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Department
Astronomy
Course
AS101
Professor
Victor Aurora
Semester
Winter

Description
1. Introduction  02/05/2014 LECTURE Learning goals: Understand how all the mass and energy in the universe is divided up Recognize different astronomical objects in the universe Identify which objects reside within the different levels of structure The Cosmos 4.9% = Atoms 68.3% = Dark energy 26.8% = Dark matter "Dark" = not visible Mostly vacuum (empty space) Everything we can possibly see is a tiny fraction Why does it seem like there are so many objects in space (if it's mostly empty)? The Sun is Our Star Massive ball of glowing gas that generates energy through nuclear fusion About 100x as wide as the Earth Indirect source of almost all energy on Earth Planets are less massive than stars Rocky = Mars Gaseous = Neptune Definition of planet Non-luminous and spherical Sphere indicates pulled together by gravity In orbit around a star "Cleared the neighbourhood" of other objects Why Pluto is no longer a planet Exoplanets Potential planets Over 900 planets confirmed to be orbiting other stars Thousands of "candidate" exoplanets, observed by Kepler Space Telescope Need to be confirmed through multiple observations Some planets have satellites An object in orbit around a planet Natural satellites are also known as moons Ice and rock = orbit around Saturn (ring) Other objects in the solar system Asteroid: A small, rocky object orbiting a star Comet: A small, icy object that orbits the sun Studying of these things can help in studying how the solar system came to be Largest known trans-Neptunian objects (TNOs) Thousands of them Called "dwarf planet" Galaxies: "Cities" of the Cosmos A large system of dark matter, stars, gas and dust, all bound together by their combined gravity Not all galaxies are spirals Pizza dough analogy Something is holding the galaxy together Gravity is not enough (spinning too fast) Best explanation is dark matter Galaxy would not be able to exist with gravity alone Nebulae Clouds of gas and/or "dust" within galaxies Raw materials for new stars from previous generations Star clusters Open clusters: 1000's of stars Gobular clusters: 100000's of stars Galaxy groups + clusters A group of galaxies (a few dozen up to thousands) all held together gravitationally Many clusters form a supercluster (make up the cosmic web) Make up a wall Empty spots are "voids" READING 2. Scale of the Universe 02/05/2014 First quiz will be online tomorrow night 2. Scale of the Universe 02/05/2014 Due next Thursday (Jan. 16) Models in Science Model = something that represents reality A scientific model is a hypothesis that describes reality and has withstood observational or experimental tests We can also develop conceptual models to help us think about how nature works Scale model: the solar system What if we shrunk the solar system by a factor of about 60 billion? Sun -> radius = 12mm Earth -> 0.1mm Jupiter -> 1 mm Neptune -> 0.4 mm The Astronomical Unit We can specify distance sin the solar system by comparing them to the average Earth- Sun distance 1 AU = 150 000 000 km = 150 000 000 000 m There has to be a better way What if we just specified how many zeroes a number has? This is known as scientific notation Ex. 2. Scale of the Universe 02/05/2014 10 = 101 100 = 10 x 10 = 102 1000 = 10 x 10 x 10 = 103 1 Googol = 1 with 100 zeroes after it (10100) Representing Large Numbers The Earth is on average 150 000 000 000 m from the sun In scientific notation, we would write this as a coefficient between one and ten, multiplied by the appropriate power of ten Hint: Just count the number of places the decimal would move 150 000 000 000 m = 1.5 x 1011 m ^ coefficient Using Exponents Made Easy To multiply two (or more) numbers written in scientific notation, just: Multiply the coefficients Add the exponents 1 1 2 Ex. 10 x 10 = 10 x 10 = 10 10 x 10 = 10 5 Dividing two numbers written in scientific notation is similar, except the divisor exponent is subtracted from the dividend exponent 10 / 10 = 10 2 How Far Away Are The Stars? The nearest star system is Alpha Centauri: 4.1 x 10 km 2. Scale of the Universe 02/05/2014 Using our scale from our model solar system 60 billion actual km = 1 scale km Light-travel and look-back time BUT light has a finite speed: 300 000 km/s The star is so far away that we are seeing it as it looked in the past Imagine you can walk at a rate of 3 km/hr; in one hour, you will walk 3 km Let's call distance this a "walk-hour" In one year (8760 hours), you will walk 26 280 km Let's call distance this a "walk-year" A light-year, is the distance light will travel in one year - about 10 km Alpha-Centauri is 4 light-years away Distances in the Universe The further an object appears in space, the further we are looking back in time The sun = about 8 light minutes The light/sun/heat that we are experiencing from the sun was developed 8 minutes prior to retrieval on earth Alpha Centauri = 4 light-years away Andromeda Galaxy = 2 million light-years