Study GuideAstronomy–Observing the Sky1.Naked-Eye AstronomyHuman understanding of the universe began when people started observing the sky with their owneyes. Long before telescopes existed, people noticed that objects in the sky changed their positionsover days, months, and years. These careful observations laid the foundation of astronomy.1.1The Sky as We See ItWhen we look up from Earth, the sky appears like a huge dome or bowl covering us from all sides.Astronomers call this imaginary dome thecelestial sphere.•Thehorizonis the circle where the sky seems to meet the ground.•Thezenithis the point directly overhead.Because stars and other objects are extremely far away, our eyes cannot judge their distances. As aresult, all objects appear to lie on the same spherical surface.1.2The Horizon (Alt-Azimuth) Coordinate SystemTo locate objects in the sky, astronomers use coordinates, just like latitude and longitude on Earth.•Altitude: how high an object is above the horizon•Azimuth: the direction of the object, measured along the horizon starting from due north andmoving eastwardThis method is called thehorizon coordinate systemoralt-azimuth system.Preview Mode
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