Astronomy Study Guide for Exam 1
Astronomy Study Guide for Exam 1 AST2002-16Fall 0001
University of Central Florida
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This 20 page Study Guide was uploaded by Becca Petersen on Thursday September 15, 2016. The Study Guide belongs to AST2002-16Fall 0001 at University of Central Florida taught by Dr. James Cooney in Fall 2016. Since its upload, it has received 174 views. For similar materials see Astronomy in Science at University of Central Florida.
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Date Created: 09/15/16
Astronomy Chapters 1 -3 (book/class notes) Picture of the day: Density of “stuff” (gas and dust) is actually very thin Hydrogen and helium – almost all of the universe is hydrogen and the parts that aren’t hydrogen are helium Scattered blue light – is light reflectant and scattered off of the new stars Red light – is omitted light from the gases 10 billion is 10^10 1 million is 10^6 10^11+10^14=10^25 (just know how that works) Earth – solar system – galaxy-‐ local group – super cluster – universe Dimensions and units: Length-‐ SI (shorthand for the metric system) – meter – distance light travels in 1/299792458 s – meter becomes too cumbersome to deal with when distances become too large. So in order to not have to carry around a bunch of powers of tens we us the following . . . Astronomical Unit (AU) – 150 million km – average distance from the earth to the sun – an average because we don’t orbit in a perfect circle around the sun (standard unit of distance for anything within our solar system) Light Year – 9.5 trillion km – the distance that light can travel in a vacuum in one year – it is not time it is a distance – good for measuring our neighborhood as far as stars are concerned Parsec – How far away a star would have to be to appear to shift by one arc second when the earth moves by one AU . 30 trillion km (definition is a little complicated but it is essentially three light years) 2 concepts that go into its definition: Concept 1: Parallax – it is something you experience in your everyday life – it is the apparent motion of something not because it’s actually moving but because the observation point has changed. We can use that to measure the distance to stars. The closer the object is, the more it will shift (when we switch eyes/observation points) Ex: hold out your finger at extended arms length and close one eye. Then switch and close the other. The point of your finger will shift. Parallax only works with nearby stars but the more the stars shift the closer it is. Concept 2: Arc minutes -‐ You are really measuring an angle when you’re measuring how something shifts. In astronomy you measure angles by arc minutes. There are 360 degrees in a circle. 1 degree = 60 arc minutes One arc minute can be divided up into 60 pieces to be called an arc second. Si second duration of 9129631770 periods of radiation given off by hyperfine transition cesium 133 Mass – SI kilogram – standard kg located in France Solar mass = mass of the sun Chapter 2: The Night Sky General patterns in the sky… (Chapter preview) There are patterns of motions in the sky The sunrises and sets (so does everything else…moon, stars, planets, etc.) The circling sky is a result of the rotation of the earth on its access The orbit of the earth around the sun combined with the earth’s access tilt is also the reason for seasons. The moons orbit around the earth causes it to go through different phases. The ancient mystery of the Planets – the various orbits around the sun. Day – earth’s access Week -‐ planets’ orbit – Mercury, Venus, Mars, Jupiter, moon, sun. The 7-‐ day week was created named after those objects^ Month -‐ the moon’s orbit around the earth Year -‐ the earths orbit around the sun Constellations: A region of the sky, within official borders set in 1928 by the IAU Often recognizable by a pattern or grouping of stars - Constellations, while interesting, are not really of that much use in astronomy. Astronomy is more concerned with borders of constellations. So stars that do not constitute the stick figure of Orion are still considered part of that constellation if they fall within the set borders. To us the sky looks like a hemispherical screen. We can only understand and see a 2 dimensional view of a 3d universe. So when you look up you perceive stars and planets to be close together but that’s not necessarily the case. There are coordinates in the sky just like there are coordinates on the earth Coordinates on the earth: Latitude is where you are north and south referencing the equator Longitude – prime meridian is in Greenwich England. (We are west of the prime meridian) Sphere of the sky is called the celestial sphere – obviously in real life the universe is not a sphere – but that’s what they used to believe. So even though it’s not a physical reality it’s useful to describe what we see. And we can apply the same latitude longitude concepts to describe the night sky (picture earth sitting smack in the middle of another sphere) Reference slide for photo Take earth’s equator and move it out that becomes the celestial equator. Same thing for the celestial poles Ecliptic – a word we use for the path that the sun takes around the celestial sphere Daily Motion: The sun rises and sets at an angle. And