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PH1600: Introductory Astronomy Lecture 5: Magnitudes and Calendars What is this? PH1600: Introductory Astronomy Lecture 5: Magnitudes and Calenders Next time: The Inner Solar System School: Michigan Technological University Professor: Robert Nemiroff Online Course WebCT pages: http://courses.mtu.edu/ This class can be taken online ONLY, class attendance is not required! You are responsible for… Lecture material Listed wikipedia entries But not higher math APODs posted during the semester APOD review every week during lecture Completing the Homework-Quizzes Homework 1 & 2 quizzes already due Homework 3 quiz is due soon See WebCT at http://courses.mtu.edu/ Wikipedia entries: Apparent magnitude Absolute magnitude Diurnal motion Celestial pole North star Right ascension Declination Equinox Solstice Season Calendar Magnitudes Apparent magnitude How bright something appears Absolute magnitude How bright something really is Apparent magnitudes: m Higher magnitude means dimmer Counterintuitive, outdated, but standard! mag 5 star dimmer than mag 0 star Scale is logarithmic 5 magnitudes means a factor of 100 mag 0 star 100 times brighter than mag 5 star Relative magnitude formula bA = 2.512(mB-mA) bB bA is the apparent brightness of star A bB is the apparent brightness of star B mA is the apparent magnitude of star A mB is the apparent magnitude of star B Relative magnitude examples Example: Star B appears one magnitude fainter than star A. How many times fainter is this? Solution:bA/bB=2.512(mB–mA) bA/bB = 2.512(1) = 2.512 times fainter Relative magnitude examples Example: Star B appears five magnitudes fainter than star A. How many times fainter is this? Solution:bA/bB=2.512(mB–mA) bA/bB = 2.512(5) = 100 times fainter Apparent magnitude table App. Mag. Celestial object −26.8 Sun −12.6 full Moon −9.5 Maximum brightness of an Iridium Flare −4.4 Maximum brightness of Venus −3.9 Faintest objects observable during the day with naked eye −2.8 Maximum brightness of Mars −1.5 Brightest star (except for the sun) at visible wavelengths: Sirius −0.7 Second brightest star: Canopus 0 The zero point by definition: This used to be Vega 3 Faintest stars visible in an urban neighborhood 5.5 Maximum brightness of Uranus 6 Faintest stars observable with naked eye 7.7 Maximum brightness of Neptune 12.6 Brightest quasar 27 Faintest objects observable in visible light with 8m ground-based telescopes 30 Faintest objects observable in visible light with Hubble Space Telescope http://en.wikipedia.org/wiki/Apparent_magnitude Sirius: The Brightest Star in the Night Credit & Copyright: Juan Carlos Casado APOD: 2000 June 11 The Brightest Star Yet Known Credit: D. F. Figer (UCLA) et al., NICMOS, HST, NASA APOD: 1997 October 8, APM 08279+5255: The Brightest Object Yet Known Credit: M.J. Irwin (RGO) et al., Isaac Newton 2.5-m Telescope APOD: 1998 August 18 Across the Universe Credit: NASA Swift Team, Stefan Immler (GSFC) et al. APOD: 2008 March 28 Rising and Setting Stars Earth’s rotation makes sky appear to rotate one full turn each day Celestial Sphere The sky: everything appears attached to a distant rotating sphere In Earth’s North: stars appear to circle the North Celestial Pole Nearly occupied by the North Star: Polaris http://commons.wikimedia.org/wiki/Image:Celestial_Sphere.gif Celestial Sphere Diagram Northern Lights Credit & Copyright: Philippe Moussette (Obs. Mont Cosmos) APOD: 2004 July 30 Star Trails Above Mauna Kea Credit & Copyright: Peter Michaud (Gemini Observatory), AURA, NSF APOD: 2005 December 20 11 Hour Star Trails Credit & Copyright: Josch Hambsch APOD: 2006 September 15 Warped Sky: Star Trails Panorama Credit & Copyright: Peter Ward APOD: 2007 June 13 Rising and Setting Stars Zenith: The point on the Celestial Sphere directly over the observer’s head Horizon: 90 degrees from your zenith in all directions Planets, the Sun, and the Moon rise and set too just like the stars Earth’s rotation faster than their orbits A Setting Sun Trail Credit & Copyright: Anglo-Australian Observatory, Photograph by David Malin APOD: 2002 July 29 Celestial Coordinates & Sky Labels Celestial Equator Right Ascension & Declination The circle midway between the north & south celestial poles Sky projection of Earth’s Equator Like celestial latitude and longitude Ecliptic The plane that the planes orbit the Sun http://en.wikipedia.org/wiki/Right_Ascension Reasons for Seasons NOT caused by Earth’s nearness to the Sun Caused by the tilt of the Earth 23.5 degree tilt points in the same direction all year long http://en.wikipedia.org/wiki/Image:North_season.jpg Seasons of Northern Earth Key Seasonal Dates Equinoxes: Equal Day and Night Vernal Equinox (March 22) Autumnal Equinox (September 22) Solstices: Maximum Day or Night Winter Solstice (North: December 22) Summer Solstice (North: June 22) Sunrise by Season Credit & Copyright: Anthony Ayiomamitis APOD: 2005 December 21 Sunrise Solstice at Stonehenge Credit & Copyright: Pete Strasser (Tucson, Arizona, USA) APOD: 2006 June 21 Manhattanhenge: A New York Sunset Credit & Copyright: Neil deGrasse Tyson (AMNH) APOD: 2007 July 13 Equinox + 1 Credit & Copyright: Joe Orman APOD: 2004 March 20 Analemma over Ukraine APOD: 2007 June 17 Calendar: Day Names Sunday = Sun Day Monday = Moon Day Tuesday = Mars (Twis) Day Wednesday = Mercury (Woden) Day Thursday = Jupiter (Thor) Day Friday = Venus (Frige) Day Saturday = Saturn Day Calendars One year = time for Earth to go once around the Sun Does NOT correspond exactly to any integer amount of Earth spins (about 365.25 days) Forgetting this causes your Calendar to drift 0.25 days a year This can add up! Calendars Roman Calendar Originally lunar, but kept changing Lost many days per year had occasional leap months Julian Calendar Introduced in 46 BC by Julius Ceasar 365 days each year One leap day every four years Occurs in years evenly divisible by 4 Calendars Gregorian Calendar Decreed by Pope Gregory XIII in 1582 Accounted for inexactness of the “0.25” in 365.25 days per year Took away a leap day every 100 years except every 400 years Leap years: 1600, 2000 Not leap years: 1700, 1800, 1900, 2100 Leap Seconds Now added many years to account for erratic spin of the Earth 11 Hour Star Trails Credit & Copyright: Josch Hambsch APOD: 2006 September 15