Celestial Spheres”**

Calculated with accuracy the relative distances of the planets to the sun using geometry Could not explain lack of “stellar parallax” when viewing stars Copernicus surmised the distances were too great to have any parallax effects People at the time couldn’t imagine such vast distances Theory rejected as a model of reality but embraced as useful for calculations Animation: The retrograde motion of Mars according to the heliocentric model

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Tycho Brahe (15461601) Build island observatory -- “Uraniborg” Invented “modern” instruments to observe planets Accurately measured the locations of the planets Gathered other data – Mars in particular Brahe’s Quadrant

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**For measuring altitude or angular distances**

Azimuthal Semicircle

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Armillary Sphere

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Tycho Brahe (15461601) Observed things that suggested heavens were changeable and complex Observed comet traveling through the orbits of several planets Shattered Aristotle’s crystalline-sphere theory Proposed different, but still incorrect, geocentric model Geo-heliocentric model The Sun and the Moon revolved around the Earth Everything else revolved around the Sun Lacked mathematical know-how to explain the motion of the planets Hired Johannes Kepler

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Johannes Kepler ( ) Challenged by Brahe to explain mathematically Mars’ orbit around the sun Used Brahe’s data on Mars to show the orbit was elliptical Put the sun at one focus resulting in 3 laws of motion

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Kepler’s First Law The orbits of the planets are ellipses with the sun at one focus of the ellipse.

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Kepler’s Second Law The line joining the planet to the sun sweeps out equal area in equal times. What does this mean? When a planet in its orbit is closer to the sun, it must speed up to sweep out an equal area…

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Kepler’s Third Law The square of the orbital period (time it takes to complete one orbit) is directly proportional to the cube of the mean (average) distance from the sun to the planet What does this mean? A larger orbit requires more time to make one complete revolution. Kepler predicted EXACTLY how much time each planet’s orbit would take.

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Johannes Kepler ( ) Brought new emphasis on finding and quantifying causes of motion United geometric or kinematic description of orbits with their physical, dynamic force that causes it Authored fictional account of a space journey, the Somnium Described trip to the Moon Inspired other fiction writers, including Jules Verne and H. G. Wells

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Galileo Galilei ( ) Refined telescope design to a state-of-the-art 20X Observed several things about the physics of motion Ushered in new era of space exploration

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**Galileo’s Discoveries**

Saw imperfections in “perfect bodies” of Aristotle’s model of the universe Mountains and valleys of the Moon Sun spots (noticed the sun rotated like the Earth) Discovered four moons orbiting Jupiter Disproved Aristotle’s assessment that everything revolved about the Earth Noticed the phases of Venus indicating it too revolved around the sun Observed the Milky Way and noted its vast size Telescope’s inability to magnify stars confirmed Copernicus’s guess about their vast distances

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**Galileo and Gravitation**

Measured time for spheres to reach bottom of an incline Dropped stones from the Leaning Tower of Pisa Weight didn’t affect time to reach the bottom –both balls had the same acceleration Previously, people thought heavier objects fall faster Theory proven during Apollo 15 using a hammer and feather

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**Galileo vs the Pope His “theories” were at odds with Church “law”**

In 1610, published “Starry Messenger” Non-technical style—readable for common class Presented as dialogue of the Copernican system between a wise teacher (him) and an unbeliever he named Simplicio (the Pope) In 1632, published “Dialogue Concerning the Two Chief World Systems” with the approval of the Inquisition Supposed to be neutral, but wasn’t In 1633, placed under house arrest for the rest of his life (church admitted its mistake in 1992)

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**Lesson review What idea did Copernicus develop and promote?**

Used planetary parallax to argue for heliocentric (Sun-centered) solar system Measured relative distances of planets from the Sun Surmised the vast distances to stars

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**Lesson review (cont’d)**

How did Tycho Brahe contribute to our understanding of space? Invented new instruments Observed and accurately recorded the locations of the planets over time Had a wacky theory of a geo-heliocentric solar system

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**Lesson review (cont’d)**

What are Kepler’s three laws? Planets move in elliptical orbits around the sun The closer to the Sun, the faster they move The larger orbit requires longer time to complete What did Galileo do? Telescope observations led to the downfall of Aristotle’s geocentric model Confirmed vast distances to stars One of the first to study gravity

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