away Observable universe = 13.82 billion light years Putting It Into Perspective What if the entire age of the universe were one calendar year? 3. Observing pt. 1 02/05/2014 Learning goals 3. Observing pt. 1 02/05/2014 By the end of class, we should be able to: Describe how the earth’s rotation causes the sky to change over a single night Use the celestial sphere model to map out the relative positions and nightly motions of the stars Identify how our location on the earth changes what we see in the sky How the Earth is moving Two basic motions 1. Revolves around (“orbits”) the Sun 2. Rotates on its (tilted) axis Which motion is responsible for rising and setting of the Sun? To a moving observer, stationary objects appear to move in the (same) direction as the observer’s motion The Earth rotates from west to east The Celestial Sphere A conceptual model of the sky An imaginary sphere of very large radius surrounding the earth Assumptions Objects in the sky appear to be attached to the rotating celestial sphere Allows us to specify the direction to an object in the sky and how it moves over a single night Celestial Sphere Model From the inside, you can see exactly one-half of the entire sky at any given time, from your horizon up to your zenith (directly overhead) The north and south celestial poles are directly above the Earth’s north and south poles 3. Observing pt. 1 02/05/2014 Your meridian is the line passing through the north point on your horizon, through your zenith, and to the south point on your horizon Inside view Some stars never appear to set – in the Northern hemisphere they are located near the north celestial pole These are called circumpolar stars because they appear to circle the pole, and never set The north celestial pole is near the star Polaris Other stars also make circles around one of the celestial poles but part of their daily path is below the horizon Stars reach their highest points when they cross the meridian Terrestrial coordinates We specify locations on Earth using latitude and longitude Longitude: in degrees East of West of the Prime Meridian Latitude: in degrees North or South of the equator On the Earth, intersecting lines of longitude and latitude form a grid Imagine this grid projected outward onto the celestial sphere We specify positions in the sky using right ascension (RA) and declination (dec) Right ascension: in hours, east of the vernal equinox From 0-24 hours Declination: in degrees, north (+) or south (-) of the celestial equator From +90 degrees to -90 degrees Astronomy-brain training Recall: we can only see one half of the sky at one time The half that we see at a given time of night changes depending on: The position of the earth along the annual orbit The latitude of the observer Summary Objects in the night-sky rotate due to the Earth’s rotation on its axis The celestial sphere is a model that helps us map out the relative positions and the nightly motions of these objects What we see in the night-sky depends on our latitude 4. Observing pt. 2 02/05/2014 Recap: 4. Observing pt. 2 02/05/2014 The earth’s rotation on its axis causes objects in the night-sky to rotate as well We can specify the positions of these objects and visualize their motion using the celestial sphere model The stars that we can see, and how they move over the course of the night, both change with latitude Learning goals: Describe some ways that we find our way around the night-sky Understand how we measure angular size in the sky and the brightnesses of stars Describe how the earth’s revolution causes what we can see in the night-sky to change over the course of a year Precession: A slow wobble The sun and moon are unevenly pulling on the slight (43 km) bulge around the earth’s equator This causes a very slow but regular wobble of the earth’s axis The orientation of the axis changes but it remains tilted at 23.5 degrees Precession changes the NCP This very slowly changes the position of the NCP, and all the other coordinates Catalogues of stellar coordinates are slightly adjusted only every 50 years The constellations Then: The constellations are arbitrary patterns of stars invented, mostly, by ancient cultures Now: Today, astronomers officially recognize 88 constellations The sky is divided into parcels that each contain the constellation and other stars or objects in the same part of the sky 4. Observing pt. 2 02/05/2014 Stars in the constellations The stars in a constellation are not at the same distance from the earth, just in the same direction All the patterns made by the stars will change as the sun (and other stars) slowly orbit the centre of the milky way Measuring angular distances You can measure the angular sizes and separations on the celestial sphere using degrees The meridian spans a distance of 180 degrees Rule of pinky: pinky held at arm length = 1 degree Rule of fist: fist held at arm length = 10 degrees Measuring apparent brightness Astronomers sometimes specify how bright an object would appear to the naked eye using the magnitude scale