History of AstronomyHistory of Astronomy

How have ideas about the solar system How have ideas about the solar system and our place in it changed?and our place in it changed?

EgyptiansEgyptians

Saw patterns in the Sun, moon, and Saw patterns in the Sun, moon, and VenusVenus

Called the sun “Ra” (Sun god) – rode in Called the sun “Ra” (Sun god) – rode in his boat across the sky dailyhis boat across the sky daily

Developed 365 day year solar calendarDeveloped 365 day year solar calendar

Ra (on right)

GreeceGreece

Eudoxus, born 400 B.C.Eudoxus, born 400 B.C.saw objects that moved in skysaw objects that moved in skycalled planets, sun, and moon “wanderers”called planets, sun, and moon “wanderers”Earth motionless and at the center of universeEarth motionless and at the center of universe

Ptolemy, AD 140Ptolemy, AD 140Leading astronomerLeading astronomer ““geocentric” theory – geocentric” theory – earth is center of is center of

universe. Lasted for 1400 years.universe. Lasted for 1400 years.

PtolemyPtolemy

“epicycles”

ArabsArabs

Caliph Harun-al-Rashid, rulerCaliph Harun-al-Rashid, rulerScholars translated Greek texts into Arabic Scholars translated Greek texts into Arabic

and preserved in “House of Wisdom” and preserved in “House of Wisdom” library – in Baghdadlibrary – in Baghdad

Measured positions of stars and planets Measured positions of stars and planets with fine instruments, astrolabe, can with fine instruments, astrolabe, can perform calculationsperform calculations

Named the red giant Betelgeuse, in OrionNamed the red giant Betelgeuse, in Orion325 light years away325 light years away

Betelgeuse – “shoulder”Betelgeuse – “shoulder”

Orion constellation

PolandPoland

Copernicus – 1540Copernicus – 1540First to suggest “heliocentric model” – First to suggest “heliocentric model” – sunsun is is

center of universecenter of universeObjects orbit sun in perfect circlesObjects orbit sun in perfect circlesStill not a perfect modelStill not a perfect model

CopernicusCopernicus

from book De Revolutionibus The English version…

What do you think?What do you think? Why do you think it was difficult for people to Why do you think it was difficult for people to

accept a heliocentric model (sun centered) over accept a heliocentric model (sun centered) over the geocentric model (Earth centered)?the geocentric model (Earth centered)?

Geocentric model Heliocentric model

DenmarkDenmarkTycho Brahe – 1576Tycho Brahe – 1576

Collected data of position of planets for 20 Collected data of position of planets for 20 yearsyears

Able to make accurate predictions of positions Able to make accurate predictions of positions without telescopeswithout telescopes

Had own “Tychonic Universe” – combination Had own “Tychonic Universe” – combination of Ptolemy and Copernicusof Ptolemy and Copernicus

Earth is stationary Earth is stationary Believed in circular orbitsBelieved in circular orbits

Tychonic UniverseTychonic Universe

Why the Circles?Why the Circles?

Why do you think these astronomers Why do you think these astronomers believed in circular orbits?believed in circular orbits?

KeplerKepler

Works with Tycho – after Tycho’s death, Works with Tycho – after Tycho’s death, Tycho’s family sued to recover instruments Tycho’s family sued to recover instruments and books of observationsand books of observations

Why?Why?Kepler was a Copernican, and they knew he Kepler was a Copernican, and they knew he

wouldn’t follow the Tychonic systemwouldn’t follow the Tychonic systemKepler kept the books! Began to study Kepler kept the books! Began to study

motion of Mars …motion of Mars …

KeplerKepler

Elliptical orbits

Kepler’s First LawKepler’s First Lawof Planetary Motionof Planetary Motion

Orbits are elliptical (oval shaped),Orbits are elliptical (oval shaped),

sun is at the focussun is at the focus

Kepler’s Second LawKepler’s Second Lawof Planetary Motionof Planetary Motion

A line from a planet to the sun sweeps A line from a planet to the sun sweeps over equal areas in equal timesover equal areas in equal times

Planets travel at different speeds – due to Planets travel at different speeds – due to pull of gravitypull of gravity

Same time, moves slower

Closer to Sun, moves faster…

Why?

Kepler’s Unanswered QuestionKepler’s Unanswered Question

What keeps planets in orbit?What keeps planets in orbit?

Galileo GalileiGalileo Galilei

Born in 1564Born in 1564Supports Copericus’ theory of heliocentric Supports Copericus’ theory of heliocentric

universeuniverseUsed telescope to observe:Used telescope to observe:

surface of Moon – was not perfect (like surface of Moon – was not perfect (like Ptolemaic model)Ptolemaic model)

Phases of VenusPhases of VenusMilky Way was made up of many starsMilky Way was made up of many starsMoons of JupiterMoons of Jupiter

GalileoGalileo

Moon surface

Phases of Venus

Isaac NewtonIsaac Newton

Born in 1654 (year Galileo dies)Born in 1654 (year Galileo dies)Three Laws of MotionThree Laws of Motion

Review from exam! Review from exam! These laws help us understand orbital These laws help us understand orbital

motionmotion11stst law – law –22ndnd law – law –33rdrd law – law –

Newton answersNewton answersKepler’s Question…Kepler’s Question…

What keeps planets in orbit?What keeps planets in orbit?

Tying it all up…Tying it all up…

MythsMythsSee patternsSee patternsPtolemyPtolemyCopernicusCopernicusKeplerKeplerGalileoGalileoNewtonNewton

HomeworkHomework

Timeline – list at least Timeline – list at least fourfour models of models of relationship between sun and planets.relationship between sun and planets. Include: dates, astronomer, modelInclude: dates, astronomer, model

Read – Chapter 16.1: pp. 526 – 530.Read – Chapter 16.1: pp. 526 – 530.