Astro 101: Navigating the Night Sky

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Astro 101Navigating the Night Sky

• presented by Paul Winalski20 April 2007

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Topics for this Evening

• Celestial Coordinates
• Star Nomenclature
• Orientating Ones Self to the Sky
• Star-hopping
• Reading Star Charts

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The Universes Four Dimensions

• At 90-degree angles to each other
• Distance (forward/backward)
• Azimuth (left/right)
• Altitude (up/down)
• Time (past/future)

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Coordinate Systems

• Two basic kinds
• Measure from a point of reference (origin)
• Cartesian linear measure in all dimensions
• Example two blocks forward, left three blocks,
  up ten stories, two hours from now
• Not useful for the sky (objects too far away)
• Polar
• Distance and Time by linear measure
• Altitude and Azimuth by angular measure

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Polar Altazimuth Concepts

• Angles measured in degrees (º) of arc
• 360º in entire circle, or 180º from origin
• 60 minutes of arc (60') per degree
• 60 seconds of arc (60") per minute
• Zenith (direction overhead 90º altitude)
• Nadir (direction underneath -90º altitude)
• Horizon (great circle at 0º altitude)
• Azimuth measured starting 0º due north, east
  positive, west negative

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Polar Altazimuth Coordinates
Zenith
Distance
Altitude
Azimuth
Horizon
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Motion of the Earth

• Rotation once around the N/S polar axis every
  24 hours
• Revolution about the Sun once every 365.25 days
• Precession (wobbling of polar axis) in 47º
  circle once every 25,800 years
• Nutation (a shorter period wobbling) not very
  significant to us

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Relativity

• Newtonian Relativity
• Observation only CHANGE in momentum (motion in
  a linear direction) can be detected
• Conclusion position and motion are relative to
  the observer (frame of reference)
• Einsteinian Relativity
• Observation the speed of light in a vacuum is
  the same for all observers
• Conclusion passage of time is relative to the
  observer

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Invoking Relativity

• Our motion
• rotational speed 800 mph
• average revolutionary speed 66,622 mph
• Nearly constant motion change in direction
  insignificantly small (at our scale)
• Thus we seem to be standing still
• By Newtonian Relativity, its valid to say that
  the sky moves around us

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The Celestial Sphere

• We are on the Earths surface
• Earths radius (4000 mi) is insignificant
  compared to stellar distances (25 trillion miles
  to nearest star)
• So we can simplify
• move our origin to the center of the Earth
• ignore distance and deal only in altitude/azimuth

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Equatorial Coordinate System

• Polar coordinate system, equivalent to geographic
  coordinate system
• Celestial poles aligned with geographic poles of
  the Earths rotation
• Independent of local latitude/longitude (unlike
  altazimuth coordinates)

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Important Celestial Great Circles

• Horizon at 0º local altitude
• Meridian intersecting local zenith and
  equatorial poles
• Celestial Equator at 90º to meridian
• Ecliptic intersecting plane of Earths orbit
  about the Sun (at 23.5º angle to equator)
• Galactic Equator intersecting center plane of
  Milky Way galaxys spiral arms

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Equinoxes

• Intersection points between equator and ecliptic
• Vernal Equinox (first point of Aries)
  position of Sun passing from Southern to Northern
  Hemisphere (our first day of Spring)
• Autumnal Equinox position of Sun passing from
  Northern to Southern Hemisphere (our first day of
  Fall)

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Declination and Right Ascension

• Declination (Dec) latitude angle north
  (positive) or south (negative) from celestial
  equator divided into degrees, minutes, seconds
  of arc
• Right Ascension (RA) longitude
• angle measured along equator east from vernal
  equinox
• divided into 24 hours (15º of arc each)
• each hour into 60 minutes (15' of arc each)
• each minute into 60 seconds (15" of arc each)

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Ecliptic Coordinate System

• Used in many sky atlases from the 1600s
• Polar coordinates
• Ecliptic is 0 latitude
• Poles are North and South Ecliptic Poles
• Longitude measured 0-360 from classical First
  Point of Aries
• Independent of Precession (but not nutation)
• No significant advantage over Equatorial System

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Galactic Coordinate System

• Polar coordinates
• 0 Latitude is galactic equator (mid line of
  Milky Way)
• Poles are North and South Galactic Poles
• Longitude measured 0-360 from galactic center

