Astro 101:Navigating the Night Sky

presented by Paul Winalski20 April 2007

Topics for this Evening

• Celestial Coordinates

• Star Nomenclature

• Orientating One’s Self to the Sky

• Star-hopping

• Reading Star Charts

The Universe’s Four Dimensions

• 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

• 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

Horizon

Azimuth

Altitude

Distance

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

• 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

Equator

Poles

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 StarAlbireo

N

Separation=34.3"

Position Angle=54°

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

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

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

• 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 LettersUranometria, 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• α Alpha

• β Beta

• γ Gamma

• δ Delta

• ε Epsilon

• ζ Zeta

• η Eta

• θ Theta

• ι Iota

• κ Kappa

• λ Lambda

• μ Mu

• ν Nu

• ξ Xi

• ο Omicron

• π Pi

• ρ Rho

• σ Sigma

• τ Tau

• υ Upsilon

• φ Phi

• χ Chi

• ψ Psi

• ω 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

• 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• 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

• 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

• 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)• 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

• 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)• 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

The Zodiac:Where the Moon and Planets Are• Pisces• Aries• Taurus• Gemini• Cancer• Leo

• 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 200710:00 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 20079:00 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 September2007

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 December2007

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

• 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

SunEarth

Greatest Eastern Elongation

InferiorConjunction

SuperiorConjunctionOpposition

Conjunction

EasternQuadrature

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)

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?