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Astronomy & Astrophysics
Michael Reid

Chapter 2Discovering the Universe for Yourself21 Patterns in the Night SkyConstellations y A constellation is a region of the sky with welldefined borders the familiar patterns of stars merely help us locate the constellationsy Every point in the sky belongs to some constellation y Names of constellations visible in the Northern Hemisphere can be traced back to civilizations of the ancient Middle East while Southern Hemisphere thconstellations carry names that originated with 17 century European explorers The Celestial Sphere y The stars in a particular constellation appear to lie close to one another but may be quite far apart in reality because they may lie at different distances from Earth This illusion occurs because we lack depth perception when we look into space a consequence of the fact that the stars are so far away The ancient Greeks mistook this illusion for reality imagining the stars and constellation to lie on a great celestial sphere that surrounds Earth y The north celestial pole is the point directly over Earths North Pole y The south celestial pole is the point directly over Earths South Pole y The celestial equator which is a projection of Earths equator into space makes a complete circle around the celestial spherey The ecliptic is the path the Sun follows as it appears to circle around the celestial sphere once each year It crosses the celestial equator at a 235r angle because that is the tilt of Earths axis The Milky Way y The 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 widest and brightest parts of the Milky Way are most easily seen from the Southern Hemispherey The Milky Way in the night sky traces our galaxys of disk of starsthe galactic planeas it appears from our location in the outskirts of the galaxy y Countless stars and interstellar clouds make up the Milky Way y The Milky Way seems somewhat wider in the direction of the constellation Sagittarius because that is the direction in which we are looking toward the galaxys central bulgey We can observe the distant universe only when we look in directions away from the galactic disk so that there are relatively few stars and clouds to block our view y The dark lanes that run down the centre of the Milky Way contain the densest clouds and they appear dark because these clouds obscure our
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