Astro study notes for test 1
Astro study notes for test 1 Astr 1010
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This 9 page Study Guide was uploaded by Alikhan Ladhani on Sunday February 7, 2016. The Study Guide belongs to Astr 1010 at Georgia State University taught by in Spring 2016. Since its upload, it has received 54 views. For similar materials see Astronomy in Science at Georgia State University.
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Date Created: 02/07/16
Our cosmic address • Earth is a planet in our solar system o Consist of the sun, planet and their moons and smaller object like rocky asteroids and icy comets • Our solar system belongs to the milky way galaxy o Galaxy is a great island of stars in space containing between a few hundred million and a trillion or more stars o Milky way one of the 2 largest among about 40 galaxies in the local group o Groups of galaxies with more than a few dozen members are often called galaxy clusters • Regions where galaxies and galaxy clusters are most tightly packed are called super clusters which are essentially clusters of galaxy clusters Scale of universe Units Powers of 10 History of universe Motions in universe Star • A large glowing ball of gas that geraerates heat and light through nuclear fusion Planet • A moderately large object that orbits a star it shines by reflected light planets may be rocky icy or gaseous in composition • Reflects sun light Galaxy • A great island of stars in space, held together by gravity and orbiting a common center Universe • The sum of all matter and energy everything within and between all galaxies 1.2 The scale of the universe • units o distance: meter (of kilometer= 1000m) o mass: gram (or kilogram= 1000g) § how much matter in an object o time: second (1 day= 24 hr x 60 min x 60 s) • powers of 10 o need to deal with big numbers: EARTH- SUN distance= 150,000,000 km= 1.5x10^8 kilometer o also small numbers: mass of hydrogen atom = 0.0000000000000000000000017 gram = 1.7 x 10^-24 gram • Distance in astronomy o Astronomical unit- the average distance between the earth and sun (1.5 x 10^8km) o Parsec- the typical distance between stars as defined by the angular wobble caused by earhts orbit (3.1 x10^13 km) o Light-year- the distance between light can travel in one year (9.5 x 10^12 km) • how big is the universe? o the milky way is one of about 100 billion galaxies that we can see o 10^11 stars/galaxy x 10^11 galaxies = 10^22 stars § as many stars as the grains of san on all of earths beaches • history of universe: 14 billion years Destination Light travel time moon 1 second Sun 8 minutes Sirius (nearby star) 8 yr Andromeda galaxy 2.5 million years § **when we see the sun we are seeing it how it was 8 minutes ago 1.3 motion of spaceship earth • earth rotates on axis >1000km/ hr • earth orbits sun >100,000km/ hr • soalr system moves among stars 70,000km/hr • smilky way raotes 800,000km/hr • milky way moves in a local group( as a whole) 2.1 PATTERNS IN THE NIGHT SKY Constellations • Constellations- region of the sky with well defined borders • Names and borders of the 88 official constellations were made in the 1928 by the members of the international astronomical union (IAU) o Most lived in the Europe or US o Constellations in the northern hemisphere can be traced back to civilization of ancient middle east while the southern hemisphere are named by the European explores in the 17thcentury. The Celestial Sphere • Constellation appear to lie close to one another but are far apart because they may lie at very different distances from earth o Due to the lack of depth perception when looking in to space • Useful to map the stars in space • North celestial pole- directly over earths north pole • South celestial pole- over south pole • Celestial equator- projection of earths equator into space • Ecliptic- path the sun follows as it appears to circle around the celestial sphere once a year. Crosses celestial equator at 23.5° angle The Local Sky • Local sky- the sky as seen form wherever you happen to be standing • Horizon- boundary between earth and sky • Zenith- point directly over head • Meridian- imaginary half circle stretching from horizon due south through zenith to horizon due north • To pinpoint a object in the sky you state its direction along the horizon (sometimes stated as azimuth degrees