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Galactic Equator and Poles
Poles
Equator
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Local MeasurePosition Angle and Separation

• Origin is primary object
• Degrees / Minutes / Seconds of arc
• Position angle measured eastward from North
• Separation likewise measured as an angle

Example Double StarAlbireo
N
Position Angle54
Separation34.3"
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Local Measure Yardsticks

• Across top of bowl of Big Dipper 10
• Across bottom of bowl of Dipper 8
• Across end of bowl of Big Dipper 5
• Across end of bowl of Little Dipper 3º
• Diameter of Moon 0.5º (30')
• Telrad 4, 2, ½º
• Rigel QuikFinder 2, ½º
• Learn field width of your finder and eyepieces

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More Local Measure YardsticksDistance with Arm
Outstretched

• Width of index finger 2
• Across clenched fist 10
• From end of thumb to end of pinky with hand
  stretched wide 30º

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Motion of Sky Objects

• Daily rotation 360º of arc (24 hours of right
  ascension) about the celestial poles per day 15º
  of arc per hour
• Culmination when an object crosses the meridian
• Sidereal Time current right ascension
  coordinate of the meridian
• Annual revolution 360º of arc in 365.25 days,
  about 4 minutes of right ascension per day
• Therefore Sidereal day is 4 minutes shorter
  than the chronological day

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Precession

• Wobble of the Earths axis
• Poles move in a 47º circle over 25,800 year
  period
• Changes orientation of equatorial coordinate
  system
• Pole star changes (Thuban ? Polaris ? Vega)
• Vernal equinox (first point of Aries) is now in
  Pisces moving westward into Aquarius
• Tropics of Cancer/Capricorn are really
  Gemini/Sagittarius today
• Star charts change orientation or Epoch (1920 vs.
  1950 vs. 2000) over time

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Proper Motion

• Motion of close stars relative to distant
  background stars
• Very slow except for a few close stars (Proxima
  Centauri, Barnards Star)
• Only significant over centuries and millennia

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The Sky Looks Confusing...
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Constellations

• Patterns of bright stars in the sky
• IAU-designated boundaries, marking out specific
  sky regions
• Named in Latin, mostly from ancient times
• There are 88 of them in all
• 53 are visible from our latitude
• About 20 or so are worth learning

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Bringing Some Order to Chaos...
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Star Nomenclature

• Proper (individual) Names
• Bayer Letters
• Flamsteed Numbers
• Variable Star Designations
• Double Star Lists and Catalogues
• Professional Catalogues

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Proper Names

• Different in different languages
• English names generally based on Greek, Latin,
  Arabic
• Only the brightest stars have proper names in
  common use
• Multiple stars have suffix A, B, C, etc. applied
  to components in order of apparent brightness

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Bayer LettersUranometria, 1603

• Greek lower case letter a - ? followed by
  genitive of Latin constellation name (e.g., a
  Centauri), allowing for 24 designations
• Usually assigned in order of apparent brightness,
  but not always
• If more needed, lowercase Roman (a - z) and then
  uppercase Roman (A - Q) these are not in use
  anymore

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The Greek Lowercase Alphabet

• a Alpha
• ß Beta
• ? Gamma
• d Delta
• e Epsilon
• ? Zeta
• ? Eta
• ? Theta
• ? Iota
• ? Kappa
• ? Lambda
• µ Mu

• ? Nu
• ? Xi
• ? Omicron
• p Pi
• ? Rho
• s Sigma
• t Tau
• ? Upsilon
• f Phi
• ? Chi
• ? Psi
• ? Omega

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Flamsteed Numbers

• Number genitive of constellation name (e.g., 66
  Geminorum)
• Assigned to stars from west to east
• Greek Bayer letters usually take precedence

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Argelander Variable Star Letters

• Upper case Roman Letters genitive of
  constellation (e.g., R Leporis)
• Assigned in order of discovery of variability
• R - Z, RR - RZ, SS - SZ, TT - TZ, UU - UZ, VV -
  VZ, WW - WZ, XX - XZ, YY, YZ, ZZ
• Then AA - AZ, BB - BZ, etc. through QZ (but J
  never used) then V335, V336, ...
• Usually not assigned to stars with Bayer Greek
  letters

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Double Star Lists and Catalogues

• F.G.W. Struve (S, S I, S II)
• Otto Struve (OS, OSS)
• John Herschel (h)
• J. Dunlop (?)
• S.W. Burnham (ß)
• T.J.J. See (?)