that angle depends where on earth that you are. Going off of that idea for that same model: If you are at either of the poles everything moves parallel to the horizon. And if you are at the equator everything goes straight up and straight down rises and sets 90 degrees to the horizon. ***Reference the photo in the slide Some stars never set (circumpolar) and others never rise Many stars (and sun and moon and planets) rise in the east and set in the west Angle at which they rise and set is determined by your latitude Zenith-‐whatever is above you if you look straight up (wherever you are, it does not matter) The Day Sidereal day – How long it takes to rotate once 23 hours, 56 minutes, and 4.1 seconds Solar day-‐ Noon to noon 24 hours Annual Motion As earth orbits the sun, the sun appears to move eastward with respect to the stars The sun circles the celestial sphere once every year. A year is a little bit complicated but it is about 365.25 days It’s not an even number – thus the need for a “leap” year every four years Seasons – Has to do with the angle that sun is shining on the earth The earth’s axis is tilted 23.5 degrees from being perpendicular to the elliptic plane Therefore, the celestial equator is tilted 23.5 to the ecliptic As seen from earth, the sun spends 6 months north of the celestial equator and 6 months south of the equator Seasons are caused by the earth’s axis tilt – Not the distance from the earth to the sun!!!! Ecliptic – the apparent path of the sun through the sky Equinox-‐ where the ecliptic intersects the celestial equator Solstice-‐ where the ecliptic is the farthest from the celestial equator Zodiac-‐ The sun is never actually directly overhead at noon in central Florida – the sun is most directly overhead at the summer solstice (June 21) In order for the sun to at some point be over your head (at some point during the year) you need to live within the tropics (cancer and Capricorn) 23.5 degrees north or south Example test questions: Describe the motion of the sun in the sky in the summer and in the winter… The sun will stay at 23.5 degrees. If you live at the North Pole you’re just sitting there most of the day. The sun won’t reach your horizon until the equinox. December 21 – the sun is 23.5 degrees below your horizon so it’s dark for 6 months Be able to describe locations of the sun depending on your location on earth How long is a day in the summer? Winter? When will the sun be directly overhead? Special lines of latitude on earth – tropic of cancer, Capricorn where at some times of the year the sun will be directly over your head. Seasons – although the solstice, which occurs around June 21, is considered the first day of summer It takes time or more direct sunlight to heat up the land and water Therefore, July and August are typically hotter than June It takes a while for things to heat up or things to cool down Why distance doesn’t matter… Small variation for earth – about 3% but distance does matter for some other planets, notably, mars and Pluto Surprisingly, seasons are more extreme in N hemisphere even though earth is closer to the Sun in the Southern hemisphere summer and farther in the southern hemisphere winter. Because of the land/ocean distribution. Precession of the equinoxes The earth’s axis processes wobbles like a top once every 26,000 years. Precession changes the positions in the sky of the celestial poles and the equinoxes Polaris wont always be the North Star The spring equinox seen by ancient Greeks in Aries moves westward and is now in Pisces. The moon Lunar moon: Phases of the moon’s 29.5 day cycle – it takes 28 days for the moon to actually orbit the earth Waxing: New – can’t see any of it Crescent – can see less than half of the face First quarter – once you can see half (called a quarter because it’s a quarter of the way through the cycle) Gibbous – more than half of the face Full – entire Waning: Gibbous Last quarter Crescent The cycle of the moon’s phases has to do with the relative position of he sun, earth, and the moon Why do we see the phases? Half of the moon is illuminated by sun and half dark We see combination of the bright and dark faces Perfectly aligned sun and moon – creates a shadow on the earth which we call a solar eclipse They are usually not perfectly aligned Lunar month – new moon to new moon Sidereal month – the time it takes for the moon to go around the sun The moon rises at 6am – (sunrise) New moons always rise around sunrise and set around sunset Most directly over your head at midnight Synchronous rotation – how long it takes to spin once – rotation period = orbital period Moon is more than 250k miles away Ancient Greeks didn’t have telescopes and couldn’t see parallax (especially with the stars) so in their eyes, the earth was at rest. Claudius Ptolemy (AD 100-170) He made the first scientific intellectual model of the universe. (A model that makes predictions on what will happen and where things will be a week from now) His model puts earth in the center of the solar system / universe (we call this a geocentric (earth centered) model How do you explain how sometimes the planets switch and travel the wrong way? (Retrograde movement) He said the planets move in little epicycles (their own circles) around the earth One thing spinning around in a circle and simultaneously