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Lecture 3

Astronomy Reading Week 3: 2.1-2.4

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University of Toronto St. George
Astronomy & Astrophysics
Michael Reid

2. Discovering the Universe for Yourself 9/27/2012 12:59:00 PM 2.1 Patterns in the Night Sky What does the universe look like from Earth?  After sunset, one sees stars fill the sky o More than 2000 stars are visible of the night sky  Constellations o = a region of the sky with well-defined borders  the familiar patters of stars merely help us locate the constellations o everyone point of the sky belongs to a constellation o Named after western names b/c they were named/discovered by Europeans/Americans  Celestial Sphere o = imaginary sphere on which objects in the night sky appear to reside when observed from Earth o Constellations lie close to each other but are actually far apart  Ancients mistaken their visible distance as reality o Earth is center of sphere o Full of illusion o 4 special points/circles on the celestial sphere  North celestial pole = point directly over Earth’s north pole  South celestial pole = point directly over Earth’s south pole  Celestial equator = projection of Earth’s equator into space, makes a complete circle around the celestial sphere  Ecliptic = path the Sun follows as it appears to circle around the celestial sphere once each year  The Milky Way o Band of light circles all the way around the celestial sphere, passing through more than a dozen constellations o The milky way in the night sky traces our galaxy’s disk of stars – the galactic plane – as it appears from our location in the outskirts of the galaxy o We’re a little more than halfway out from the center o Countless stars, vast interstellar clouds o Band of light makes fun circle around our night sky o Dark lanes that run around the center contain densest clouds  The Local Sky o = the sky as seen from wherever you happen to be standing  the half of the celestial sphere that you see at any time o Key features of the local sky  Horizon = boundary b/w Earth and the sky  Zenith = Point directly overhead  Meridian = imaginary half circle stretching from the horizon due south, through the zenith, to the horizon due north o Can pinpoint position of any object in local sky by stating its direction along the horizon and its altitude from the horizon  Direction = (sometimes stated Azimuth)= degrees clockwise from due north  Angular Sizes and Distances o No way to judge true sizes or separations of the objects we see but we can describe angular sizes or separations of objects w/o knowing how far away they are o Angular size = angle it appears to span in your field of view  Does not by itself tell us an object’s true size, b/c it also depends on distance  Sun is 400 large in diameter than Moon but they have same angular size b/c our sky is also 400x farther away o Angular distance = angle that appears to separate a pair of objects in the night sky o Use of arcminutes and arcseconds for precise astronomical measurements Why do stars rise and set?  Earth rotates, not the rest of universe  Stars relatively near the north celestial pole remain perpetually above the horizon o Never set or rise but instead make daily counterclockwise circles around the north celestial pole which is why we say that these stars a circumpolar o Stars relatively near the south celestial pole never rise above the horizon at all (hidden side) o All other stars have daily circles that are partly above the horizon and partly below it o b/c earth rotates from west to eat, stars appear to rise in the east and set in the west Why do constellations we see depend on latitude and time of year?  If you move from one place to another, you see diff constellations and if you stay, you see diff ones at diff times of the year  Variation with Latitude o Latitude = measures north-south position  “lines of latitude” are actually circles running parallel to the equator  it affects which constellations we see b/c it affects the locations of the horizon and zenith elative to the celestial sphere o Longitude = measures east-west position  “lines of longitude” are semicircles extending from the north pole to the south pole o This pinpoints location on earth o The altitude of the celestial pole in your sky is equal to your latitude  Variation with Time of Year o Night sky changes throughout the year b/c of Earth’s changing position in its orbit around the Sun o As we orbit the Sun, the Sun is also is also completing its circuit, gradually moving along with ecliptic  Constellations along the ecliptic are called constellations of the Zodiac  Sun’s location along ecliptic determines which constellations we se at night 2.2 Reason for the Seasons What causes the seasons?  The tilt of the earth’s axis causes sunlight to fall differently on Earth and diff times of the year  Earth’s axis points in the same direction (to Polaris) all year round, so its orientation relative to the Sun changes as Earth orbits the Sun  Summer occurs in your hemisphere whens sunlight hits it more directly; winter occurs when the sunlight is less direct  Direct heat causes more heating  Axis tilt changes directness of sunlight during the year  Sun’s altitude also changes with seasons o Sun’s position at noon in summer: higher altitude means more direct sunlight o Sun’s position at noon in winter: lower altitude means less direct sunlight  AXIS TILT is the key to the seasons; without it, we would not have seasons on Earth How do we mark the progression of the seasons?  We define four special points: o summer solstice o winter solstice
 o spring (vernal) equinox o fall (autumnal) equinox  We recognize solstices and equinoxes by Sun’s path across the sky: o Summer solstice: Highest path, rise and set at most extreme north of due east o Winter solstice: Lowest path, rise and set at most extreme south of due east o Equinoxes: Sun rises precisely due east and sets precisely due west  Seasonal changes are more extreme at high latitudes How does the orientation of Earth’s axis change with time?  Although the axis seems fixed on human time scales, it actually pre
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