The Renaissance era (c.1400AD-c.1700AD) of European civilisation was characterised by a renewed interest in classical scholarship and the reinterpretation of the wisdom, ideals and philosophy of ancient Greek scholars such as Plato, Aristotle and Ptolemy. One manifestation of this renewed interest was the decision by Pope Julius II….
…to adorn a wall of the newly redecorated Apostolic Palace in the Vatican City with a fresco depicting the famous philosophers of Ancient Greece. ‘The School of Athens’ (fresco on the right) was painted by Raffael Sanzio da Urbino between 1509 and 1511.
Two of the greatest ancient Greek philosophers, Plato and Aristotle, can be seen in animated conversation in the center of the fresco about to descend some steps.
Plato is depicted as old, wise-looking with bare feet. By contrast Aristotle is younger, wears sandles and is dressed in gold.
Another of the ancient philosophers, Ptolemy, stands in the bottom right hand corner; with his back to the audience he is dressed in green and yellow and holds a globe in his left hand.
Claudius Ptolemy (c. AD 100 – c. AD 170) was a Greek speaking Roman citizen who lived in the city of Alexandria in Egypt.
We know little of his life but he is best remembered for promoting a geocentric (Earth centered) view of the Universe in which the Sun and planets revolve round the Earth. Ptolemy drew on the ideas of his predecessors and almost certainly carried out his own astronomical observations to test his ideas.
He summarized his ideas about the geocentric motion of the Sun and planets in his mathematical and astronomical treatise known by its Arabic title ‘Almagest’ meaning ‘The Greatest’. This image shows a page of his treatise translated from Arabic into Latin.
Building on the work of earlier philosophers Ptolemy’s great idea was that the Earth lay at the center of the universe; all other celestial bodies revolved round the earth in perfect circles called ‘deferents’.
The stars, he assumed, revolved round the Earth in a fixed orbit beyond Saturn, then the last known planet.
Ptolemy had to make certain adjustments to explain the observed motion of the planets. In order to explain the apparent retrograde motion….
…of the planets….
…Ptolemy introduced the idea of the ‘epicycle’. Not only did the planets revolve round the Earth in perfect big circles called ‘deferents’, but the planets also revolved in perfect little circles called ‘epicycles’.
The Ptolemaic, Earth centered model of the Universe prevailed for around 1,300 years until Copernicus ( 1473AD -1543AD) came onto the scene.
The fact that the earth centered (‘geocentric’) view of the universe prevailed for so long bears testament to false conceptions about the nature of motion in the pre Newtonian world.
These are just some of the arguments provided over the course of 1,300 years ‘proving’ the geocentric nature of the Universe:
The Earth just had to be stationary and at the center of a geocentric Universe!
Even as late as the 19th century there was serious misconceptions about the nature of motion. One Dr Dionysus Lardner (1793-1859), an early Victorian academic, is reputed to have said, “rail travel at high speed is not possible because passengers, unable to breathe, would die of asphyxia.”
Copernicus was an intermediate figure in the scientific revolution. In one important respect he more closely resembled the ancient Greek philosophers rather than modern scientists.
Copernicus did not conduct scientific experiments or make meaningful observations of the heavens. Neither did he expect anyone else to test his ideas. The notion that scientific evidence should be observed and tested was unheard of in the 15th century.
His great contribution to science was what we might call today ‘a thought experiment’. Copernicus presented a simpler, more elegant way to explain the movement of the planets across the night sky.
The idea of the Earth revolving round the Sun was not new, having been proposed by Aristtarchus way back in the third century BC. Until the arrival of Copernicus such an idea had never found favor.
His father, a wealthy merchant, died in c.1483. The young Nicholas was raised in the household of his mother’s brother, his uncle Lucas Waczenrode, later to become the Bishop of Ermeland.
In 1491 Nicolaus studied at the University of Krakow where he developed a lifelong interest in astronomy. In 1496 he moved to Italy to study the classics, mathematics, law and medicine. He was awarded a doctorate in canon law from the University of Ferrara in 1503.
It was whilst studying in Italy that Copernicus was appointed by his Uncle Lucas as Canon of Frombork Cathedral in Poland. This position amounted to a sinecure which Copernicus would keep for the rest of a life.
Copernicus returned to Poland in 1506 where he worked as secretary and physician to Uncle Lucas until 1512, the year that Uncle Lucas died.
Developing a keen interest in astronomy, Copernicus was heavily influenced by the work of German Johannes Mueller, also known as ‘Regiomontanus’.
In 1496 Regiomontanus published a book called the Epitome of the Almagest. The Epitome not only summarized the contents of Ptolemy’s Almagest but also provided a critical commentary on the Ptolemaic view of the motion of the heavenly bodies through space.
The frontispiece of the Epitome depicts Ptolemy and Regiomontanus sitting beneath an armillary sphere. Ptolemy (on the left) reads from the Almagest while Regiomontanus (on the right) listens attentively as he points to the ordered pattern of the heavens that Ptolemy’s great work, the Almagest, describes.
This frontispiece emphasizes the continuity of knowledge and understanding from the second century AD to the beginning of the sixteenth century AD – a continuity of ‘knowledge’ and ‘understanding’ spanning 1,500 years!
