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ASTR STUDY GUIDE

# ASTR STUDY GUIDE - Lecture 1 definition of a star planet o...

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Lecture 1 definition of a star, planet o Star= a celestial body of hot gases that radiates energy from thermonuclear reactions in the interior o Planet= wandering stars; an object large enough to have settled into round shape and has cleared neighborhood around its orbit celestial sphere Celestial Sphere = a huge imaginary sphere which holds all the celestial objects o Geocentric model – earth is at center of Universe o Heliocentric model – Sun is at center of universe o diurnal motion – eastward rotation of earth (makes sky appear to be moving toward west during day) angular separation = angle between two lines extending toward the two objects (measured from the observer) o right ascension = longitudinal coordinate on celestial sphere (measured in units of time) 1 hour of R.A. = 15 ° o declination = latitudinal coordinate on celestial sphere (measured from 0-90 ° N or S) Ex: Find Celestial coordinates for star Sirius (R.A = 6h 45m 9s; Dec = -16 ° 43m) R.A. o 6 + (45/60) + (9/3600) = 6.7525h o 6.7525h x 15 ° /1h = 101.2875 ° o R.A. Coordinate: 101.2875 ° Dec. o -16 + (43/60) = -16.72 ° o Dec Coordinate: -16.72 ° diurnal motion/annual motion o Diurnal motion = eastward motion of earth o Annual motion = eastward motion of sun along ecliptic o Motion along ecliptic: Ecliptic = path of sun in sky; Motion along ecliptic = eastward path sun traces in sky during year Equinoxes/Solstices o Equinoxes 2 times year when night and day same length sun crossing equator and is equal distance from N pole and S pole VERNAL EQUINOX: sun is moving south to north across celestial equator. Marks 1 st day of spring Northern hemisphere – Mar 21 Southern hemisphere – Sept 23 AUTUMNAL EQUINOX: sun is moving north to south across celestial equator

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Marks 1 st day of fall N hemisphere – Sept 23 S hemisphere – Mar 21 o Solstice = Sun's apparent position in the sky reaches its northernmost or southernmost extremes SUMMER SOLSTICE: sun as far north of equator Marks 1 st day of Summer June 21 N Hemisphere longest day S Hemisphere shortest day WINTER SOLSTICE: sun as far south of equator Marks 1 st day of Spring Dec 22 nd N Hemisphere shortest day S Hemisphere longest day Lecture 2 Moon Phases (waxing = growing, waning = shrinking) 1. NEW MOON 2. WAXING CRESCENT 3. FIRST QUARTER 4. WAXING GIBBOUS 5. FULL MOON 6. WANING GIBBOUS 7. THIRD QUARTER 8. WANING CRESCET Why does Lunar Phase occur? o Lunar phases are created by changing angles of the earth, the moon and the sun, as the moon orbits the earth o ½ of Moon ALWAYS illuminated by sun o Both sunlit and shadowed portions create various moon phase shapes Synodic/Sidereal period o Synodic Period = ~29 days (time required for moon to move to same position as seen by observer on earth) Lunar Month o Sidereal Period = ~27 days (time required for moon to move to same position as seen from outside solar system) True Orbital Period o Why Synodic period different from Sidereal period?
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