Historical AstronomyHistorical Astronomy

HistoryHistoryAristotle – Aristarchus - Aristotle – Aristarchus -

PtolemyPtolemy

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Who cares about Astronomy???Who cares about Astronomy???

HuntersHunters- Hunted animals who’s migration - Hunted animals who’s migration depended on the seasonsdepended on the seasons

GatherersGatherers- Harvested fruit and nuts at - Harvested fruit and nuts at certain times of the yearcertain times of the year

Development of AgricultureDevelopment of Agriculture- Important to - Important to harvest crops at the right times of the yearharvest crops at the right times of the year

Annual meetings of nomadic tribes based Annual meetings of nomadic tribes based on position of stars and planetson position of stars and planets

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F.D.G.F.D.G.

Famous Dead GuyFamous Dead Guy

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Aristotle (384-322 B.C.)Aristotle (384-322 B.C.)

Developed models to explain Developed models to explain the Universe without fully the Universe without fully understanding it.understanding it.

Based his models on beautiful Based his models on beautiful crystalline spherescrystalline spheres

Taught Earth was the Taught Earth was the immobile center of the immobile center of the UniverseUniverse

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Aristotle (384-322 B.C.)Aristotle (384-322 B.C.)

Crystalline Crystalline SpheresSpheres

Earth as center of Earth as center of UniverseUniverse

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Continued …Continued …

Aristotle’s model was believed for nearly Aristotle’s model was believed for nearly 2000 years because no one observed 2000 years because no one observed parallax of starsparallax of stars– Concluded Earth did not moveConcluded Earth did not move

Parallax: The change in the apparent position of an Parallax: The change in the apparent position of an object due to a change in the location of the object due to a change in the location of the observerobserver

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The problem of the unseen parallaxThe problem of the unseen parallax

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No stellar parallaxes were detected No stellar parallaxes were detected in ancient timesin ancient times

Two possible explanationsTwo possible explanations

1.1. The stars are very far away, so that their The stars are very far away, so that their parallaxes were too small to measure in parallaxes were too small to measure in ancient timesancient times

2.2. The earth did not go around the sunThe earth did not go around the sun

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Aristarchus of Samos (310 – 230 Aristarchus of Samos (310 – 230 B.C.)B.C.)

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Aristarchus of Samos (310 – 230 Aristarchus of Samos (310 – 230 B.C.)B.C.)

Predicted that Predicted that the Sun was the Sun was very far away very far away and much and much larger than the larger than the moon and moon and EarthEarth

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Aristarchus of Samos (310 – 230 Aristarchus of Samos (310 – 230 B.C.)B.C.)

Observed the Earth’s shadow on the moon Observed the Earth’s shadow on the moon during a lunar eclipseduring a lunar eclipse

Thought it was absurd to assume that the Thought it was absurd to assume that the Sun revolved around so small a body Sun revolved around so small a body (Earth)(Earth)– First person to present a good argument for a First person to present a good argument for a

“sun at the center of the Universe” model!“sun at the center of the Universe” model!Sometimes known as the “Greek Copernicus”Sometimes known as the “Greek Copernicus”

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Aristarchus of Samos (310 – 230 Aristarchus of Samos (310 – 230 B.C.)B.C.)

Believed in the “stars to far away to see the Believed in the “stars to far away to see the parallax” theoryparallax” theory

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Ptolemy (Claudius Ptolemaeus, ca. Ptolemy (Claudius Ptolemaeus, ca. 90 – ca. 168 A.D.)90 – ca. 168 A.D.)

Lived in Alexandria, the center of learning Lived in Alexandria, the center of learning at the timeat the time

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PtolemyPtolemyContinued to Continued to support the support the Geocentric Geocentric model of the model of the UniverseUniverse– Geocentric = Geocentric =

Earth centeredEarth centered

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Planetary MotionPlanetary Motion

Attempted to explain planetary motion with Attempted to explain planetary motion with his model of the Universehis model of the Universe

Prograde MotionPrograde Motion-The apparent forward -The apparent forward motion of a planet with respect to the motion of a planet with respect to the background starsbackground stars

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Planetary Motion Cont…Planetary Motion Cont…

Retrograde MotionRetrograde Motion-The apparent -The apparent backword motion of a planet with respect backword motion of a planet with respect to the background starsto the background stars– Examples: Mars, Venus, Uranus, many of Examples: Mars, Venus, Uranus, many of

Saturn and Jupiters moonsSaturn and Jupiters moons

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Retrograde MotionRetrograde Motion

Over a period of 10 weeks, Mars appears to stop, back up, then go forward again.

Understandable in a heliocentric model / how about in a geocentric model ?

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Increased ComplexityIncreased Complexity

Additional need for accuracy in Additional need for accuracy in predictions led to further and further predictions led to further and further complexity of models, ultimately codified complexity of models, ultimately codified by Ptolemyby Ptolemy

Ptolemy adopted a “wheels within Ptolemy adopted a “wheels within wheels” approach to his geocentric wheels” approach to his geocentric model of the Universemodel of the Universe

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Wheels within WheelsWheels within Wheels

EpicyclesEpicycles: The smaller cirlces that move : The smaller cirlces that move along the larger circles called the along the larger circles called the deferentdeferent

DeferentDeferent: The larger circle that the smaller : The larger circle that the smaller epicycles move aroundepicycles move around

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Continued …Continued …

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Size of the UniverseSize of the Universe

Models of Aristotle / Ptolemy envisioned (required) a Models of Aristotle / Ptolemy envisioned (required) a relatively small universerelatively small universe

– Even the grandest, Ptolemy’s, put the rough size at about Even the grandest, Ptolemy’s, put the rough size at about 50,000,000 miles (one light year = six trillion miles)50,000,000 miles (one light year = six trillion miles)

Why so small ?Why so small ?

– If earth does not move, the rest of the Universe must !If earth does not move, the rest of the Universe must !– Ptolemy’s universe would complete a cycle around the earth Ptolemy’s universe would complete a cycle around the earth

each 24 hourseach 24 hours– Speed required ~ 10,000,000 mph !Speed required ~ 10,000,000 mph !– If larger, then speed larger still …If larger, then speed larger still …