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F11_PHYS-1600 - Week 1.docx

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PHYS 1600
Saba Safdar

PHYS*1600 Unit/ Week 1: Geocentric: earth-centered universe Cosmology: the study of the formation, organization and evolution of the earth 2-2 The Belief that in a Sun-Centered Cosmology formed Slowly: - Planet comes from Greek word “wandered” - Direct Motion: the gradual, eastward apparent motion of a planet against the background stars as seen from Earth - Retrograde Motion: the occasional backward (that is westward) apparent motion of a planet against the background stars as seen from Earth  Retrograde motion is an optical illusion - Both are best understood through pictures - All planetary motions on the celestial sphere are much slower than the apparent daily movement of the entire sky caused by earth’s rotation - Ancient Greek astronomer Aristarchus of Samsos proposed a more straightforward explanation of planetary motion, that the planets revolve around the Sun - The retrograde motion of Mars in a heliocentric cosmology occurs because faster moving Earth over-takes and passes Mars - Heliocentric is a misleading word: the Sun and the bodies that orbit it all orbit the center of our Milky Way Galaxy Guided Discovery – Earth-Centered Universe: - Earth spins on its axis - Gravitational force of the planets is held by the sun - Called this background information a conceptual framework (contains all the info we take for granted) Geocentric Explanation of the Planets’ Retrograde Motion: - Ptolemy had some basic concepts: 1. Each planet is assumed to move in a small circle (epicycle), the center of which moves in a larger circle is called a deferent, whose center Is offset from Earth  As viewed from Earth, the epicycle moves eastward along the deferent, and both it and the planet on it revolve in the same direction (counter- clockwise)  Most of the time, the motion of the planet on its epicycle adds to the eastward motion of the deferent  Planet seen from earth to be direct motion (to the left or eastward) 2. However when a planet is on the part of the epicycle where its nearest earth, its motion along the epicycle subtracts from the motion along the deferent  The planet thus appears to be slow, then halts its regular motion to the left (eastward) along the constellations, then moves right (westward) Downloaded free at www.uofgexamnetwork.com  Looks like its looping back (retrograde motion) 3. Ptolemy: work Almagest, in which the positions and paths of the Sun, Moon, and planets were described with unprecedented accuracy  Seen as the astronomers bible for 1000s of years 2-3 Copernicus Devised the First Comprehensive Heliocentric Universe: - Configurations: geometric arrangements amoung Earth, , another planet, and the Sun - Inferior Conjunction: the conjunction when Mercury or Venus is directly between the Sun and Earth - Superior Conjunction: the conjunction when Mercury or Venus is on the opposite side of the Sun from Earth - Elongation: the angle between a planet and the sun as seen from Earth  At a greatest eastern or western elongation, Mercury would be about 28 degrees, 47 for Venus - During sunrise/ sunset Venus is often ½ way up in the sky making it more visible during its orbit - When one planet is behind the sun, it is said to be in conjunction - When the planet is opposite in the sky to the sun, that planet is at opposition ------------------------------------------------------------------------------------------------------------------------------------------ - Sidereal Period: the orbital period of one object about another measured with respect to the stars  Mercury would be 0.241 in Earth Years - Synodic Period: the interval between successive occurrences of the same configuration of a planet seen from Earth  Time between Earth and Mercury is 116 days (0.318 in Earth years) Downloaded free at www.uofgexamnetwork.com 2-4 Tycho Brahe made Astronomical Observations that Disproved Ancient Ideas About the Heavens - Supernova Explosion: the violent death of a certain type of star - Parallax: nearby objects are viewed at different angles from different places. They also appear to be in different places with respect to the more distant objects when viewed at the same time  Both effects are called parallaxes’ and are used by astronomers, surveyors, and sailors to determine distances - Parallax of a Nearby Object in space:  thought that earth did not rotate so if the new star is nearby, its position should shift against the background stars over the course of the night  he failed to disclose any parallaxes 2-5 Kepler’s Laws describe Orbital Shapes, Changing Speeds, and the Lengths of Planetary Years - Kepler’s First Law: the orbit of a planet around the Sun is an ellipse with the Sun at one Focus.  Shapes of ellipses have two extremes  The roundest ellipses occur when the two foci (focus) merge, is a circle  The most elongated one approaches being a straight line  The shape of a planets orbit around the sun has been described by its orbital eccentricity, designated by the letter e, which ranges from 0 (a circular orbit) to 1.0 (nearl
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