Astronomy 171
Solar System Astronomy
Prof. Paul Martini

Lecture 13: 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

The Dark Ages in Europe

Most classical knowledge was lost or dimly remembered in Europe after the fall of Rome in c. 400 AD
The rise of Islam c. 750 AD led to survival of classical learning in Arabic translation: Systematic translation into Arabic of classical texts preserved in Syria; Arabs advanced mathematics and astronomy, invented algebra and advancing trigonometry

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; Principal centers were Toledo and Cordoba
Europeans rediscovered their heritage:: Aristotle and Ptolemy rediscovered in the 12th century; Christian scholars (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

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

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
These were not, however, of most immediate concern to Copernicus

Scientific Objections

Rotating and Revolving Earth was an absurdity to strict Aristotelians as it requires very large speeds: Speed of rotation at Columbus: 1280 km/hour; Orbital speed: 107,000 km/hour (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.