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The Many Names of a Star

• Proper name Castor (A, B, C)
• Bayer a Geminorum (Alpha Geminorum)
• Flamsteed 66 Geminorum
• Variable (Castor C) YY Geminorum
• Multiple Star S1110 (Struve 1110)
• Professional GI 278, HR 2891, FKS 287,BD 32
  1581/2, HD 60178/60179,SAO 60198, ADS 6175, HIP
  36850, etc.

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The Magnitude System

• Originally established by Hipparchus Ptolemy
• Magnitude rank or importance
• 1st magnitude brightest stars
• 6th magnitude barely visible naked eye
• 5 magnitudes 100x brightness
• Each order of magnitude 2.512x brightness
  (fifth root of 100)

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The 21 First Magnitude Stars

• Sirius (Canis Major), -1.42
• Canopus (Carina), -0.72
• Alpha Centauri (Centaurus),-0.27
• Arcturus (Boötes), -0.06
• Vega (Lyra), 0.04
• Capella (Auriga), 0.06
• Rigel (Orion), 0.14
• Procyon (Canis Minor), 0.35
• Achernar (Eridanus), 0.53
• Hadar (Centaurus), 0.66
• Betelgeuse (Orion), 0.70

• Altair (Aquila), 0.77
• Aldebaran (Taurus), 0.86
• Acrux (Crux), 0.87
• Antares (Scorpius), 0.92
• Spica (Virgo), 1.00
• Pollux (Gemini), 1.16
• Fomalhaut (Piscis Austrinus), 1.17
• Deneb (Cygnus), 1.26
• Beta Crucis (Crux), 1.28
• Regulus (Leo), 1.36

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Polaris The North Star

• 2nd (1.99) magnitude star a Ursae Minoris
• ¾ degree of arc from celestial north pole
• Barely moves with time of day or season
• Its altitude your geographic latitude
• Useful for aligning equatorial mounts
• A double star (S93)
• A cepheid variable star

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NH Constellations (53)

• Andromeda
• Aquarius
• Aquila
• Aries
• Auriga
• Boötes
• Camelopardalis
• Cancer
• Canes Venatici
• Canis Major
• Canis Minor
• Capricornus
• Cassiopeia
• Cepheus
• Cetus
• Coma Berenices
• Corona Borealis
• Corvus
• Crater

• Hydra
• Lacerta
• Leo
• Leo Minor
• Lepus
• Libra
• Lynx
• Lyra
• Monoceros
• Ophiuchus
• Orion
• Pegasus
• Perseus
• Pisces
• Piscis Austrinus
• Puppis
• Sagitta
• Sagittarius
• Scorpius

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The Important Ones (24)

• Andromeda
• Aquila
• Auriga
• Boötes
• Canis Major
• Canis Minor
• Capricornus
• Cassiopeia
• Cepheus
• Cygnus
• Gemini
• Hercules

• Leo
• Lyra
• Ophiuchus
• Orion
• Pegasus
• Perseus
• Sagittarius
• Scorpius
• Taurus
• Ursa Major
• Ursa Minor
• Virgo

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The ZodiacWhere the Moon and Planets Are

• Pisces
• Aries
• Taurus
• Gemini
• Cancer
• Leo

• Virgo
• Libra
• Scorpius
• Sagittarius
• Capricornus
• Aquarius

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Circumpolar Constellations

• Always above the horizon (though often very low)
• Everything with declination higher than 90º minus
  your latitude (90 - 43 47ºN for us)
• For us Ursa Minor, Ursa Major, Cassiopeia,
  Cepheus, Camelopardalis, Draco, Lynx

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The Spring Sky

• Constellations (12) Ursa Major, Ursa Minor,
  Cepheus, Cassiopeia, Perseus, Auriga, Taurus,
  Gemini, Orion, Canis Major, Canis Minor, Leo,
  Boötes
• Stars (11) Capella, Aldebaran, Betelgeuse,
  Rigel, Sirius, Adhara, Procyon, Regulus,
  Arcturus, Castor, Pollux
• Asterisms, etc. Pleiades, Hyades

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21 March 20071000 PM
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The Summer Sky

