The Beginnings of Astronomy (or, How do we know that?)
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Ancient Astronomy Earliest recorded astronomy Babylonian /
Egyptian Chinese Mayan Early religion and astronomy Points of light
in sky associated with gods Sun / Moon dominant Planets often
associated with lesser gods
Calendars Year not even number of days (or months) Problem:
Calendar year too short Spring (vernal equinox) occurs later and
later Solution: Add leap days (or months); lengthen average year
want average length = sidereal year (365.2422 days)
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Early Calendar Sumerians & Babylonians Year 360 days
(actual value = 365.24) Divide Circle into 360 degrees Each degree
divided into 60 minutes of arc Each minute divided into 60 seconds
of arc 12 months per year (actual value = 12.37) Origin of 12
Zodiacal signs (sun in each for 1 month) Month begins at first
appearance of crescent moon Month is 29 or 30 days (modern value =
29.53) Leap months added every few years lengthen year, so average
= 12.37 months Named after common activities (planting, harvest,
etc.) Day divided into 12 hours Egyptians later adopt 24 hour day
Base 60 numbers
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The Week Originally, time between market days Sumerian &
Babylonian: 4-8 days Differed by city / region Greeks (&
Egyptians): 10 day week Romans 8 days (market week) Egyptians: 7
days (planet worship) Romans abroad bring home 7 day week permitted
by Caesar Augustus Officially adopted by Emperor Constantine Days
named after planets
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LanguageSaturnSunMoonMarsMercuryJupiterVenus Greek (Hemera)
KhronuHeliouSelenesAreosHermuDiosAphrodites Latin
(Dies)SaturniSolisLunaeMartisMercuriiJovisVeneris
FrenchSamediDimancheLundiMardiMercrediJeudiVendredi
SpanishSabadoDomingoLunesMartesMircolesJuevesViernes
PortugueseSbadoDomingoSegundaTeraQuartaQuintaSexta Saxon
(day)SaternesSunsMoonsTiwsWodensThorsFriggs
NorwegianLrdagSndagMandagTirsdagOnsdagTorsdagFredag
GermanSamstagSonntagMontagDienstagMittwochDonnerstagFreitag
EnglishSaturdaySundayMondayTuesdayWednesdayThursdayFriday Days of
Week Theory for order (Egyptian): 24 hour day; each hour ruled by
planet Order planets by decreasing period; repeat Planet in first
hour rules day
Solar Calendars Months ignore phase of moon Leap days maintain
synchronization with seasons Example: Gregorian Calendar (our
calendar)
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Lunar Calendars Months Start at new (or full) moon Alternate 29
and 30 days Leap day when necessary average month = 29.5306 days
Two types: Lunar Calendar (1 year = 12 months) Beginning of year
slips (about 1/3 month per year) New year occurs in different
seasons (in 33 year cycle) Example: Islamic Calendar Luni-Solar
Calendar (1 year = 12 or 13 months) Add leap months Maintain
synchronization with solar year Average year = 12.3683 months
Examples: Hebrew & Chinese Calendars
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Modern Calendar Julian Calendar (adopted by Julius Caesar)
Normal year = 365 days Add leap day (Feb 29) every 4 years Average
length of year = 365.25 days Months at given time of year not
linked to moon phase 30 or 31 days (except February)
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Modern Calendar Gregorian Calendar (adopted by Pope Gregory) 10
day error accumulated by 1582 10 days eliminated from calendar
October 4, 1582 followed by October 15, 1582 Modified leap day
rules Remove one leap day every hundred years (i.e., no leap year
in century years, e.g., 1800, 1900) Average length of year = 365.24
days Except every 400 years (so 2000 was a leap year!) Average
length of year = 365.2425 days
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Beginnings of Astrology Babylonians believed: Location of
planets influence destiny of kings Greeks: Adopted Babylonian
astrology Believed individuals (personality/destiny) affected by
planets Horoscope - chart of planet locations Natal signs -
zodiacal location of sun/planets at birth House - location in the
sky
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Science vs. Astrology Astrology: planets exert force at birth
Science: gravity only known planetary force Doctor produces larger
effect than planet! Astrology: planets affect personality people
born in given month share similar personalities Science: people w/
similar traits have birthdates distributed equally through year
There is NO scientific support for astrology!!
