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Early Astronomers Physics 113 Goderya Chapter(s): 4 Learning Outcomes:

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The Roots of Astronomy Already in the stone and bronze ages, human cultures realized the cyclic nature of motions in the sky. Monuments dating back to ~ 3000 B.C. show alignments with astronomical significance. Those monuments were probably used as calendars or even to predict eclipses.

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Stonehenge Alignments with locations of sunset, sunrise, moonset and moonrise at summer and winter solstices Probably used as calendar. Summer solstice Heelstone Constructed: 3000 – 1800 B.C.

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Pythagoras 580BC BC Calls the heavens cosmos. Suggest that Earth is spherical. The First Scientific Theory The daily motion of the sun and the starry heavens is due to the earth rotating in a circle

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Eudoxus 408BC BC Homo centric model of the Universe What is wrong with the model? Observation of Mars Homocentric hollow Spheres

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Aristotle 348 BC Lunar Eclipses Shape of Earth –Shadow during Lunar eclipse Earth is round –Different sky for A and B. –The ocean liner in the sea A B

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Aristarchus 300 BC BC –From earth shadow in Lunar eclipse: determines Moon-Earth diameter –From Trigonometry and phases of the Moon: Sun-Earth-Moon distance –Proposes Heliocentric Universe /3 Moon Sun Earth Earth Shadow Moon

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Eratosthenes 276 BC BC –Size of the earth. when the sun is directly overhead at noon on some summer day in Syene the sunray made a 7-degree angle for an obelisk in Alexandria.

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Eratostheness Experiment (SLIDESHOW MODE ONLY)

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Calculations –Eratosthenes calculation Distance between Syene and Alexandria = 500 miles One full circle = 360 degrees Obelisk angle difference = 7 degrees 7/360 = 500/C C = ( ) / 7 = 26,000 miles

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Hiparchus 200 BC BC –Catalogs the positions of stars –Brightness scale for stars: Magnitude scale –Motion of Sun: Tropical year Sidereal year

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Ptolemy 100 AD -200 AD –Wrote Astronomy text Almagest. –Geocentric model Earth center of the universe and the moon, mercury and Venus are in between the earth and sun.

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Later refinements (2nd century B.C.) Hipparchus: Placing the Earth away from the centers of the perfect spheres Ptolemy: Further refinements, including epicycles

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Epicycles The Ptolemaic system was considered the standard model of the Universe until the Copernican Revolution. Introduced to explain retrograde (westward) motion of planets

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Epicycles (SLIDESHOW MODE ONLY)

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Copernican Revolution Copernicus ( ) Supported heliocentric model of the Universe. Explained Retrograde Motion. –The apparent retrograde motion of Mars is created by the fact that the earth passes Mars. This occurs every 26 months. Determined distances to the planets.

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Copernicus new (and correct) explanation for retrograde motion of the planets This made Ptolemys epicycles unnecessary. Retrograde (westward) motion of a planet occurs when the Earth passes the planet.

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Tycho Brahe ( ) He proposed a model of the solar system. –The Sun and the Moon orbit the earth while the other planets orbit the Sun. Invented instruments for planetary motion studies. Discovered exploding stars and comets. Debated the validity of the heliocentric model. Realized that the Universe is changing and is complex.

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Johannes Kepler (1571 – 1630) Used the precise observational tables of Tycho Brahe (1546 – 1601) to study planetary motion mathematically. 1.Circular motion and Planets move around the sun on elliptical paths, with non-uniform velocities. Found a consistent description by abandoning both 2.Uniform motion.

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Galileo Galilei ( ) Discovered the law of falling bodies. –All objects fall to earth at the same rate in the absences of air resistance. In other words all falling objects experience the same acceleration due to gravity in vacuums. First person to use the telescope for Astronomy. –Studied in detail the moon surface, sunspots, suns rotation, Jupiters satellite, Saturn (not including the rings) the Milky Way and the phases of Venus.

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Issac Newton ( ) Discovered the laws of Motion (Details to come later) Discovered the law of Gravity (Details to come later)

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The Modern Ideas –Einstein and Weird physics Weird Physics - Einstein Theory of Special Relativity General Theory of Relativity Space-Time continuum Source: Wikipedia

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