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Understanding the Starry Sky
Ch. 01
Celestial Sphere
A model of the universeWe picture the sky as a hollow globe
surrounding earthAll stars are positioned inside the globe
facing earth
Celestial Sphere
http://www.astro.columbia.edu/~archung/labs/fall2001/lec02_fall01.html
Celestial Sphere Animation
Animation 1.1: Celestial Sphere
Celestial Sphere Features
“Sphere” appears to revolve around earth each day
Earth’s equator extended becomes celestial equator, earth’s poles (N&S) become celestial poles
Ecliptic is the path of the sun around the celestial sphere, requires one year for a revolution
Ecliptic & Celestial Sphere
http://www.utm.utoronto.ca/services/gallery/img/ecliptic.gif
Constellations
88 officialMany ancient, 48 Greek constellationsTypes
SeasonalZodiacCircumpolar
In order to more easily locate objects in the sky, we In order to more easily locate objects in the sky, we divide the sky into regions named after familiar divide the sky into regions named after familiar patterns of stars called patterns of stars called constellationsconstellations..
Ancient constellations were imaginary pictures outlined by familiar patterns of stars (stick figures).
Modern astronomers divide the sky into 88 official constellations or regions of space, many of which contain the ancient star patterns.
Sphere image of Constellation Boundaries
Zodiac
12 Constellations (traditional) along the eclipticPisces, Aries, etc.
Circumpolar Constellations
North and SouthNever setting (N), or never rising (S)In north, appear to wheel eternally about
Polaris, the North StarEx: Ursa Major (Big Dipper)
Circumpolar Stars
http://www.nmm.ac.uk/upload/img/circumpolar.jpg
“Star Trails”
N & S Circumpolar Stars
http://www.mhhe.com/physsci/astronomy/fix/student/images/01f19.jpg
Star Maps
Star Maps represent portions of the celestial sphere
Seasonal star maps provided in your textbook (appendix)
Horizon Star Map
http://www.drypen.net/Star_Map_3.jpg
Equatorial Star map
http://www.starlight-theatre.ca/images/MAP-C.GIF
Bright Star Names
About 50 of the brightest starsEx: Aldebaran, Sirius Mostly Arabic, some Greek & LatinBayer (1600), Greek letter names of stars
Ex: Alpha () Tauri means brightest star in constellation Taurus
26 Brightest Stars
http://www.astro.wisc.edu/~dolan/constellations/extra/brightest.html
Star Magnitude (Brightness)
Apparent Magnitude: Brightness of a star as seen from earth
Traditional: Greek Hipparchus, assigned 6 categories with 1 being the brightest (first magnitude)
About 100x brightness difference between 1 and 6
To calculate magnitude differences:2.5199(m2-m1)
Apparent Magnitude Scale
Terrestrial Coordinates
http://www2.shastacollege.edu/dscollon/images/Figures,%20Diagrams%20and%20Maps.htm
Celestial Coordinates
Celestial Equator: Extension of earth’s equatorial plane into space along the celestial sphere
N & S Celestial PolesVernal Equinox-Sky’s “prime meridian”Coordinates:
Declination (Dec)Right Ascension (RA)
Celestial Globe
http://www.onr.navy.mil/focus/spacesciences/observingsky/sphere1.htm
RA & Dec
http://abyss.uoregon.edu/~js/ast122/lectures/lec02.html
Summary
Earth Globe Celestial Sphere
North Pole North Celestial Pole
South Pole South Celestial Pole
Equator Celestial Equator
Prime Meridian Vernal Equinox
Latitude Declination
Longitude Right Ascension
Horizon System
Coordinate system based upon an observer’s horizon, not the celestial equator
GuidelinesHorizon-”Straight ahead”, line where earth and sky
appear to meetZenith-point straight upMeridian-Line “due south”, passes from north horizon
through zenith to south horizonNadir-point straight down
CoordinatesAltitudeAzimuth
Horizon System Diagram
http://www.yorku.ca/phall/P1070W05/L04/bea02-05.jpg
Altitude & Azimuth
http://www.nmm.ac.uk/upload/img/altaz-star2.jpg
Degrees in the Sky
Angular MeasureBased on degrees of a circle = 360ºFinger tip = 1ºFist = 10ºOutstretched hand = 20º
From horizon to zenith = 90ºThe moon and sun = 1/2º, your finger tip
will cover them (try this with the moon only)
Angular Measure
http://www.physics.capcollege.bc.ca/stan/Angular_measure.jpg
Polaris & Latitude
The height of Polaris (North Star) in degrees above the horizon is equal to an observer’s latitude (distance above the equator)
Our latitude 42-degreesPolaris will be about four fist-lengths
above horizonUseful for navigation
Geometric Explanation
p = Location
x° = Latitude
a° = Altitude of Polaris
x° = a°
http://homepage.mac.com/kvmagruder/images/polarislat.gif
Earth’s Motions
RotationRevolutionPrecession
Earth’s Motions: Rotation, Revolution
Revolution
Earth’s Seasons
Sun’s Apparent Yearly MotionSun travels around eclipticAbout 1° per day
360°/365 days 1° per day
Sun high in summer, low in winter
Seasonal Heights of Sun
Seasonal Sun Paths
http://www.madison.k12.wi.us/planetarium/sunpath.gif
SEPTEMBER MARCH
Seasonal Constellations
Earth’s Seasons
http://www.learn.londonmet.ac.uk/packages/clear/visual/daylight/sun_sky/images/seasons.png
Earth’s Seasons
Seasons Animation
Animation 1.2: The Seasons
Seasons & Celestial Sphere
Rotation: Solar and Sidereal Day
http://www.nmm.ac.uk/upload/img/sidereal-mean-day_20030415143827.gif
Sidereal Day
Precession
Earth’s other motionOne wobble takes 26,000 yearsAxis points in new directions during this
timeResult, the “North Star” changes over time
Thuban, 3000 BCPolaris, PresentVega, 14,000 AD
Precession
http://earthobservatory.nasa.gov/Library/Giants/Milankovitch/milankovitch_2a_low.html
Earth’s Precession
http://apollo.lsc.vsc.edu/classes/met130/notes/chapter18/graphics/precession.psd.gif
14,000 AD
Present
+ North Ecliptic Pole
+
Ancient Egyptian polestar
Inconstant North Star
“But I am as constant as the North Star…”
-Shakespeare, Julius Caesar
http://abyss.uoregon.edu/~js/images/bz990323.gif