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Rise of Science Greek Philosophy - Rational Thought The
Universe is knowable The Universe is understandable Observe the
Universe Think about it Hypothesize / Explain Test
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Earth is Round Aristotle Observers farther N or S see different
stars altitude of pole star (= latitude) Shape of earths shadow
(seen during lunar eclipse) always round Ships sailing over horizon
Hull disappears first Mast disappears last
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A Modern View
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Size of Earth Eratosthenes (Greek astronomer) Sun far away All
observers point same direction to sun
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Size of Earth Eratosthenes Altitude of sun at noon on Jun21 At
zenith in Syene 7 south in Alexandria 7/360 = 1/50 circle Distance
between cities 5000 stadia Circumference of Earth 50 x 5000 =
250,000 stadia (about 40,000 km)
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First Star Catalog Hipparchus (Greek astronomer) Measured
Positions of Stars in Sky Classified Stars by brightness Magnitudes
Brightest is 1st Magnitude Discovered Precession North Celestial
Pole moves (earths axis wobbles) Complete circle 26,000 years
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Models of Universe Must Explain Night & Day Motion of Sun
Motion and Phases of Moon Motion of Planets Inferior planets
(Mecury & Venus) always close to sun never at opposition
Superior planets (Mars, Jupiter & Saturn) Seen both near sun
& at opposition Retrograde motion Superior planets: only near
opposition
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Greek Cosmology Geocentric Model Earth at center of Universe
Philosophy Spheres / Circles simplest objects Motion of planets on
circles Rate of motion is uniform Fastest objects are closest
Ptolemy Greek Astronomer Alexandria, Egypt
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Ptolemaic System Start with circular path - deferent Does not
produce retrograde motion Add epicycle - circle on deferent Planet
moves backward on inside of epicyle
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Epicyclic Motion
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Ptolemaic System
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The Dark Ages Fall of Greek / Roman Empire Library at
Alexandria burns Most astronomical records lost Ptolemys Book The
Almagest Preserved in Arabic countries Renaissance Christian Church
adopts geo- centric cosmology of Greeks
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Birth of Modern Astronomy Copernicus Ptolemaic system too
complicated Suggested Heliocentric model Sun at center Planets
orbit sun Orbits are circles Planets in order Mercury, Venus,
Earth, Mars, Jupiter, Saturn
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Copernican Model
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Copernicus & Retrograde Motion
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You observe Mars and find that it is undergoing retrograde
motion. What time does it transit? A.6 am B.Noon C.6pm D.Midnight
Where is the Sun? At highest point Opposition
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Testing - the key to science Greeks Rational thought was
sufficient Inconsistencies blamed on faulty perception Modern
Science Models make predictions Verification provides support for
model A Theory is a generally accepted model
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Geocentric vs. Heliocentric Predicted Phases of Venus
Geocentric: only crescent phases Heliocentric: both full and
crescent phases Observations show both!
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Galileo Built Telescope Observed: the Moon, Jupiter, Jovian
moons, Saturn, and Venus. Saw: 1.Sunspots Sun not perfect
2.Mountains, Craters, and Valleys on Moon Moon not perfect 3.Moons
orbiting Jupiter Objects can orbit other bodies, not Earth!
4.Phases: Venus had both crescent and gibbous Verified Heliocentric
model predictions
Kepler Worked for Tycho (in Prague) Believed Heliocentric model
(Copernicus) To match Tychos data, model needed modification
Keplers Laws 1.Orbit is Ellipse Sun at focus 2.Planet sweeps out
equal areas in equal times 3.Period (P) - size (a = semi-major
axis) relationship P 2 = a 3 P in years a in astronomical units
(AU) (1 AU = size of earths orbit)
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Ellipse focus a a = length of semi-major axis
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Keplers 2 nd Law time from 1 to 2 = time from 3 to 4 area 1-2-S
= area 3-4-S
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Keplers 2nd Law
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Equal area law implies that planets move: Faster when closer to
sun Perihelion = planet closest to sun Slower when farther from sun
Aphelion = planet farthest from sun
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Keplers Model
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Keplers 3rd Law P 2 = a 3 Relation between period of orbit and
distance from Sun The farther a planet is from the Sun, the longer
it takes to go around its orbit.