Constellations, the Seasons, the Moon - Chapter 2 (Sept 25th).docx

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University of Toronto St. George
Astronomy & Astrophysics
Michael Reid

1 Thursday September 25th 2012AST101Y1Astronomy The night sky constellations the seasons The Moon Moon phases eclipsesTextbook pg 26 Chapter 2Discovering the Universe for Yourself 21 Patterns in the Night Sky What does the universe look like from Earth ConstellationsConstellationA region of the stellar sky with welldefined borders 88 official constellations chosen in 1928 by members of the IAU and named for familiar concepts in the western world cover the celestial sphere Every point in the sky belongs to some constellation or anotherThe Celestial SphereThe stars and constellations appear to lie on a celestial sphere that surrounds Earth but this is merely an illusion created by our lack of depth perception in space since the stars are far away and is useful for mapping the sky Celestial SphereThe imaginary sphere on which objects in the sky appear to reside when observed from Earth For reference we identify four special points and circles on the celestial sphereNorth Celestial PoleThe point on the celestial sphere directly above Earths North PoleSouth Celestial PoleThe point on the celestial sphere directly above Earths South PoleCelestial EquatorThe extension of Earths equator onto the celestial sphereEclipticThe Suns apparent annual path among the constellations It crosses the celestial equator at a 235 angle because that is the tilt of Earths axisThe Milky WayThe band of light that we call the Milky Way circles all the way around the celestial sphere passing through more than a dozen constellations The Milky Way in the night sky traces our galaxys disk of stars the galactic plane as it appears from our location on the outskirts of the galaxy The dark lanes that run down the centre of the Milky Way contain the densest clouds and prevent us from seeing more than a few thousand lightyears into our galaxys disk The Local SkyLocal SkyThe sky as viewed from a particular location on Earth or another solid object Objects in the local sky are pinpointed by the coordinates of altitude and direction or azimuthKey reference features of the local sky includeHorizonA boundary that divides what we can see from what we cannot see ZenithThe point directly overhead which has an altitude of 90MeridianA halfcircle extending from your horizon altitude 0 due south through your zenith to your horizon due north We can pinpoint the position of any object in the local sky by stating its direction along the horizon and its altitude above the horizon Direction in local skyOne of the two coordinates the other is altitude needed to pinpoint an object in the local sky It is the direction such as north south east or west in which you must face to see an object Altitude above horizonThe angular distance between the horizon and an object in the skyAngular Sizes and DistancesOur lack of depth perception means we have no way to judge the true sizes or separations of the objects we see in the sky however we can describe their angular sizes without knowing how far they are Angular Size DistanceA measure of the angle formed by extending imaginary lines outward from our eyes to span an object or the space between two objects Angular size does not by itself tell us an objects
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