Astronomy 161
Introduction to Solar System Astronomy
Prof. Paul Martini

Lecture 12: Copernican Revolution

Key Ideas

Copernicus' Heliocentric System: Earth rotates on its axis once a day; Earth and Planets revolve around the Sun; Retained epicycles, but purged Ptolemy's equant and restored uniform circular motion
Objections: "Impossibility" of a moving Earth; Non-observation of stellar parallaxes

Components of Ptolemy's Model

Eccentric: Moved the Earth off the center of the deferent to account for non-uniform motion (Hipparchus)
Epicycle: With eccentric, produce retrograde motion and explain brightness changes of superior planets (Hipparchus)
Equant: Uniform angular motion (no longer uniform circular motion) and no longer centered on the deferent. Introduced by Ptolemy to account for observed changes in speed

After the Fall

After the fall of Rome in 476 AD, most classical knowledge was lost in Europe
Many of these works were preserved and studied by Islamic scholars starting in the 8th century: Systematic translation into Arabic of classical Greek texts preserved in Syria; Arabs advanced mathematics and astronomy, invented algebra and advanced trigonometry; Our word Algebra comes from the Arabic 'al jabr'; Essentially all manuscripts of the ancient Greeks today are translations of Arabic translations of the original Greek

Rediscovery in Europe

Spain became the center for translation of Arabic texts into Latin in the 11th century: Primarily Jewish scholars working between the Christian and Islamic worlds; Christian students from all over Europe flocked to Spanish universities such as Cordova, Seville, and Granada to learn from Muslim scientists and scholars; Principal centers were Toledo and Cordoba
Europeans rediscovered their heritage:: Aristotle and Ptolemy were rediscovered in the 12th century; Christian scholars (e.g. Aquinas) reconciled these with Christian dogma (13th century)

From Rediscovery to Rebirth

The period betwen the Middle Ages and the Renaissance were times of great social and intellectual change: The rise of the great Universities; Invention and spread of printing; Challenges to the spritual and political authority of the Catholic Church by Protestant reformers; Extended ocean voyages of discovery and trade by Portugal and Spain

Nicolaus Copernicus (1473-1543)

Born in Torun, Poland
Educated at Krakow, Bologna, and Padua in mathematics, medicine, law, astronomy, and philosophy
Canon at Frauenberg Cathedral in Poland
Aristotelian

Copernicus the Revolutionary

Copernicus sought to purge Ptolemy's system of the messy expedient of the Equant: Violated the Aristotelian ideal of uniform circular motion; Felt that a good system must please the mind as well as "preserve appearances"
1514: Circulated a brief "Commentariolus" describing a new system of the heavens.

The Heliocentric System Revived

Revived Aristarchus' Heliocentric system, which he knew only through Archimedes' description in The Sand Reckoner: The Sun, not the Earth, is at the center; The Earth rotates around its axis, producing the daily motions; The Earth revolves around the Sun, producing annual motions
These were radical ideas for their time

De Revolutionibus Orbium Coelstium (1543)

On the Revolution of the Heavenly Orbs: Dedicated to Pope Paul III; Printed with Church approval
It was not a bestseller: Long, difficult Latin treatise; It was not widely circulated
Despite this, his ideas got serious attention, both for and against
Presented as a different calculation tool, rather than a different model of reality

The Reluctant Revolutionary

Copernicus still clung to Aristotelian ideas:: Retained epicycles, centered on the Sun; Required uniform circular motion
This made his system more complex: 48 epicycles, compared to 40 in the Ptolemaic geocentric system; But, he had eliminated the complicated equants
Still only described the motions of the planets without explaining them physically

Simple in Principle

Inferior and Superior Planets: Mercury and Venus orbit closer to the Sun; Mars, Jupiter and Saturn are in larger orbits
Retrograde Motion: Consequence of observing moving planets from a moving Earth; By contrast, Ptolemy required epicycles to get retrograde motion; Copernicus needed them for non-uniform speeds

Geometric Distances to Planets

Heliocentric geometry provies a natural way to measure the distances of the planets from the Sun.
Inferior Planets: Use geometry at maximum elongation
Superior Planets: Measure time from Opposition to Quadrature - more complicated but quite tractable

Opposition to Copernicus

The Copernican Heliocentric System met with almost immediate opposition
Religious Objections:: Luther, Calvin, and Melancthon objected that a moving Earth contradicted scriptures; Catholic Church was initially silent
Scientific Objections:: Rotating and Revolving Earth was an absurdity to strict Aristotelians; Requires very large speeds (speed of rotation at Columbus: 1280 km/hour, orbital speed: 107,000 km/hour or 30 km/sec); No observational evidence of orbital motion (stellar parallaxes were not observed, stars were not brighter at opposition)

The Power of Ideas

In detail, the Copernican System was flawed and unwieldy, and was only an incremental improvement over the Ptolemaic system
But it made up for this by offering greater conceptual simplicity
In a world of increasing change, the idea behind it was to prove powerful, and truly revolutionary

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.