• Constellations (15) Cepheus, Ursa Minor, Ursa
  Major, Cygnus, Leo, Boötes, Hercules, Corona
  Borealis, Lyra, Aquila, Ophiuchus, Virgo,
  Scorpius, Hydra, Corvus
• Stars (9) Deneb, Vega, Altair, Arcturus,
  Regulus, Spica, Antares, Castor, Pollux
• Asterisms, etc. Keystone of Hercules Summer
  Triangle

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21 June 2007900 PM
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The Autumn Sky

• Constellations (19) Ursa Major, Ursa Minor,
  Cassiopeia, Cepheus, Boötes, Hercules, Corona
  Borealis, Lyra, Cygnus, Aquila, Delphinus,
  Ophiuchus, Sagittarius, Capricornus, Aquarius,
  Piscis Austrinus, Pisces, Pegasus, Andromeda
• Stars (5) Arcturus, Vega, Deneb, Altair,
  Fomalhaut
• Asterisms, etc. Summer Triangle, Keystone of
  Hercules, Great Square of Pegasus, Teapot of
  Sagittarius, Water Jar of Aquarius, Circlet
  of Pisces

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21 September2007900 PM
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The Winter Sky

• Constellations (16) Ursa Major, Ursa Minor,
  Cepheus, Cassiopeia, Perseus, Andromeda, Pegasus,
  Cetus, Pisces, Auriga, Aries, Taurus, Orion,
  Canis Minor, Canis Major, Gemini
• Stars (9) Vega, Deneb, Aldebaran, Betelgeuse,
  Rigel, Sirius, Procyon, Castor, Pollux
• Asterisms, etc. Great Square of Pegasus,
  Circlet of Pisces, Pleiades, Hyades

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21 December2007900 PM
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Star Hopping

• Finding objects without artificial aid
• Use the major guideposts to orient the sky
• Follow pointers to zero in on the destination

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Star Hop to M31, M32, M110

• Find the Great Square of Pegasus
• Northeast corner is Alpha Andromedae
• Count two stars along each string of Andromeda
• Follow pointers north the same distance, and
  there you are

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Telescopic Star HopNGC 2261 (Hubbles Variable
Nebula)

• Locate Gemini
• Go to foot of Pollux ? (Xi) Geminorum
• Move south to NGC 2264 (Christmas Tree Cluster)
• South to double star S953 (Struve 953)
• Southwest to Hubbles Variable Nebula

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2º View
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Solar System Guideposts

• 5 Naked-eye Major Planets
• Mercury, Venus, Mars, Jupiter, Saturn
• Very bright (1st magnitude or brighter)
• Earths Moon (magnitude -12.6 when full!)
• They move relative to the stars
• Your geographic position may be significant

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Orbital Motion of Planets and Moon

• Orbital planes all very near Ecliptic
• Direct motion west to east
• Retrograde motion east to west (when we pass
  an outer planet in our orbit)
• Mercury and Venus shuttle back and forth across
  the Suns position

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Significant Planetary Positions

• Conjunction both objects at same RA
• Opposition objects 12 hours apart in RA
• Quadrature objects 6 hours apart in RA
• Transit smaller object passes across larger
  objects disk
• Occultation larger object hides smaller one
• Eclipse shadow of one object hides another

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Solar Conjunction

• Culmination at local Noon
• Inner planets do it twice
• Superior Conjunction opposite side of Sun
• Inferior Conjunction same side of Sun
• For our Moon, is called New Moon
• Planet/Moon not visible near conjunction

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Solar Opposition

• Midnight culmination
• Not possible for inner planets closest they get
  is Greatest Elongation
• Eastern (furthest away from Sun at evening
  twilight)
• Western (furthest away from Sun at morning
  twilight)
• For our Moon, is called Full Moon
• Outer planets at their brightest (because at
  their closest)
• Generally best time to observe outer planets

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EasternQuadrature
Greatest Eastern Elongation
Sun
Earth
SuperiorConjunction
InferiorConjunction
Opposition
Conjunction
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Observing Resources

• Planisphere
• Sky and Telescope magazine
• Celestial Atlases (Sky Atlas 2000.0)
• Software (The Sky, Sky Tools)
• Web Sites
• Heavens Above (www.heavens-above.com)
• CalSKY (www.calsky.com)

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Using a Sky Atlas

• Very much like reading a map
• Useful for plotting difficult star hops
• www.CalSKY.com - online star atlas
• prints finder charts at many angular diameters
• charts oriented for your latitude/longitude/time
  of night
• planetary/moon positions and motions plotted

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Questions?