traveling in a bigger circle The reason for this was that he thought that in the midst of traveling around in those little epicycles, that it would momentarily travel the opposite direction – reference-‐mastering astronomy online models to better understand this movement In his model the celestial sphere was outside our solar system As time went on and they found flaws in his theory, he and his followers just adjusted their hypothesis adding more and more random, arbitrary epicycles and circles that didn’t really make sense Copernicus – came up with a sun centered, heliocentric model/hypothesis – much simpler Occam’s Razor The idea that if you have two different ideas that correctly explain the same thing, the simpler one is probably more accurate. With that idea in mind, we adopted the Copernicus model to replace Ptolamine’s Copernicus – Polish scientist and founder of our modern view of the solar system His uncle was a bishop and got him a position as a canon of frauenburg cathedral which he held for the rest of his life In that day and age when he was working on this model is was dangerous to suggest that the sun was at the center – the religious community found his theory to be troublesome to say the least He wrote a book of his hypotheses, which he did not publish until the year he died The Catholic Church actually banned this book There is still something wrong with the Copernicus system Tycho Brahe Super rich Danish guy He wanted to build the greatest astronomical abservatory in the world – he built these great instruments – not telescopes because those didn’t exist – but they had instruments to measure things He was the greatest observational astronomer of his day His assistant was Kepler He collected all this data about things like how venus moves day to day Johannes Kepler Kepler found flaws with the Copernicus theory -‐ he realized Tycho Brahe had all the best data – he needed Brahe’s data but he wouldn’t give it to him, even though he was his ‘assistant’ … Tycho Brahe mysteriously died from mercury poisoning… After his death he got ahold of all of his data to test his own ideas of what was happening but after years of sort of playing around with Brahe’s data, he finally found a hypothesis to match the data He was also an astrologer as well as an astronomer He came up with a set of three laws to be tacked on to the Copernicus theory that made it work perfectly 1. He recognized that one of the principle problems with the Copernicus theory, as so many thought before him, was that everything in heaven moves in perfect circles Each planet’s orbit around the sun is in an ellipse with the sun at one focus. An ellipse is sort of just an elongated circle. The one “focus” is the sun and the other focus is just empty– reference pic on slides Semi major axis – is essentially just the radius of an ellipse Perihelion – the point on the ellipse when the object is closest to the sun Eccentricity of an ellipse – spreading the focus points out will increase the eccentricity 2. As a planet is orbiting the sun it doesn’t always move at the same speed. It moves faster when its closer to the sun and slower whenever it is farther away. (qualitative explanation) Text book definition -‐ “A planet moves along its orbit with a speed that changes in such a way that a lone from the planet to the sun sweeps out equal areas in equal intervals of time.” That is to say that in that picture, all of the intervals will have exactly the same area 3. a^3 = P^2 When comparing two planets – lets say Mars to Jupiter… The planets that area farther out (like Jupiter) orbit more slowly than the planets (like Mars) that are closer to the sun in our solar system The ratio of the cube of a planet’s average distance from the sun to the square of its orbital period is the same for each planet P is the period of the orbit – so for example, earth’s orbital period is one year Jupiter has a semi-‐major axis of about 5 AU (remember 1 au is the average distance from the earth to the sun) Galileo – Galileo popularized the Copernicus model and Kepler’s laws – up until then they were not very accessible – so he went around giving tours, and lectures and writing books. He eventually got into trouble for that with the church and was placed under house arrest and forced to recant He is most famous for being the first astronomer to look through a telescope – it was made for military purposes so no one else was really using it for astronomy so he made all kinds of discoveries with that. Almagest Star catalogue Instruments Some basic physics – Kepler’s laws seem kind of arbitrary – there are basic laws of nature and physics that underlie Kepler’s laws Motion Position Requires a coordinate system (ex: it lies 3 meters above …..) Displacement – change in the position of things Velocity Rate of change of position – describes how quickly that displacement takes place A vector Not only the speed but the direction** (ex: 60 miles per hour, north) Acceleration Rate of change of velocity (ex – if you go from one meter per second to to meters per second then your acceleration was 2 meters per second-‐ per second) Also a vector – has a direction Understand that…. You can accelerate without changing your speed – going around in a circle where your speed is the same but your direction changes Moving in a