Historical Astronomy History Aristotle – Aristarchus - Ptolemy

2 Who cares about Astronomy??? Hunters- Hunted animals who’s migration depended on the seasons Gatherers- Harvested fruit and nuts at certain times of the year Development of Agriculture- Important to harvest crops at the right times of the year Annual meetings of nomadic tribes based on position of stars and planets

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4 F.D.G. Famous Dead Guy

5 Aristotle ( B.C.) Developed models to explain the Universe without fully understanding it. Based his models on beautiful crystalline spheres Taught Earth was the immobile center of the Universe

6 Aristotle ( B.C.) Crystalline Spheres Earth as center of Universe

7 Continued … Aristotle’s model was believed for nearly 2000 years because no one observed parallax of stars –Concluded Earth did not move Parallax: The change in the apparent position of an object due to a change in the location of the observer

8 The problem of the unseen parallax

9 No stellar parallaxes were detected in ancient times Two possible explanations 1.The stars are very far away, so that their parallaxes were too small to measure in ancient times 2.The earth did not go around the sun

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

11 Aristarchus of Samos (310 – 230 B.C.) Predicted that the Sun was very far away and much larger than the moon and Earth

12 Aristarchus of Samos (310 – 230 B.C.) Observed the Earth’s shadow on the moon during a lunar eclipse Thought it was absurd to assume that the Sun revolved around so small a body (Earth) –First person to present a good argument for a “sun at the center of the Universe” model! Sometimes known as the “Greek Copernicus”

13 Aristarchus of Samos (310 – 230 B.C.) Believed in the “stars to far away to see the parallax” theory

14 Ptolemy (Claudius Ptolemaeus, ca. 90 – ca. 168 A.D.) Lived in Alexandria, the center of learning at the time

15 Ptolemy Continued to support the Geocentric model of the Universe –Geocentric = Earth centered

16 Planetary Motion Attempted to explain planetary motion with his model of the Universe Prograde Motion-The apparent forward motion of a planet with respect to the background stars

17 Planetary Motion Cont… Retrograde Motion-The apparent backword motion of a planet with respect to the background stars –Examples: Mars, Venus, Uranus, many of Saturn and Jupiters moons

18 Retrograde 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 ?

19 Increased Complexity Additional need for accuracy in predictions led to further and further complexity of models, ultimately codified by Ptolemy Ptolemy adopted a “wheels within wheels” approach to his geocentric model of the Universe

20 Wheels within Wheels Epicycles: The smaller cirlces that move along the larger circles called the deferent Deferent: The larger circle that the smaller epicycles move around

21 Continued …

22 Size of the Universe Models of Aristotle / Ptolemy envisioned (required) a relatively small universe –Even the grandest, Ptolemy’s, put the rough size at about 50,000,000 miles (one light year = six trillion miles) Why so small ? –If earth does not move, the rest of the Universe must ! –Ptolemy’s universe would complete a cycle around the earth each 24 hours –Speed required ~ 10,000,000 mph ! –If larger, then speed larger still …