202 Lecture 2 - Lecture 2 Basic Patterns and Motions in the...

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Lecture 2: Basic Patterns and Motions in the Sky. The Celestial Sphere: o Relative positions and patterns of stars seem to remain unchanged to the naked eye over the years. o Project the position of stars onto a Celestial Sphere surrounding the Earth. o The Surface of the Earth limits our view of the Celestial Sphere at the Horizon. o At any given time, we can only see at most half of the Celestial Sphere. o We call Zenith the point of the Celestial Sphere right above the observer. The opposite point in the Celestial Sphere is called Nadir . o As a guide, we introduce the Meridian as an imaginary circle on the Celestial Sphere that starts in the South point in the Horizon, through the Zenith, crosses the Horizon again at the North point, continues through the Nadir completing a major circle. Patterns and Constellations: o Constellations (old definition): recognizable pattern of stars (nowadays called Asterism by astronomers ) . Myths and legends associated to them. o Constellations (contemporary astronomical definition): particular region in the sky. o Celestial Sphere divided into 88 Constellations. o In its annual motion through the Celestial Sphere, the Sun passes through 12+1 constellations. This group of constellations is called the Zodiac constellations. Diurnal Motion: o Due to the rotation of the Earth on its axis, the Sun, the Moon, the celestial objects and the whole Celestial Sphere seem to rotate from East to West. o In mid-latitudes, many stars rise in the East at the Horizon, reach their highest point in the sky (transit) when they cross the Meridian and set in the West when they reach the Horizon. o Some stars never rise or set: they follow circles around a fixed point in space. They are called Circumpolar stars. The North Star: o A star in the Northern Hemisphere does not seem to move significantly overnight; the Northern Star . o The Northern Star lies on the Meridian. Can be used to find due North. o The Angle between the North Star and the Horizon corresponds to the latitude of the observer. Navigational tool. The Sun: o The Sun has an apparent Westward motion. o Reaches highest point near the Meridian: Defines noon. o AM: Ante Meridian (before Meridian) o PM: Post Meridian (after Meridian) Different latitudes see the motion of the stars differently: o At the North Pole, all stars circumpolar and North Star in Zenith. o At the Equator, all stars rise and set and North Star in Horizon. o The North Star is below the Horizon at the Southern Hemisphere.
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Celestial Coordinates: o Coordinates on the Celestial Sphere. Similar to Latitude and Longitude on Earth o Celestial Poles, Extensions of Earth’s North and South poles. o Celestial Equator: Extension of Earth’s equator. o Declination: North-South position (+90 degrees to -90 degrees).
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