Ancient AstronomyMany ancient cultures were interested in the night sky

• Calenders

• Prediction of seasons

• Navigation

1 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 popularidea at the time)

• ∼ 240 B.C. Eratosthenes estimates Earth’s circumference

• ∼ 130 B.C. Hipparchus develops first accurate star map (one of the firstto use R.A. and Dec)

2 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 east-

ward

• Short periods of westward motionof planets

• Then continuation eastward

How did the early Greek philosophersmake retrograde motion consistent withuniform circular motion?

3 Ptolemy

Ptolemy’s Geocentric Model• Planet moves around a small circle called

an epicycle

• Center of epicycle moves along a larger cir-cle 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 thusPtolemy 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 tomove 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 joiningthe Sun and Earth

• Theory did not fit data as accuracy of observations increased

• Assumption of uniform circular motion not based on observations but onphilosophy

Predictive value of Ptolemaic modelPrediction of phases of Venus

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No full phase !

4 Copernicus

Nicolaus Copernicus• 16th century Polish cleric

• Dissatisfied with inconsistencies of Ptole-maic 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 ModelCopernican model explains:

• Retrograde motion

• Varying brightness of planets

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Copernican model

• However, Copernicus kept the idea of uniform circular motion and epicy-cles

• Copernicus’s astronomical tables were no better than ones calculated usingPtolemy’s methods

Copernican modelWhy 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 modelPrediction of phases of Venus

5 Galileo

Galileo Galilei• Italian mathematician and philosopher

(1564-1642)

• Willing to test ideas

• Built a telescope in 1609 and aimed it atthe 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 Coperni-can view

Istoria dimostrazioni intorno alle macchie so-

lari (1613)

Galileo’s observations

• Sunspots

• Inferred that the Sun was rotating

Istoria dimostrazioni intorno alle macchie so-

lari (1613)

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