clockwise from due north) and altitude Angular Sizes and Distances • Angular size- the angle it appears to span in your field of view o Does not tell us the true size of an object because it also depends on distance • Size of sun and moon ½° o The sun is 400x bigger than the moon but it is also 400x farther away which gives the sun the same size as the moon • Angular distance-the angle that appears to separate objects • Precise astronomical measurements subdivide each degree into 60 arcminutes and subdivide that into 60 arcseconds o 1°=60’ (arcseconds) o 1’=60’’ (arcminutes) o Ex. 35° 27’ 15’’ read as 35 degrees, 27 arcminutes, 15 arcseconds • small angle formula (valid only when the angular size is small) o ▯▯▯▯▯▯▯ ▯▯=▯▯▯▯▯▯▯▯▯ ▯▯▯▯ ▯▯▯° ▯▯ ▯ ▯▯▯▯▯▯▯▯ o ???????????????????????????? ???????????????? = ????ℎ???????????????????????? ???????????????? ???? ▯▯▯° ▯▯ ▯ ▯▯▯▯▯▯▯▯ • stars near the north celestial pole are circumploar. They remain perpetually above the horizon circling (counterclockwise) around the north celestial pole each day • stars near the south celestial pole never rise above the horizon at all • all other stars have daily circles that are partly above the horizon and partly below it meaning they appear to rise in the east and set in the west Variation with Latitude • latitude- measures north- south position o 0° at equator increasing to 90°N at the north pole and 90°S at the south pole o latitude affects the constellation we see because it affects the locations of the horizon and zenith relative to the celestial sphere • longitude measures east- west position o 0° along the prime meridian which passes through Greenwich England 2.1 patterns in the Night sky • The Celestial Sphere • Stars at different distances all appear to lie on the celestial sphere (projection of earths postions into space) o It is similar to earths equator • Ecliptic is suns apparent path through the celestial sphere • North celestial pole- straight up into space from the north pole • The local sky • An objects altitude (above horizon) and azimuth or direction (along horizon) specifies its location in your local sky • Altitude is measured in degree. • Azimuth is When looking north is 0 degree and N to S is 180 degree • Sky Angular Measurements • Full circle= 360 ° • 1 °= 60’ (arcminutes) • 1’= 60’’ (arcseconds) • Angular size • Angular size/ 360 °= physical size/ circumference • Angular size= physical size * (360°/2pi* distance) o An objects angular size appears smaller the further away it is • Why do stars rise and set? • Earth rotates west to east so stars appear to circle from east to west –Diurnal motion • Why do the constellations we see depend on latitude and time of year? • Latitude : position on earth determines which constellations remain below the horizon • Time of year: earth orbit changes the apparent location of the sun among the stars • Review: coordinates on earth • Latitude: position north or south of equator • Longitude: position east or west of prime meridian (runs through Greenwich, England) • What causes eclipses? • The earth and moon cast shadows • Moon enters earths shadow : lunar eclipse • Moon casts shadow on earth: solar eclipse • Lunar eclipse • Moon appear reddish in totality • When can eclipses occur? • Lunar eclipse Occur only at full moon o can be penumbral, partial or total • Solar eclipses occur only at new moon o Can be partial, total or annular (when the moon is far away so it does not block the sun completely ) Solar eclipse • When the moon is in-between the sun and earth • When the moon passes through earth and the suns plane it is called node • 2 condition must be met to have an eclipse • must be full moon(lunar eclipse) or new moon (solar eclipse) • moon must be at or near one of the two points in its orbit where it crosses the ecliptic plane (nodes) 2.4 The Ancient Mystery of the Planets • mercury o difficult to see; always close to sun in sky • Venus o very bright when visible; morning or evening “star” • mars o noticeably red • Jupiter o Very bright • Saturn o Moderately bright 3.1 What did ancient civilizations achieve in astronomy? • Daily timekeeping • Tracking the seasons and calendar • Monitoring lunar cycles • Monitoring planets and stars o Ancient civilization found out planets moved • Predicting eclipses • Navigation • Days of the wk were named after planets • Sun- Sunday • Moon- Monday • Mars- Tuesday • Mercury- Wednesday • Jupiter- Thursday • Venus- Friday • Saturn – Saturday 3.2 Ancient Greek Science • Greeks were the first people known to make models of nature • they tried to explain patterns in nature without resorting to myth or the supernatural • how did greeks explain planetary motion? • Thought that every thing revolved around us (earth center) in a circular motion Geocentric model • What are keplers three laws of planetary motion • First law: orbit of each planet around the sun is an ellipse with the sun at one focus • Second law: as a planet moves around its orbit it sweeps out equal areas in equal times o means that a planet travels faster when it is nearer to the Sun and slower when it is farther from the Sun. o near perihelion any particular amount of time (such as 30 days) a planet sweeps out an area that is short but wide § planet travels faster when it is close to the sun but travels a longer distance o near aphelion in the same amount ot time a planet sweeps out of an area that is long but narrow § when the planet is farther away from the sun it will travel slower over a short amount of distance § • third law: more distant planets orbit the sun at slower average speeds, obeying the relationship o ???? = ???? ▯ § p= orbital period in years § a= average distance from sun in AU (astronomical units) Qusetions • An asteroid orbits the Sun at an average distance a = 4 AU. How long does it take to orbit the Sun? o P^2=A^3 o P^2=4*4*4 o P=sqrt64 o P=8 • How did Galileo solidify the Copernican revolution? • Galileo (1564-1642) overcame major objections to Copernican view. Three key objections rooted in Aristotelian view were: o 1. Earth could not be moving because objects in air would be left behind. o 2. Non-circular orbits are not “perfect” as heavens should be. o 3. If Earth were really orbiting Sun, we’d detect stellar parallax • Overcoming the first objection (nature of motion): • Galileo’s experiments showed that objects in air would stay with a moving Earth. o Aristotle thought that all objects naturally come to rest. o Galileo showed that objects will stay in motion unless a force acts to slow them down (Newton’s first law of motion). • Overcoming the second objection (heavenly perfection): • Tycho’s observations of comet and supernova already challenged this idea. • Using his telescope, Galileo saw: o Sunspots on Sun (“imperfections”) o Mountains and valleys on the Moon (proving it is not a perfect sphere) • Overcoming the third objection(parallax) • Tycho thought he had measured stellar distances, so lack of parallax seemed to rule out an orbiting Earth. • Galileo showed stars must be much farther than Tycho thought — in part by using his telescope to see the Milky Way is countless individual stars. • If stars were much farther away, then lack of detectable parallax was no longer so troubling. • Galileo also saw four moons orbiting Jupiter, proving that not all objects orbit the Earth • Galileo’s observations of phases of Venus proved that it orbits the Sun and not Earth. • The idealized scientific method • 1. Make observation • 2. Ask a Q • 3. Suggest a hypothesis o Educated guess • 4. Make a prediction • 5. Perform a test: experiment or additional observation o Test supports hypotheses; make additional predictions and test them o Test does not support hypotheses revise hypotheses or make a new one • Hallmark of science #1 • Modern science seeks explanations for observed phenomena that rely solely on natural causes. • Hallmark of science# 2 • Science progresses through the creation and testing of models of nature that explain the observations as simply as possible. o (Simplicity = “Occam’s razor”) • Hallmark of Science: #3 • A scientific model must make testable predictions about natural phenomena that would force us to revise or abandon the model if the predictions do not agree with observations. • What is a scientific theory? • The word theory has a different meaning in science than in everyday life. • In science, a theory is NOT the same as a hypothesis, rather: • A scientific theory must: • Explain a wide variety of observations with a few simple principles, AND • Must be supported by a large, compelling body of evidence. • Must NOT have failed any crucial test of its validity. •
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