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ASTRONOMY 101 Introduction to Solarr System Astronomy D Taylor Greenwich HS, CT Rowan University University, NJ ©2008 Astronomy 2008 8: The Web Page www.DTF Fizzix.com Astronomy y Picture of the Day y (APOD): ( ) http://antwrp.gsffc.nasa.gov/apod/ The Celesttial Sphere Thursday, September S 11 The Celestial Spheere: Key Concepts (1) The sky as seen from Earth is divided into 88 constellations. (2) It is convenient to pretend thhe stars are attached to a celestial sphere. p (3) The celestial sphere appearss to rotate about the celestial poles (1 day). (4) The Sun appears to move west to east relative to stars (1 year) year). (5) The Moon appears to move west to east relative to stars (1 month) month). (1) The sky is divided into 88 constellations Mesopotamia, circa 3000 BC: oldest know constellaations Ptolemy, Pt l 2nd 2 d century t A AD: AD 48 constellations in northern n skyy 16th to 18th century AD D: unmapped regions off sky filled in Constellations aree largely arbitrary Other cultures, other constellations: Example: Ursa Major = bear, b dipper, bull's leg, grain scoop, scoop wagon, wagon plo ow etc ow, etc. Stars in a constellation usu ually are not at the same distance from us. At a diff differentt place l in i ourr Galaxy, G l we would ld see different star patterns. A modern star chart of Ursa Major: (2) Stars are “attachedd” to a celestial sphere Distances to stars are hard h to measure. However, we can preteend all stars are at the same distance di t from f u attached us, tt h d to t a large l celestial sphere. p Position on the celestiaal sphere is known even when the distance in unknown. u C l ti l Sphere: Celestial S h A large l imagin i inary sphere h centered t d on Earth E th Special locations onn the celestial sphere North Celestial Pole = pooint directly above Earth’s N hP North Pole l ((near the h starr Polaris) P l i) South Celestial Pole = pooint directly above Earth Earth’ss South Pole (no nearby bright b star) Celestial Equator = circle directly above Earth’s Equator Distances betweeen points on the celestial sphere are measured in d degrees, arcminute i tes, and d arcseconds d 360 degrees in a circle 60 arcminutes in a degree 60 arcseconds in an arcmiinute ½ degree = angular size of Sun & Moon Celestial navigattion made simple At Earth’s North Pole: P l i is Polaris i directly di l overh h d head At Earth’s Earth s Equator: Polaris is due north, on the t horizon In Earth’s Northern hem misphere: Polaris is due north - height above the horizon (in degrees) is equal to your y latitude (in degrees) (3) The celestial sphere appears to rotate about the celestial poles (1 day cycle) Observation: Stars, Suun, Moon and planets move in counterclockkwise circles around north (south) celestial pole. pole Objects j near the celestiial equator q move east to west when above the horizon (“rising” in east, t “setting” “ tti ” in i westt) t). t What causes these circu ular motions? Explanations of the 1 day cycle: HYPOTHESIS #1 HYPOTHESIS #2 (P l (Ptolemy, 2nd century): ) (Copernicus, (C i 16th cent.): ) Earth is stationary; stars Stars are stationary; Earth are attached to a sphere rotates about its axis per day. y that revolves around once p the Earth once per day. RIGHT! WRONG! Nicolaus Coperniicus (1473-1543) Foucault'ss Pendulum Li ki Link kicus k (4) The Sun appears to move west to east relative to starss (1 year cycle) Today the Sun is “in” Sagittarius, next month in Capricornus, etc. Sun’s path on the celesttial sphere = ecliptic Constellations through which w the ecliptic runs = zodiac The ecliptic is NOT thee same as the celestial equator! Observation: Sun movves west to east relative to stars (about 1 degrree per day). What causes this annuaal motion? Explanations of o 1 year cycle: HYPOTHESIS #1 (P l (Ptolemy): ) Sun revolves around Earth at a slightly slower rate than the celestial sphere. WRONG! HYPOTHESIS #2 (C (Copernicus): i ) Earth revolves around the Sun, once per year. RIGHT! (5) The Moon appearss to move west to east relative to stars (1 month cycle) Today the Moon is “in” Scorpius In two weeks: Gemini In four weeks: Scorpius, again. a Observation: Moon movees west to east relative to stars, taking 27.3 days to o complete cycle. What h causes this hi monthly hl motion? i ? Explanations off 1 month cycle: HYPOTHESIS #1 (P l (Ptolemy): ) Moon revolves around Earth at a significantly slower rate than the celestial sphere. WRONG! HYPOTHESIS #2 (C (Copernicus): i ) Moon revolves around Earth, once per month. RIGHT! Few closing g questions: 1) Can the Sun come closee to the North Celestial Pole? 2)) Can the Moon come clo ose to the South Celestial Pole? 3) Y You are on th the E Earth’s th’ equator---how e t h hi high h is i Polaris above the horizzon? 4) How often does the Sun n cross the plane of the ecliptic? Few closing quesstions continued: 5) How often does the Moo on cross the plane of the ecliptic? li i ? 6) How often does the Sun cross the Celestial Equator? 7) How often does the Moo on cross the Celestial Equator? NEW TEXTBOOK?????