Download 1 Timeline 2 Geocentric model

Ancient Astronomy
Many ancient cultures were interested in the night sky
• Calenders
• Prediction of seasons
• Navigation
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Timeline
Astronomy timeline
• ∼ 3000 B.C. Stonehenge
• 2136 B.C. First record of solar eclipse by Chinese astronomers
• 613 B.C. First record of Halley’s comet by Zuo Zhuan (China)
• ∼ 270 B.C. Aristarchus proposes Earth goes around Sun (not a popular
idea at the time)
• ∼ 240 B.C. Eratosthenes estimates Earth’s circumference
• ∼ 130 B.C. Hipparchus develops first accurate star map (one of the first
to use R.A. and Dec)
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Geocentric model
The Geocentric Model
• Greek philosopher Aristotle (384-322 B.C.)
• Uniform circular motion
• Earth at center of Universe
Retrograde Motion
• General motion of planets eastward
• Short periods of westward motion
of planets
• Then continuation eastward
How did the early Greek philosophers
make retrograde motion consistent with
uniform circular motion?
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Ptolemy
Ptolemy’s Geocentric Model
• Planet moves around a small circle called
an epicycle
• Center of epicycle moves along a larger circle called a deferent
• Center of deferent is at center of Earth
(sort of)
Ptolemy’s Geocentric Model
• Ptolemy invented the device called the eccentric
• The eccentric is the center of the deferent
• Sometimes the eccentric was slightly off center from the center of the Earth
Ptolemy’s Geocentric Model
• Uniform circular motion could not account for speed of the planets thus
Ptolemy used a device called the equant
• The equant was placed the same distance from the eccentric as the Earth,
but on the opposite side
• From the viewpoint of the equant the center of the epicycle appears to
move with uniform angular motion
Problems with Ptolemaic model
• Inconsistent use a various devices (eccentric, deferent, equant)
• Centers of the epicycles for Mercury and Venus are fixed on a line joining
the Sun and Earth
• Theory did not fit data as accuracy of observations increased
• Assumption of uniform circular motion not based on observations but on
philosophy
Predictive value of Ptolemaic model
Prediction of phases of Venus
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No full phase !
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Copernicus
Nicolaus Copernicus
• 16th century Polish cleric
• Dissatisfied with inconsistencies of Ptolemaic model
• Displeased at use of equant
• Reexamined heliocentric model
Simple Copernican Model
• Earth and planets orbit Sun
• Sun and stars stationary
• Only the Moon orbits Earth
Simple Copernican Model
Copernican model explains:
• Retrograde motion
• Varying brightness of planets
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Copernican model
• However, Copernicus kept the idea of uniform circular motion and epicycles
• Copernicus’s astronomical tables were no better than ones calculated using
Ptolemy’s methods
Copernican model
Why the Copernican model was not initially accepted
• No observable stellar parallax (at the time)
• Predictive accuracy was not better than using Ptolemy’s methods
• Earth does not feel as if it is moving
Copernican model
Prediction of phases of Venus
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Galileo
Galileo Galilei
• Italian mathematician and philosopher
(1564-1642)
• Willing to test ideas
• Built a telescope in 1609 and aimed it at
the sky
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Galileo’s observations
• Observed that the Moon has mountains,
valleys, and craters
Moon watercolor from Sidereus Nuncius
manuscript(1610)
Galileo’s observations
• Moons of Jupiter
Galileo’s observations
• Phases of Venus consistent with Copernican view
Istoria dimostrazioni intorno alle macchie solari (1613)
Galileo’s observations
• Sunspots
• Inferred that the Sun was rotating
Istoria dimostrazioni intorno alle macchie solari (1613)
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