Astronomy Week 2 Readings: 3.1-3.4, All of chapter 4

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Astronomy & Astrophysics
Michael Reid

3. The Science of Astronomy 9/25/2012 6:21:00 PM 3.1 The Ancient Roots of Science In what ways do all humans use scientific thinking?  Baby lets go of ball – it falls, she drops something – it falls too o Through the power of observation, the baby learns about the physical world, finding that things fall when they aren’t supported  One day she lets go of a helium balloon - it rises o Understanding of nature is reversed o Not everything falls, but mostly everything does  Sci. is a way of learning about nature through careful observation and trial-and-error experiments  Develop. of sci. is a gradual process for humanity How did astronomical observations benefit ancient societies?  Begins in central Africa, where ppl. made observations on weather based on the moon o Watching movements of moon to see when rainfall would come  Made these observations to satisfy curiosity  Used changing positions of Sun, Moon and stars to keep track of time/seasons o Crucial for agriculture industry What did ancient civilizations achieve in astronomy?  Built structures for observing sky  Determining the Time of Day o In daytime, ancient ppl. would tell time by observing Sun’s path through the sky  Used shadows cast by sticks to tell time  Egyptians built huge obelisks that served as clocks o In nighttime, they could estimate the time from the position/phase of the Moon o Origin of modern clock can be traced to ancient Egypt  Divided daylight and night into 12 equal parts (24 in total)  They then began using water clocks (hour-glass) which were then used by Greeks, Romans, and Chinese  Slowly developed to mechanical clocks and so on  Marking the Seasons o Built structures to marks seasons  Ex. Stonehenge (one of the oldest)  Ex. Templo Mayor in Aztec city of Tenochtitlan o Aligned their buildings with cardinal directions (North,E,S,W) o Other structures used to mark Sun’s position on special dates like winter and summer solstice  Lunar Calendars o Ancient civ. Also tracked lunar phases and used the lunar ccle as a basis for their calendar  Basic lunar calendar = 12 months, each month lasting 29-30 days o Some cultures didn’t like the idea of their months cycling through the seasons over time so they modified their calendar to take advantage of the lunar phases repeating every 19 days  Metonic Cycle o In addition to lunar phases, some cultures discovered lunar cycles, eclipse prediction, etc.  Ancient Structures and Archaeoastronomy o …  From Observations to Science o … 3.3 The Copernican Revolution How did Copernicus, Tycho, and Kepler challenge the Earth-centered universe?  Copernicus o While trying to find a better way to predict planetary positions, he decided to try Aristarchus’s Sun-centered idea  Did the mathematical work, calculated the Earth’s orbit time around the Sun and the distance from the Sun- Earth  His success meant the idea was right  His published book spread widely but gained few converts b/c it was based on the assumption/calculations that heavenly motion must occur in perfect circles  Tycho o Nobleman with a hidden passion for astronomy o Observed a nova “new star”  Proved it was far away from the Moon  He actually saw a supernova o Made naked-eye observations extremely accurately  Before the telescope o He was convinced that the planets must orbit the Sun but didn’t have proof so we created his own theory that the Earth is still and the Sun orbits the Earth and the other planets orbit the Sun  Kepler o Hired by Tycho o Extremely religious so he too tried to match circular motions to Tycho’s data (perfect circles) o After observation Mar’s position, he tried to find it’s orbit but his calculations weren’t matching Tycho’s so he abandoned the idea of circular orbits and found the right solution to planetary motion (this marked transition point) o Key discovery = planetary orbits are not circles but are special type of oval called ellipse  Location of 2 ends of the oval is called foci (focus)  The long axis is called its major axis, each half of which is called a semimajor axis. Short axis = minor axis o Kepler’s model withstood the test of time and became accepted as model of nature/underlying truth about planetary motion What are Kepler’s 3 laws of planetary motion?  A.K.A. Kepler’s laws of planetary motion  Kepler’s First Law o Tells us “the orbit of each planet around the Sun is an ellipse with the Sun at one focus”  Tells us that a planets distance from the Sun varies during the orbit  Closest point = perihelion and the farthest = aphelion  Kepler’s Second Law o States “as a planet moves around its orbit, it sweets out equal areas in equal times”  Means that the planet travels faster when it is nearer to the Sun and slower when it is farther from the Sun  Kepler’s Third Law o Tells us that “more distant planets orbit the Sun at slower average speeds, obeying a precise mathematical relationship” How did Galileo solidify the Copernican revolution?  Although the success of Kepler’s laws in matching Tycho’s data was sufficient enough evidence to prove Copernicus’s place of the Sun at the center, there were still some objections o 3 basic objections  Aristotle said that the Earth couldn’t be moving b/c objects such as birds and clouds would be left behind as the Earth moved  Idea of noncircular orbits contradicted Aristotle’s claim that the heavens (realm of sun, moons, stars, and planets) must be perfect and unchanging  No one detected the stellar parallax that should occur if Earth orbits the Sun  Galileo Galilei answered all 3 objections o 1 STby experiments using rolling balls that showed that moving objects remain in motion unless a force acts to stop it o 2 ndby using Tycho’s supernova and comet observations to show that the heavens could change  Galileo built a telescope and shattered the idea of heavenly perfection  He saw spots on sun/moon that were seen as “imperfections”  3rd by proving that the stars were a lot more distant then Tycho had said  Galileo’s invention of the telescope o He observed 4 moons orbiting Jupiter by which he showed that moon can orbit planets o He observed Venus goes though phases which only make sense if it orbits the Sun, not Earth 3.4 The Nature of Science How can we distinguish science from nonscience?  