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Frontiers in Astronomy-Week 2 notes

by: Kathryn Notetaker

Frontiers in Astronomy-Week 2 notes PHY 21430

Marketplace > Kent State University > Physics > PHY 21430 > Frontiers in Astronomy Week 2 notes
Kathryn Notetaker
GPA 3.3

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About this Document

All the notes from week 2- includes material from the 3 lectures
Spyridon Margetis (P)
Class Notes
astronomy, Physics, Science
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This 8 page Class Notes was uploaded by Kathryn Notetaker on Friday September 9, 2016. The Class Notes belongs to PHY 21430 at Kent State University taught by Spyridon Margetis (P) in Fall 2016. Since its upload, it has received 26 views. For similar materials see FRONTIERS IN ASTRONOMY in Physics at Kent State University.


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Date Created: 09/09/16
Frontiers in Astronomy Week 2 Seasons, moon-phases, etc-Chapter 1  Sun and seasons  the sun’s apparent path on the sky (ecliptic) is making 23.5 degrees angle with the celestial equator  if there was not a tilt of earth’s axis (0 degrees), the sun would always move on the celestial equator=same or no seasons  with tilt- Sun shines more on the northern hemisphere during the summer and less in winter  there are 2 points where the sun is on both planes  vernal equinox  21 of march (beginning of spring)  autumnal equinox st  21 of September (beginning of autumn)  these seasons are for the north hemisphere  the south hemisphere are opposite  these 2 dates, the length of the day and night is the same for every place on earth  summer and winter solstice  2 other points on this travel path  Solstice- meaning the sun stops  summer solstice  the sun reaches the maximum height above the celestial equatot  Winter Solstice  the sun reaches the lowest point  mark the beginning of summer and winter  opposite for the southern hemisphere  Axis  as earth moves around the sun, its axis of rotation stays the same  due to a fundamental law in physics  tilt of axis cause the seasons  Sunlight during the summer solstice hits earth’s north hemisphere almost perpendicularly  why it’s hotter in summer, colder in winter/ hotter in midday and cooler in afternoon  Motion of the moon  phases are due to different amounts of sunlit portion being visible from earth  always start counting from new moon, when the moon is completely hidden since it is in the direction of the sun  waxing (increasing) crescent is the next phase  after the new moon, it starts getting illuminated from the right as we see it from earth until full moon  then gets dark again from right side  synodic month- takes moon about 29.5 days (about a month) to go through whole cycle  sidereal month- 2 days shorter (27.3 days) time it takes for a full revolution around the earth  2 day difference comes from earth’s motion around the sun  Eclipses  Lunar eclipse  earth is between moon and sun, instead of full moon it is a dark moon  partial eclipse-when only part of moon is in earth’s shadow  total eclipse- when all of the moon is in earth’s shadow  Solar eclipse  moon is between earth and sun  partial eclipse- only part of the sun is covered by the moon  total eclipse- when the sun is completely covered by the moon  eclipses occur when earth, moon and sun form a straight line  does not happen in every new/full moon  the moon’s plane of rotation is inclined with respect to sun-earth’s plane (5.2”)  earth’s and moon’s orbits are not in the same plane The Copernican Revolution-Chapter 2  Ancient astronomy  over 6000 years ago  almost every major culture and civilization studied the celestial objects and their move on the sky  religion was one of the driving forces  also navigation, time-keeping, weather changes, animal behavior- things that linked to human survival  only a few select people were allowed to learn about astronomy  people were fascinated with the endless repeating patterns of change in the world around them  practical knowledge  Egyptians relied on astronomical observation to plan the planting and harvesting of crops  knew that the rising of Sirius just before the sun heralded the flooding of the nile river  we believe that megalithic monuments were the first observatories  Stonehenge mark astronomical events the sunrise of the summer solstice when the sun shines through a specific stone, only happens at this time  big horn medicine wheel spokes are aligned with the rising and setting of the sun and other stars thin streak of light aligns perfectly with center of the pattern at noon of summer solstice  Caracol temple built by mayans many windows that align with astronomical events looks like an observatory  Chinese astronomers left behind astronomical events like supernova explosions  main contributors to astronomy as a science were the greeks and muslims  Islamic world provided a link through the dark ages  Most names of sky objects are either greek, muslim or latin greek: orion, helios (sun), planet (wanderer) muslim: zenith, azimuth, rigel, betelguese, vega latin: mars, Jupiter, Neptune  The geocentric universe  ancient astronomers observed:  sun moving daily and yearly on ecliptic  moon doing its phases and move near ecliptic  stars moving only daily, otherwise fixed on sky  5 planets were known:  mercury, venus, mars, Jupiter, Saturn  moved on/near ecliptic, never far from it  Sun, moon and stars have simple movements in sky  planets  move with respect to fixed stars from west to east  change in brightness  change speed  undergo retrograde motion- an apparent change in the direction of motion of a planet a puzzle to early astronomers trying to figure out how heavens work  Inferior and superior planets  inferior-mercury, venus  stay close to sun  have orbits