For historical reasons, the scale runs backwards Our eyes are non-linear detectors: each step in magnitude is a factor of 2.5x brighter Professional astronomers measure the “flux” (the rate of light energy collected per unit area) If we don’t know how far away the star is, its apparent brightness does not tell us about its true (absolute) power output Sidereal and solar days Recall: the two basic motions of the earth are revolution and rotation How can we define what a “day” should be? Two ideas: Solar day: the average time between successive meridian crossings from the sun (24 hours) Sidereal days: the time between successive meridian crossings for any other star (23h 56m) Yearly variation 4. Observing pt. 2 02/05/2014 A slight difference means that the stars rise about 4 minutes earlier each night From dividing 24 hours (of right ascension) by ___ days Over the course of a year, the constellations we see at night time change The underlying cause of the change is the earth’s revolution around the sun Summary Constellations are invented patterns used to help demarcate and find our way around the sky The apparent brightness of stars is measured using a non-linear scale that runs backwards The earth’s revolution around the sun causes different constellations to be visible through the year Predict when star will appear at certain time Star appears at 3AM Want to see it at 10PM Stars rise 4 minutes earlier each night (r) The number of hours between when the star appears and when we want to see it (h) Minutes in an hour (m) Days in a month one would have to wait to see the star at preferred time (d) h x m = ____ / r = d Ex. 5 x 60 = 300 / 4 = 75 5. Seasons 02/05/2014 Recap: Objects in the sky move in circles around the celestial poles due to the earth’s rotation on its axis What we see in the night­sky depends on our latitude The earth’s revolution around the sun causes different constellations to be visible at night through the year Learning goals Identify what two factors are responsible for the earth’s seasons and how they each contribute Explains how the position of the sun on the celestial sphere changes over the course of a year Recall: the orbit of the earth The earth’s orbit is nearly circular By what percent does the distance from the sun vary from the average through the year? 0.033 or 3.3% “Mythbusters”: the real­world The average daytime high temperatures in Waterloo (latitude 43 degrees N) and Christchurch, New  Zealand (latitude 43 degrees S) Opposite hemispheres have opposite seasons at the same time of year Conclusion: distance from the sun does not cause the seasons Also, the earth’s axial tilt does not bring on hemisphere any closer to the sun than the small variation in  distance Angle of light Hypothesis: changing the distance of the sun (angle above horizon) changes the amount of energy on the  ground Annual motion of the earth The earth revolves around the sun in 365.26 days and remains tilted at 23.5 degrees The ecliptic is the plane of the earth’s orbit around the sun 5. Seasons 02/05/2014 The path of the earth around the sun is a nearly perfect circle The amount of solar energy received in either hemisphere varies through the year due to the tilt Northern summer solstice: 6/21 Northern hemisphere: sunlight hits surface at nearly perpendicular angle Energy is more concentrated Start of summer Southern hemisphere: sunlight hits the surface at a grazing angle Energy is spread out Start of winter Northern winter solstice: 12/21 Northern hemisphere: sunlight hits the surface at a grazing angle Energy is spread out Start of winter Southern hemisphere: sunlight hits the surface at a nearly perpendicular angle Energy is more concentrated Start of summer Equinoxes: 3/21 or 9/21 Days with roughly equal amounts of daylight and darkness, everywhere on Earth Mark the beginning of spring (vernal) or fall (autumnal) Apparent annual motion of sun Recall: the stars rise 4 minutes earlier every night ▯ they move east 5. Seasons 02/05/2014 On the celestial sphere, the sun moves eastward with respect to the “background stars” We can also think of the ecliptic as the apparent yearly path of the sun amongst the stars The sun spends half the year north of the celestial equator (CE) and half the year south of the CE 1. Vernal equinox: sun crosses CE heading north 2. Summer solstice: sun at furthest point north from CE 3. Autumnal equinox: sun crosses CE heading south 4. Winter solstice: sun at furthest point south from CE Length of days The sun rises and sets at different times through the year and in different cardinal directions In the northern hemisphere, there are more daylight hours when the sun is further north More daylight hours in the summer means more energy from the sun is received at the surface per day Summary Observations: Earth keeps nearly the same distance from the sun The seasons are opposite in opposite hemispheres Model: the sun is highest in the sky in the summer Warming from sunlight is more efficient when it is nearly perpendicular to the earth’s surface More hours