Science = knowledge  The idealized scientific method o Hypothesis = educated guess o Make observations -> ask q’s -> suggest a hypothesis -> make a prediction -> perform a test: experiment or additional observation o It is a useful idealization but science barely works that way (in orderly fashion)  Hallmarks of Science o Everything we consider science shares the following 3 basic characteristics:  Seeks explanations for observed phenomena that rely solely on natural causes  Progresses thought creation and testing of models of nature that explain the observations as simply as possible  Makes testable predictions about natural phenomena. If predictions do not agree with observations, model must be revised or abandoned. What is a scientific theory?  Theory = powerful yet simple model makes predictions that survive through repeated/varied testing o Ex. Newton’s theory of gravity Charles Darwin’s theory of evolution o Sci. theory can’t be proved true beyond all doubt b/c future observations may disagree but it also must be supported by a large body of evidence o Only can replace/discard theory if alternate explanation is provided 3.5 Astrology How is astrology different from astronomy?  Apparent positions of the Sun, Moon, and planets among the stars in our sky influence human events o Position of the Sun determines the seasons and the times of daylight/darkness o Position of Moon determines the tides  Ancient astrologers tried to link the positions of the Sun, Moon and planets with events that take place on Earth o Natural disasters, King’s illness  They did this so that they could forecast human events with the same reliability that astronomical observations of the Sun could forecast Spring  Ptolemy’s book, Tetrabiblios, remains the foundation for astrology  Astrologers held high positions as political advisors since they could “predict” what is to come o They were given resources needed to chart observations which helped make advances in astrology  Kepler cast a number of horoscopes, but did so only for income o Didn’t believe in it  Galileo also cast a horoscope for the Duke of Tuscany, saying he would live a long life but it was wrong and he died weeks later Does astrology have any scientific validity?  Astronomers gave up on it centuries ago, but it remains popular with the general public  Testing Astrology o Hard to validate it b/c astrologers themselves can’t come to acre on what it is and what it can predict  “Western astrology” is diff from astrology practiced in India and China o Very vague guidance, rather than testable predictions are given which means the question of validity doesn’t apply o Prediction of future events and person’s personality/life can be checked for accuracy  A way of testing this is by evaluating the horoscopes and comparing their accuracies to pure chance o Never been proved accurate by substantially a great margin than expected from pure chance o This method/science is useless 4. Making Sense of the Universe 9/25/2012 6:21:00 PM 4.1 Describing Motion: Examples from Daily Life  Objects are pulled to the Earth by gravity, slowed by air resistance and pushed by wind  We can predict where things are going to land before we throw them and this basic trick is used to predict the motions of objects throughout the universe o Land spaceship on Mars How do we describe motion?  Speed, Velocity and Acceleration (these concepts show how ind. objects move) o Speed = the rate at which an object is moving  Its units are distance divided by time  Ex. 100 km per hour o Velocity = combination of speed and direction of motion  Ex. 100 km per hour due north o Acceleration = rate at which an object’s velocity changes  Meaning a change in either speed of direction  Ex. speeding up, turning and slowing down  The Acceleration of Gravity o = acceleration of a falling object  abbreviated as “g” o By dropping weights from the Leaning Tower of Pisa, Galileo demonstrated that gravity accelerates all objects by the same amount, regardless of their mass  This is without air resistance factored in o The acceleration of gravity is about 10 meters per second squared = 10 m/s^2  Momentum and Force (shows interactions b/w objects) o Momentum = Product of an objects mass and velocity  Momentum = mass x velocity o Force = only way to change an objects momentum, causing it to accelerate o Understanding these concept using ex. of collisions  Bug hitting your windshield vs. head on collision with 2- ton truck, both at 30km/hr  Truck’s great mass = more momentum than bug, even though they have the same velocity  During collision, momentum is transferred  Bug has little momentum to give to your car  Truck imparts enough of its momentum to cause a sudden change in your car’s momentum = force o Just presence of force doesn’t cause change in momentum  Car experiences forced of air resistance, friction but maintains its momentum and velocity o Forces of some kind are always present  Net force (overall force) acting on an object represents the combined effect of all the ind. forces put together o Changing an object’s momentum means changing its velocity, as long s its mass remains constant  Planets are always accelerating as they orbit the Sun b/c their direction of travel constantly changes as they go around their orbits  The force that causes this is gravity  Moving in Circles o Ex. Ice skater spins in place, isn’t going anywhere, has no velocity and therefore, no momentum but every part of her body is moving o Therefore, the total momentum from each part of her body gives her Angular momentum, also called “circling momentum” or “turning momentum”  Any object that’s either spinning or moving along a curved path has angular momentum o Earth is rotating so it has a rotational angular momentum o Also
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