Astro 101: Navigating the Night Sky presented by Paul Winalski 20 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 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 • • • Used in many sky atlases from the 1600 s 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
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 • • 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 Quik. Finder: 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 • • • 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 • • • α β γ δ ε ζ η θ ι κ λ μ Alpha Beta Gamma Delta Epsilon Zeta Eta Theta Iota Kappa Lambda Mu • • • 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 V 335, V 336, . . . • Usually not assigned to stars with Bayer Greek letters
Double Star Lists and Catalogues • • • F. G. W. Struve (Σ, 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: Σ 1110 (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” • 1 st magnitude = brightest stars • 6 th magnitude = barely visible naked eye • 5 magnitudes = 100 x brightness • Each order of magnitude = 2. 512 x 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 • • 2 nd (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 (Σ 93) 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 Minor Lepus Libra Lynx Lyra Monoceros Ophiuchus Orion Pegasus Perseus Pisces Piscis Austrinus Puppis 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 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 M 31, M 32, M 110 • 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 Σ 953 (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 (1 st 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
Eastern Quadrature Greatest Eastern Elongation Earth Opposition Inferior Conjunction Superior Conjunction
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) – Cal. SKY (www. calsky. com)
Using a Sky Atlas • Very much like reading a map • Useful for plotting difficult star hops • www. Cal. SKY. 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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