of sunlight means more time for warming 6. Lunar Phases and Eclipses 02/05/2014 Recap: The sun reaches its highest altitude in the summer because the earth is tilted on its axis  Warming is more efficient and longer in daily duration Distance is not a major factor in the seasons Learning goals Identify the phases of moon and understand their relationship to the position of the moon in its orbit The earth and the moon are together The moon orbits the earth in a slightly elliptical path once in approximately one month How does the moon shine? The moon shines by reflecting light from the sun Only half of the spherical moon is illuminated at any given time (just like the earth) The moon rotates! This must be the case since we only ever see same side of moon There is no permanently “dark side” of the moon, only a near­side and far­side The moon’s librations allow us to see 59% of the surface over the course of a lunar cycle How long is a moon­th? 1. How long does the moon take to orbit the earth once? Reference point: background stars Time: 27.32 days ▯ sidereal month 2. How long does the lunar phase cycle take? Reference point: the sun Time: 29.53 days ▯ synodic month Summary The phases of the moon are a result of different parts of the near­side of the moon becoming  illuminated when it is at different points in its rotation 7. Story of Astronomy + Scientific Method 02/05/2014 Recap:  7. Story of Astronomy + Scientific Method 02/05/2014 The phases of the moon are caused by the location of the moon along its orbit Different parts of the earth­facing side become illuminated The moon rises and sets at different times depending on its position in the orbit If the moon crosses the ecliptic plane during the appropriate phase, we have either a lunar or solar eclipse Learning goals Explain how and why the scientific method gives self­consistent explanations about the natural world Identify observations that supported and weakened the geocentric model of the universe  Suggest reasons that the heliocentric model of the universe became the preferred model Scientific method Science is a systematic way to investigate the physical world Theory ▯ test the idea (in a controlled and reproducible experiment) ▯ observations (data from observing  nature) ▯ hypothesis (the simplest idea that explains the data) ▯ implications (what does the hypothesis  predict?) ▯ test the idea ▯ etc. Scientific theory If the outcome of the test… Supports the hypothesis, then it should be tested again by multiple independent researchers Weakens the hypothesis, then the hypothesis may need to be refined or discarded entirely In order for an idea to become accepted as a scientific theory, it must: Be tested repeatedly by multiple groups, and not shown to be false A scientific theory is much stronger than a hypothesis All scientific knowledge is provisional But extraordinary claims require extraordinary evidence (Carl Sagan) 7. Story of Astronomy + Scientific Method 02/05/2014 Classical Greek Civilization Ancient peoples observed that: The sun, moon and stars appear to revolve around us Different stars are visible at different times of the year Classical Greek civilization believed in building and testing models to explain nature Pythagoras (500 BCE): the earth is spherical Boats disappeared over the horizon Not only did this idea agree with observations, but it also agreed with the classical Greeks’ notion of  “geometric perfection” from Plato Geocentric model Aristotle Model The earth is motionless Heavenly bodies move on circular paths at constant speeds around the earth (each planet is on a different  sphere) How did his beliefs bias his thinking? Idea of heavenly perfection: spheres, uniform motion Is the earth in a special place? Observations: retrograde motion As viewed from the earth, planets generally move eastward along the ecliptic with respect to the stars over  a time­period of months (or years) Sometimes planets appear to stop moving eastward, and move backwards in a loop, before continuing  eastward About how long does it take to make this loop? Planets moving in epicycles? 7. Story of Astronomy + Scientific Method 02/05/2014 Ptolemy Proposed a mathematical model that kept a motionless earth at the centre of the universe Hypothesis: Planets follow a smaller circle (“epicycle”) that itself spins on a larger circle (“deferent”) Also, try moving the earth slightly off centre Also, add more “epicycles” Problems with geocentric model Even with these complex revisions, the model could not predict planetary positions accurately In spite of this, the model remained popular because of the high regard in which Aristotle’s ideas were held Although the model was based in mathematics, it turned out to be physically incorrect For example, it was assumed that the earth was motionless since the stars do not appear to shift relative to  each other over the course of a year But the angular size of the shift would get smaller as the distance from earth get larger So if the
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