The Beginnings of Astronomy (or, How do we know that?)

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

Planets and Gods PlanetBabylonianGreekRomanGerman/Norse SunShamashHeliosSôlSôl MoonSinSelenêLuna MercuryNabûHermesMercuriusWotan/Odin VenusIshtarAphroditêVenusFria/Freda/Frigg MarsNergalAresMarsTiw JupiterMardukZeusIuppiterThor SaturnNinurtaKronosSaturnus

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 ( days)

Early Calendar Sumerians & Babylonians –Year 360 days (actual value = ) 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 = months Named after common activities (planting, harvest, etc.) –Day divided into 12 hours Egyptians later adopt 24 hour day Base 60 numbers

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

LanguageSaturnSunMoonMarsMercuryJupiterVenus Greek (Hemera) KhronuHeliouSelenesAreosHermuDiosAphrodites Latin (Dies)SaturniSolisLunaeMartisMercuriiJovisVeneris FrenchSamediDimancheLundiMardiMercrediJeudiVendredi SpanishSabadoDomingoLunesMartesMiércolesJuevesViernes PortugueseSábadoDomingoSegundaTerçaQuartaQuintaSexta Saxon (day)Saterne’sSun’sMoon’sTiw’sWoden’sThor’sFrigg’s NorwegianLørdagSøndagMandagTirsdagOnsdagTorsdagFredag 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

HourSaturdaySundayMondayTuesdayWednesdayThursdayFriday 1SaturnSunMoonMarsMercuryJupiterVenus 2JupiterVenusSaturnSunMoonMarsMercury 3MarsMercuryJupiterVenusSaturnSunMoon 4SunMoonMarsMercuryJupiterVenusSaturn 5VenusSaturnSunMoonMarsMercuryJupiter 6MercuryJupiterVenusSaturnSunMoonMars 7MoonMarsMercuryJupiterVenusSaturnSun 8SaturnSunMoonMarsMercuryJupiterVenus 9JupiterVenusSaturnSunMoonMarsMercury 10MarsMercuryJupiterVenusSaturnSunMoon 11SunMoonMarsMercuryJupiterVenusSaturn 12VenusSaturnSunMoonMarsMercuryJupiter 13MercuryJupiterVenusSaturnSunMoonMars 14MoonMarsMercuryJupiterVenusSaturnSun 15SaturnSunMoonMarsMercuryJupiterVenus 16JupiterVenusSaturnSunMoonMarsMercury 17MarsMercuryJupiterVenusSaturnSunMoon 18SunMoonMarsMercuryJupiterVenusSaturn 19VenusSaturnSunMoonMarsMercuryJupiter 20MercuryJupiterVenusSaturnSunMoonMars 21MoonMarsMercuryJupiterVenusSaturnSun 22SaturnSunMoonMarsMercuryJupiterVenus 23JupiterVenusSaturnSunMoonMarsMercury 24MarsMercuryJupiterVenusSaturnSunMoon

Solar Calendars –Months ignore phase of moon –Leap days maintain synchronization with seasons –Example: Gregorian Calendar (our calendar)

Lunar Calendars Months –Start at new (or full) moon –Alternate 29 and 30 days –Leap day when necessary average month = 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 = months Examples: Hebrew & Chinese Calendars

Modern Calendar Julian Calendar (adopted by Julius Caesar) –Normal year = 365 days –Add leap day (Feb 29) every 4 years Average length of year = days –Months at given time of year not linked to moon phase 30 or 31 days (except February)

Modern Calendar Gregorian Calendar (adopted by Pope Gregory) –10 day error accumulated by 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 = days Except every 400 years (so 2000 was a leap year!) –Average length of year = days

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

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!!

Rise of Science Greek Philosophy - Rational Thought –The Universe is knowable –The Universe is understandable Observe the Universe Think about it Hypothesize / Explain Test

Earth is Round Aristotle –Observers farther N or S see different stars –altitude of pole star (= latitude) –Shape of earth’s shadow (seen during lunar eclipse) always round Ships sailing over horizon –Hull disappears first –Mast disappears last

A Modern View

Size of Earth Eratosthenes (Greek astronomer) –Sun far away –All observers point same direction to sun

Size of Earth Eratosthenes –Altitude of sun at noon on Jun21 At zenith in Syene 7º south in Alexandria –7º/360º = 1/50 circle –Distance between cities 5000 stadia –Circumference of Earth –50 x 5000 = 250,000 stadia (about 40,000 km)

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 (earth’s axis wobbles) Complete circle 26,000 years

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

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

Ptolemaic System Start with circular path - deferent –Does not produce retrograde motion Add epicycle - circle on deferent –Planet moves backward on inside of epicyle

Epicyclic Motion

Ptolemaic System

The Dark Ages Fall of Greek / Roman Empire –Library at Alexandria burns –Most astronomical records lost Ptolemy’s Book –The Almagest –Preserved in Arabic countries Renaissance –Christian Church adopts geo- centric cosmology of Greeks

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

Copernican Model

Copernicus & Retrograde Motion

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

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

Geocentric vs. Heliocentric Predicted Phases of Venus –Geocentric: only crescent phases –Heliocentric: both full and crescent phases Observations show both!

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

Galileo - Sunspot Observations

Galileo Observes Moons of Jupiter

Tycho Brahe Danish Astronomer Accurately measured positions –Mars Enabled Quantitative Testing

Tycho’s Model

Kepler Worked for Tycho (in Prague) Believed Heliocentric model (Copernicus) –To match Tycho’s data, model needed modification Kepler’s 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 earth’s orbit)

Ellipse focus a a = length of semi-major axis

Kepler’s 2 nd Law time from 1 to 2 = time from 3 to 4 area 1-2-S = area 3-4-S

Kepler’s 2nd Law

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

Kepler’s Model

Kepler’s 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.