In his Eptiome Regiomontanus drew attention to a significant flaw in Ptolemy’s model of the universe- a flaw that did not go unnoticed by Copernicus. According to Ptolemy’s model the Moon should be significantly closer to the Earth on some days of the month than on others. However Copernicus noticed that the Moon does not change size in any significant way.
The Epitome planted doubts in Copernicus’s mind about the veracity of the geocentric Universe.
Copernicus is known to have completed his model of the sun centered Universe by 1510, but the book containing his radical new theory was not printed until 1543.
Copernicus was very busy; as a medical doctor he tended to the community living around Frombork Cathedral; as a mathematician he worked on a plan to reform the local currency; as a lawyer his legal acumen was put to good use to help the cathedral authorities;..
…as commander of a castle at Allerstein he had to withstand a seige to prevent the capture of the town by warring teutonic knights in the Polish-Teutonic war of 1519-1521.
Not only did Copernicus harbor doubts about the geocentric model of the Universe, but he also had serious doubts about the plausibility of his heliocentric Universe.
As the Earth orbited the Sun, Copernicus expected to see the apparent motion of the ‘fixed’ stars through the phenomenon known as the ‘parallax effect’.
But Copernicus found no evidence of any ‘parallax effect’.
The coachman sees the tree to the righthand side of the distant mountain before journeying a little further and viewing the same tree on the lefthand side of the distant mountain.
The tree and mountain have not moved yet the tree, as a result of being closer to the coachman than the distant mountain, appears to have shifted from one side of the mountain to the other.
This is an example of the parallax effect where, as a result of motion, nearby objects appear to move whereas distant objects appear stationary.
Copernicus searched for evidence of the apparent motion of stars through the ‘parallax effect’ but found none. This diagram gives an indication of what Copernicus thought a stellar ‘parallax’ might look like.
The only explanation Copernicus could think of for not finding any stellar parallax was that the stars were very much further away than he previously thought. In the age before telescopes any parallax effect would be far too small to see with the naked eye.
It was not until 1608 that the first telescopes were used to examine the night sky- a full 65 years after the publication of the Copernican theory of the Sun centered Universe. However these early telescopes were too rudimentary to detect any parallax effect.
It was in 1838 that the first parallax motion was detected by F. W Bessel (1784 – 1846), a German astronomer, who measured the parallax angle of a binary star 61 Cygni…
…using a ‘Fraunhofer heliometer’.
Bessel measured the parallax angle of 61 Cygni at 0.314 arc seconds. Bessel was now able to use his measurement of the parallax angle to calculate the distance of the binary star from Earth. Bessel’s calculations indicated that 61 Cygni was 10.3 light years away from Earth.
We now know that 61 Cygni is in fact 0.292 arc seconds, or 11.4 light years distant, showing that Bessel’s original calculation was out by 9.6%.
0.292 arc seconds is a tiny,tiny angle when you consider that there are 1,296,000 arc seconds in one 360° circle. A single degree is divided into 60 arc minutes and one arc minute is divided into 60 arc seconds.
By way of comparison a human with 20/20 vision has the spatial awareness allowing him/her to distinguish a mere one minute of arc division
Use the geometry of the Earth’s orbit round the Sun, it is possible to use the backdrop of distant ‘fixed’ stars to calculate distances to stars closer than 100 light-years away.
As the distance to a star increases its parallax angle decreases. This phenomenon is summarized below:
The largest parallax angle observed for any star is that of our nearest star neighbor, Proxima Centauri. At a distance of 4 light years from Earth Proxima Centauri has a parallax of 0.772 arc seconds
Today parallax are best measured from space. Through its Gaia spacecraft the European Space Agency is currently measuring the parallax and distances, relative to the Earth, of millions of stars.
It was Georg Joachim von Lauchen, (also known as Rheticus) Professor of Mathematics from the University of Wittenberg, who was instrumental in persuading Copernicus to publish his radical new idea.
It was agreed that the book would be printed in Nurenberg under the supervision of Rheticus. However a move to Leipzig meant that Rheticus did not stay in Nurenberg long enough to oversee the publication of the book.
He deputised this task to Andreas Osiander, a Lutherian minister. Osiander added an unsigned preface explaining how the heliocentric model described in the book was not intended as a description of how the universe really is, but was merely a mathematical device to simplify calculations involving the motion of the planets.
Osiander had every reason to fear that the book would not be well received by others in the religious establishment.
Copernicus died in 1543 the year his great work, ‘De Revolutionibus Orbium Coelestium’, (On the Revolutions of the Heavenly Spheres) was published.
The heliocentric model of the universe had arrived at last!
Some 30 years after the publication of De Revolutionibus the English astronomer Thomas Digges (1546-1595) made additions to the Copernican model. Digges was the first scientist to contend that the Universe was infinite; he produced a diagram showing a multitude of stars extending to infinity in all directions.
Since ancient times it had been an enigma how Mercury and Venus could only be seen around dawn and dusk, while the other three known planets could be seen any time of the night. The Copernican model provided an explanation why this should occur.
By making allowances for Earth’s motion, Copernicus could calcuate the length of time it took each planet to orbit once round the Sun. These orbital periods formed a neat sequence, with Mercury having the shortest, and Saturn the longest year.
The Copernican model was simpler and more elegant than the convoluted, stodgy Ptolemaic model. And it was all true!!