straight line in uniform speed – you have a velocity but no acceleration You can have acceleration not equal to zero but velocity equal to zero – if you toss something up in the air – its direction of its velocity is going up but the acceleration is going down – it slows down as it travels higher in the air – and that statement is the case at its very peak Issac Newton Between 1665-‐1667 (he was 22 in 65) Known for… *He sort of co-‐invented calculus *Newton’s laws *Basis of all physics until Einstein *Law of universal gravitation He wanted to understand the nature of the universe and why Kepler's laws worked the way they did. So he basically found the fundamental laws of nature Came up with the universal law of gravitation Newton’s Laws of Motion 1. A body remains at rest or moves with constant velocity unless acted upon by an outside force – I.e. whatever state of motion they are in, they are going to keep that exact velocity until you do something to it. There are things in our every day lives (friction, gravity, air resistance) that will act on objects to change their velocity or stop them. Being at rest is essentially the same thing as being in perfect constant motion (think about being on an airplane-‐ do you know you’re moving? – the only way you know you’re moving is by gaging it on other things. So motion is very relative. 2. The change in a body’s velocity due to an applied force is in the same direction as the force and proportional to it. But is inversely proportional to the body’s mass. F=ma Force = mass times acceleration Mass is a measure of how much something does not want its motion changed. High mass thing will have a very low acceleration and vise versa. So pushing a brick will move more slowly than pushing a stack of sticky notes. Think of the example from class – if you out a lead brick over your hand (lets say its 30 pounds) it has a higher mass and doesn’t want to move so you can hit it really hard with a hammer and feel no pain. And vice versa with a block of wood. It has less mass and is much easier to move because of the less resistance so you will experience much more pain 3.For every applied force, a force of equal size but opposite direction arises. Object A acts on object B and object B acts on object A So the earth is pulling you down by 160lbs and you are pulling the earth up by 160lbs. Astronomy Chapter 2 Book Notes Key: Blue – vocab term Yellow – definition Italics – important info Bold – big umbrella vocab terms Constellations-‐ A region of the sky with well-‐defined borders Patterns of stars (ex the little dipper) just help us locate these regions but are not actually the constellations in the eyes of the International Astronomical Union who chooses official constellations. -‐88 total -‐every point in the sky belongs to some constellation Celestial Sphere-‐ An illusion that the Greeks mistook for a reality. Now we just use it as a visual to better understand a map of the sky as seen from earth. -‐Earth sits in the center -‐Earth’s equator matches up with the celestial equator “projection of earth’s equator into space” -‐ makes a complete circle North Celestial Pole-‐ The point directly over earth’s north pole South Celestial Pole-‐ The point directly over earth’s south pole Ecliptic-‐ The path the sun follows as it appears to circle around the celestial sphere once a year It crosses the celestial equator at 231/2 degrees because that is the tilt of the earth’s axis. Milky Way-‐ The band of light that circles all the way around the celestial sphere *Note that this is different than the Milky Way Galaxy Relationship between the Milky Way and the Milky Way Galaxy – It traces our galaxy’s disk of stars – the galactic plane – as it appears from our location within the galaxy. -‐Our galaxy is shaped like a thin pancake with a bulge in the middle -‐Stars and interstellar clouds make up the milky way we see in the night sky – Which is the band of light that makes a full circle around our sky -‐The dark parts are the parts with the densest clouds They prevented us from seeing more than a few thousand lightyears into our galaxy’s disk until recent advancements in technology. The celestial sphere is a useful diagram but its not a good representation of what we actually see whenever we go outside. Local Sky -‐The sky that you see from wherever you happen to be standing Appears to take the shape of a hemisphere or dome We only see half of the celestial sphere because the ground blocks the other half. (Imagine you are on flat ground with nothing intervening in your vision of the horizon that would surround you.) Horizon-‐ The boundary between earth and sky Zenith-‐ The point directly overhead from wherever you’re standing – 90 degrees Meridian-‐ An imaginary half-‐circle stretching from the horizon due south through the zenith, to the horizon due north Direction-‐ We can pinpoint the position of any object in the local sky by stating its direction along the horizon (sometimes referred to as the azimuth, which is degrees clockwise from due north) Altitude-‐ This one is pretty straightforward – but how high up something is It is pretty hard to know off the bat how far away things are from each other. Depth perception is a difficult thing to gage when you’re speaking in terms of light years. But we can use angles to describe relative locations. Angular Size-‐ The angular size of an object is the angle it appears to span in your field of view The farther away an object is, the smaller its angular size will be *View diagram on page 28 to better understand Angular distance-‐ Is measured between a pair of objects in the sky. It is the angle that appears to separate them. Arc minutes-‐ Exist as a subdivision between degrees for a more precise measurement. 