Astro 101:
Navigating the Night Sky
presented by Paul Winalski
20 April 2007
Topics for this Evening
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Celestial Coordinates
Star Nomenclature
Orientating One’s Self to the Sky
Star-hopping
Reading Star Charts
The Universe’s Four Dimensions
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At 90-degree angles to each other
Distance (forward/backward)
Azimuth (left/right)
Altitude (up/down)
Time (past/future)
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
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
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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
Polar Altazimuth Coordinates
Zenith
Distance
Altitude
Azimuth
Horizon
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
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
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, it’s valid to say
that the sky moves around us
The Celestial Sphere
• We are on the Earth’s surface
• Earth’s 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
Equatorial Coordinate System
• Polar coordinate system, equivalent to
geographic coordinate system
• Celestial poles aligned with geographic
poles of the Earth’s rotation
• Independent of local latitude/longitude
(unlike altazimuth coordinates)
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 Earth’s orbit
about the Sun (at 23.5º angle to equator)
• Galactic Equator: intersecting center plane
of Milky Way galaxy’s spiral arms
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)
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)
Ecliptic Coordinate System
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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
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
Galactic Equator and Poles
Poles
Equator
Local Measure:
Position 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 Star
Albireo
N
Position Angle=54°
Separation=34.3"
Local Measure Yardsticks
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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
More Local Measure Yardsticks
Distance 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
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
Precession
• Wobble of the Earth’s 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
Proper Motion
• Motion of close stars relative to distant
background stars
• Very slow except for a few close stars
(Proxima Centauri, Barnard’s Star)
• Only significant over centuries and
millennia
The Sky Looks Confusing...
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
Bringing Some Order to Chaos...
Star Nomenclature
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Proper (individual) Names
Bayer Letters
Flamsteed Numbers
Variable Star Designations
Double Star Lists and Catalogues
Professional Catalogues
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
Bayer Letters
Uranometria, 1603
• Greek lower case letter α - ω followed by
genitive of Latin constellation name (e.g., α
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]
The Greek Lowercase Alphabet
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α
β
γ
δ
ε
ζ
η
θ
ι
κ
λ
μ
Alpha
Beta
Gamma
Delta
Epsilon
Zeta
Eta
Theta
Iota
Kappa
Lambda
Mu
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ν
ξ
ο
π
ρ
σ
τ
υ
φ
χ
ψ
ω
Nu
Xi
Omicron
Pi
Rho
Sigma
Tau
Upsilon
Phi
Chi
Psi
Omega
Flamsteed Numbers
• Number + genitive of constellation name
(e.g., 66 Geminorum)
• Assigned to stars from west to east
• Greek Bayer letters usually take precedence
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
Double Star Lists and Catalogues
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F.G.W. Struve (Σ, Σ I, Σ II)
Otto Struve (OΣ, OΣΣ)
John Herschel (h)
J. Dunlop (Δ)
S.W. Burnham (β)
T.J.J. See (λ)
The Many Names of a Star
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Proper name: Castor (A, B, C)
Bayer: α Geminorum (Alpha Geminorum)
Flamsteed: 66 Geminorum
Variable (Castor C): YY Geminorum
Multiple Star: Σ1110 (Struve 1110)
Professional: GI 278, HR 2891, FKS 287,
BD +32 1581/2, HD 60178/60179,
SAO 60198, ADS 6175, HIP 36850, etc.
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)
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
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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
Polaris: The North Star
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2nd (1.99) magnitude star α 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 (Σ93)
A cepheid variable star
NH Constellations (53)
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Andromeda
Aquarius
Aquila
Aries
Auriga
Boötes
Camelopardalis
Cancer
Canes Venatici
Canis Major
Canis Minor
Capricornus
Cassiopeia
Cepheus
Cetus
Coma Berenices
Corona Borealis
Corvus
Crater
Cygnus
Delphinus
Draco
Equuleus
Eridanus
Gemini
Hercules
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Hydra
Lacerta
Leo
Leo Minor
Lepus
Libra
Lynx
Lyra
Monoceros
Ophiuchus
Orion
Pegasus
Perseus
Pisces
Piscis Austrinus
Puppis
Sagitta
Sagittarius
Scorpius
Scutum
Serpens
Taurus
Triangulum
Ursa Major
Ursa Minor
Virgo
Vulpecula
The Important Ones (24)
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Andromeda
Aquila
Auriga
Boötes
Canis Major
Canis Minor
Capricornus
Cassiopeia
Cepheus
Cygnus
Gemini
Hercules
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Leo
Lyra
Ophiuchus
Orion
Pegasus
Perseus
Sagittarius
Scorpius
Taurus
Ursa Major
Ursa Minor
Virgo
The Zodiac:
Where the Moon and Planets Are
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Pisces
Aries
Taurus
Gemini
Cancer
Leo
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Virgo
Libra
Scorpius
Sagittarius
Capricornus
Aquarius
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
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
21 March 2007
10:00 PM
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”
21 June 2007
9:00 PM
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
21 September
2007
9:00 PM
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
21 December
2007
9:00 PM
Star Hopping
• Finding objects without artificial aid
• Use the major guideposts to orient the sky
• Follow pointers to zero in on the destination
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
Telescopic Star Hop:
NGC 2261 (Hubble’s Variable Nebula)
• Locate Gemini
• Go to “foot of Pollux”: ξ (Xi) Geminorum
• Move south to NGC 2264 (“Christmas
Tree” Cluster)
• South to double star Σ953 (Struve 953)
• Southwest to Hubble’s Variable Nebula
2º View
Solar System Guideposts
• 5 Naked-eye Major Planets
– Mercury, Venus, Mars, Jupiter, Saturn
– Very bright (1st magnitude or brighter)
• Earth’s Moon (magnitude -12.6 when full!)
• They move relative to the stars
• Your geographic position may be significant
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 Sun’s position
Significant Planetary Positions
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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
object’s disk
• Occultation: larger object hides smaller one
• Eclipse: shadow of one object hides another
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
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
Eastern
Quadrature
Greatest Eastern
Elongation
Earth
Opposition
Sun
Inferior
Conjunction
Superior
Conjunction
Conjunction
Observing Resources
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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)
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
Questions?
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The 22 First Magnitude Stars