closer to sun than earth’s  they can never be in opposition with the sun  2 conjunctions 1 behind the sun (superior) 1 in front (inferior)  Superior- Mars, Jupiter, Saturn  have orbits that are further away  can be in opposition with the sun (earth in between)  have only 1 conjunction (superior-like)  1 opposition  Geocentric universe  developed by greek philosopher Aristotle-1 theoretical model trying to explain the motion of known celestial objects  called geocentric because earth was at the center of universe the center is where observer is- makes sense  things were supposed to move in perfect circles with constant speed  model employed nested spheres with the stars, planets, etc., attached on them  earth in center, moon, mercury, Venus, sun, mars, Jupiter, Saturn, fixed stars, sphere of the prime mover  Aristotle’s arguments  earth doesn’t feel like it’s moving- why it’s at the center  if it moved, why don’t we feel a head wind  if earth moved, then the stars should have stellar parallax but they don’t- stars should have an apparent movement on the sky  Aristotle’s model  could not account for 2 important things variation in planetary brightness the retrograde motion  1 step towards resolving these issues was to introduce epicycles- a small circle whose center remained on the main path of the planet (deferent)  many more fine adjustments until Ptolemy’s model (~140 AD)  Ptolemy’s (refined) geocentric model  preserved earth at the center, circular motion  interested in accurate calculations, not laws mechanisms or consistency  introduced eccentric (off center) motion, epicycles, deferent and equant to deal with various effects  very accurate in predictions but very complex and inconsistent  violated the law of simplicity- the truth is simple  violated occam’s razor- law of economy  between 2 explanations, the simpler one is probably the correct one Heliocentric model of the solar system  Aristarchus of samos  the sun is the center of the solar system, only moon orbits around earth; planets orbit around the sun  used logic to explain things in a more convincing way  the earth makes 2 motions, 1 around itself and 1 around the sun  still, the Ptolemaic picture survived for almost 14 centuries  Nicolaus Copernicus  challenged ptolemy’s model  re-introduced the heliocentric model  motivation was simplicity  the critical realization that earth is not the center of the universe (Copernican revolution)  explained retrograde motion in a natural way  foundations of the Copernican revolution  earth is not at the center of everything  center of earth is the center of moon’s orbit  all planets revolve around the sun  in circular orbits  the stars are very much farther away than the sun  the apparent motion of the sun and stars is due to earth’s rotation around itself  retrograde motion  Geometry and earth moves faster  Birth of modern astronomy  telescope was invented around 1600  Galileo built his own, he improved the telescope  was inspired by kepler’s work  made detailed observations  published dialogo in 1632  condemned by inquisition  sentenced to house arrest until death  1992 john paul II pardons  Galileo’s observations:  moon has mountains and valleys like earth  sun has sunspots and rotates  Jupiter has moons-smaller objects rotate around larger  venus has phases  Laws of planetary motion  galileo demolished ptolemy’s system and established the Copernican heliocentric system  Copernican approach is a classic example of the scientific method  the laws of gerning the motion of celestial objects were still unknown  the dynamics were missing  Johannes Kepler  kepler’s laws derived data using observations made by tycho brahe  formulated 3 laws that explain everything  Kepler’s laws  the orbits of the planets are ellipses, not circles, with the sun at one focus  the line joining the sun and the planet sweeps out equal areas in equal times  square of period of planet’s orbital motion (P) is proportional to cube of semimajor axis (a)  ratio P^2/a^3 should be constant  properties of planetary orbits  perihelion: closest approach to the sun  Aphelion: farthest distance from sun  Isaac Newton  revolutionized all of science and mathematics  probably the greatest physicist of all time  2 year vacation- there he revolutionized science and math  3 laws of motion and 1 law of gravity  classical mechanics/ newtonian mechanics  Newton’s laws  explain how objects interact with the world and with each other  law of gravity identifies the responsible force behind celestial motion  & provides the formula to estimate it  Principia Mathematica (1686)  explained all motion  most important book in history  Mass-quantity of matter- how much stuff there is (not weight)  force- an action that changes motion (the cause of acceleration  1 law-law of motion  if an object is at rest, it will stay at rest  if an object is in motion, it will not change its motion unless an external force acts on it  2ndlaw  when force (F) is exerted on an object its acceleration (a) is inversely proportional to its mass (m)  a=f/m rdf=ma  3 law-action/reaction law  mutual forces of 2 objects upon each other are equal and directed in opposite directions  Newton’s gravitational law  for 2 massive objects, the gravitational force is proportional to the product of their masses divided byt the square of the distance between them  the farther the objects are, the weaker the force  newton realized the universality of the gravitational force  the same force that makes an apple fall, keeps earth revolving around the sun  Newtonian mechanics  kepler’s laws a r a consequence of newton’s laws  some fine-tuning was done to kepler’s laws due to newton’s laws  einstein’s theory of relativity super-tuned Newtonian celestial mechanics


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