1 degree is broken down into 60 arc minutes (‘). Each arc minute is broken down into 60 arc seconds (“) Stars “rising and setting” – Although this is not physically the case, it appears that the celestial sphere rotates around earth. – every object in the celestial sphere appears to circle around earth in a simple daily motion. The motion looks a little more complex from the local sky because the motion is cut in half since we only ever see half of the celestial sphere at any given time. -‐stars near the north celestial pole are circumpolar, meaning that the 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 the other stars have daily circles that are partly above the horizon and partly below, which means they appear to rise in the east and set in the west. Earth’s west to east rotation makes stars appear to move from east to west though the sky as they circle around the celestial poles. If they are large enough, the circles cross the horizon, so that the stars rise in the east and set in the west. You will see different constellations at different times of the year. Variation with latitude-‐ Latitude-‐ Measures north-‐south position on earth Defined to be 0 degrees at the equator Longitude-‐ Measures east-‐west positions Defined to be 0 degrees along the prime meridian Latitude effects the constellations that we see because it affects the locations of the horizon and zenith relative to the celestial sphere Although the sky varies with latitude, it does not vary with longitude. At the north pole, you can only see objects that lie on the northern half of the celestial sphere, and they are all circumpolar. The sun remains above the horizon for 6 months at the north pole for this reason. It lies north of the celestial equator for half of each year. The Reason for Seasons: The earth’s rotation makes the sky appear to circle us. – the combination of the earth’s rotation and orbit leads to the progression of seasons The tilt of the earth’s axis causes sunlight to fall differently on earth at different times of the year The tilt of the earth’s axis remains pointed in the same direction in space (towards Polaris) throughout the year. The orientation relative to the sun changes over the course of each orbit: The Northern hemisphere is tipped toward the Sun in June and away from the sun in December while the reverse is true for the Southern Hemisphere – that is why the two hemispheres experience opposite seasons. A sidereal day – is how long it takes any star to make one full circuit through our sky. Solar Day – our 24 hour day – the average time it takes for the sun to make on circuit through the sky The earth is only about 3% farther from the sun at its farthest point than at its nearest. The difference in the strength of sunlight due to this small change in distance is overwhelmed by the effects caused by the axis tilt. – Which is just reiterating that the distance from the sun DOES NOT cause seasons To help mark the changing seasons, we define four positions in earth’s orbit throughout the year 1. June Solstice – summer solstice 2. December Solstice – or winter solstice 3. March equinox – or spring equinox 4. September Equinox – fall equinox Precession – a gradual wobble that alters the orientation of earth’s axis in space Lunar Phases – a time period of about 29 ½ days where the moon orbits earth and returns to the same position relative to the sun in our sky To understand phases understand that… Sunlight essentially comes at both the moon and the earth from the same direction 2 basic facts: 1. Half of the moon always faces the sun and therefore, is bright, while the other half faces away from the sun and is dark 2. As the moon orbit its just different combinations of its bright and dark faces. The moons phase is directly related to the time it raises, reaches its highest point in the sky, and sets. For example, the full moon must rise around sunset because it occurs when the moon is opposite the sun in the sky. See figure 2.21****** (discuses waxing and waning) Although we see many phases of the moon, we do not see many faces. From earth we always see nearly the same face of the moon. This is because the moon rotates on its axis in the same amount of time it takes to orbit earth. This is called Synchronous rotation. Eclipse – when the sun, moon, and earth fall in a straight line Lunar eclipse – when earth lies directly between the sun and the moon so earth’s shadow falls on the moon Solar eclipse – occurs when the moon lays directly between the sun and the earth, so the moon’s shadow falls on earth. Nodes-‐ two points in each orbit at which the moon crosses the surface of the ecliptic plane Eclipses can only occur when the phase of the moon is full for a lunar eclipse or new for a solar eclipse and the new or full moon occurs at a time when the moon is very close to a node
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