Chapter 2

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Astronomy & Astrophysics
Michael Reid

2 Discovering the Universe for Yourself21 Patterns in the Night SkyWhat does the universe look like from EarthConstellations A region of the sky with welldefined borders the familiar patterns of stars merely help us locate the constellationsEvery point in the sky belongs to some constellationThe Celestial Sphere The imaginary sphere on which objects in the sky appear to reside when observed from EarthThe celestial sphere allows us to map the sky as seen from EarthFour special points and circles on the celestial sphereNorth celestial pole The point directly over Earths North PoleSouth celestial pole The point directly over Earths South PoleCelestial equator A projection of Earths equator into space making a complete circle around the celestial sphereEcliptic The path the Sun follows as it appears to circle around the celestial sphere once each year It crosses the celestial equator at a 235 angel 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 constellationsThe Milky Way in the night sky traces our galaxys disk of starsthe galactic planeas it appears from our location in the outskirts of the galaxyThe Local Sky The half of the celestial sphere that you see at any time the sky as seen from wherever you happen to be standingHorizon The boundary between the Earth and sky altitude0 degreeZenith The point directly overhead altitude 90 degrees no directionMeridian An imaginary half circle stretching from the horizon due south through the zenith to the horizon due northWe can pinpoint the position of any object in the local sky by stating its direction along the horizon sometimes stated as azimuth which is degree clockwise from due north and its altitude above the horizonAngular Sizes and DistancesAngular size The angle an object appears to span in your field of viewThe angular size does not by itself tell us an objects true size because angular size also depends on distanceThe angular distance between a pair of objects in the sky is the angle that appears to separate themYou can use your outstretched hand to make rough estimates of angles in the skyWe subdivide each degree into 60 arcminutesand subdivide each arcminute into arcseconds Why do stars rise and setStars relatively near the north celestial pole remain perpetually above the horizon They never rise or set but instead make daily counterclockwise circles around the north celestial pole We say that such stars are circumpolarStars near the south celestial pole never rise above the horizon at allAll other stars have daily circles that are partly above the horizon and partly below it Because Earth rotates from west to east counterclockwise as viewed from above the North Pole the stars appear to rise in the east and set in the westWhy do the constellations we see depend on latitude and time of yearVariation with Latitude
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