The Astronomical Renaissance

by Tim Harding BSc BA

(An edited version of this essay was published in The Skeptic magazine, March 2015, Vol 35 No 1, under the title ‘Rebirth of the Universe’.  The essay is based on a talk presented to the Mordi Skeptics, Tuesday 11 November 2014).

This article follows on from my previous one on ancient astronomy and astrology (‘An Eye to the Sky’, The Skeptic, Vol. 33, No. 4, December 2013).  That story began about 4000 years ago in Babylon, then moved to the first scientific revolution in ancient Greece, ending with Ptolemy’s complicated geocentric (Earth-centred) model of the cosmos in the 2nd century CE.

We now make a great leap forward to the second scientific revolution beginning with the publication of Nicolaus Copernicus’ heliocentric (Sun-centred) model of the cosmos in 1543 CE.  Why the huge gap?  Because astonishingly, nothing much happened in cosmology for about 1400 years between Ptolemy and Copernicus.  (The reasons for this are complex and best left to a possible future article).

After the Fall of Constantinople in 1453 CE, there was a rediscovery of ancient Greek texts written by philosophers and scientists such as Plato, Aristotle, Aristarchus, Archimedes and Ptolemy.  That is why this subsequent period is described as an astronomical renaissance (alongside the cultural renaissance), from the French word meaning ‘re-birth’.  The midwives of this re-birth were the development of scientific methods and the invention of printing, which would improve access to learning, allowing a faster propagation of new ideas.

Copernicus’s astronomical observations were complemented and improved in accuracy by those of Tycho Brahe.  His heliocentric model was later adopted by Galileo Galilei and then refined by Johannes Kepler.

Nicholas Copernicus (1473- 1573 CE)

Copernicus was born 1473 in the Polish city of Torun.  His father was copper merchant – the name ‘Copernicus’ is thought to be derived from this occupation.  He studied mathematics, philosophy and astronomy at the University of Krakow; then medicine at Padua in Italy.  He was also a lawyer, physician, classics scholar, translator and economist.

Nicolaus Copernicus

As well as being a polymath, Copernicus was also a polyglot, which gave him access to the ancient Greek texts.  From these writings he would most likely have known that Aristarchus of Samos had some 1800 years earlier proposed a heliocentric model of the cosmos in the third century BCE.  Aristarchus had also calculated the diameters of the Sun and Moon, as well as their distances from the Earth in Earth radii.  This regression from the correct heliocentric to the incorrect geocentric model presents a serious challenge to our notions of inevitable human progress.

In around 1510, Copernicus moved to one of the defensive towers of the cathedral town of Frombork on the Baltic Sea coast, where he did most of his astronomical observations and writing.  His attempts at retrofitting cosmological theory to seemingly endless observational anomalies eventually became just too complex.  Simplification became a major motivation for Copernicus to construct his revolutionary heliocentric model.  His colleague Andreas Osiander, a Lutheran theologian, wrote an anonymous preface to Copernicus’ published major work De Revolutionibus.  This preface stated that Copernicus’ system was merely mathematics intended to aid computation and not an attempt to declare literal truth.  Both Copernicus and Osiander probably feared the reaction not only of other astronomers but also the Roman Church – a fear that was later justified by the trial of Galileo, of which more will be said later.  The delay in publication until the eve of Copernicus’ death is thought to be due to these fears.

Thirty years earlier in about 1514, Copernicus had written the Commentariolus – an unpublished outline of his later De Revolutionibus. In this outline, he proposed seven axioms, all of which are true:

1. Heavenly bodies do not all move around same centre.
2. The Earth is not centre of the cosmos – only the Moon’s orbit.
3. The Sun is the centre of the planetary system.
4. The Stars are much further away than the Sun.
5. The apparent daily revolution of the stars and planets is due to the Earth’s rotation on its own axis.
6. The apparent annual motion of the Sun is due to the revolution of Earth around the Sun.
7. The apparent retrograde motion of the planets has same cause.

Copernicus’ heliocentric model of the cosmos

However, Copernicus clung to the erroneous theological belief that all the orbits of celestial bodies must be perfect circles.  This forced Copernicus to retain Ptolemy’s complex system of planetary epicycles, thus leading him astray.  At first, Copernicus initially proposed that only 34 epicycles were needed in his model, but he was later forced to modify the model by increasing this number to 48 – eight more cycles than the 40 in Ptolemy’s model.  These anomalies led Kepler to subsequently propose elliptical rather than circular planetary orbits, as will be discussed later.

Copernicus also modified his model to account for the apparent absence of stellar parallax during the Earth’s orbit around the Sun.  He did this by postulating that the distance of the fixed stars was so immense compared to the diameter of the earth’s orbit that stellar parallax was unnoticeable by the accuracy of astronomical observations at that time.  This modification subsequently turned out to be correct in reality, but at the time it was an ad hoc modification made for the purpose of correcting an imagined observational anomaly.

Tycho Brahe (1546 – 1601 CE)

Tycho Brahe was a colourful character, born 1546 into an aristocratic family in Scania which was then part of Denmark but is now in Sweden. He studied law and astronomy at University of Copenhagen.  He is notorious for losing part of his nose in sword fight, so he had to wear a brass prosthetic nose.  Another piece of irrelevant trivia is that Tycho had a pet Elk that once drank too much beer at one of his friends’ dinner parties.  Sadly, the inebriated Elk fell down some stairs and died.

Tycho’s observatory on the island of Hven in Sweden

In 1597, Tycho fell out with Danish King Christian IV and became court astronomer to Holy Roman Emperor Rudolph II in Prague, who funded the building of a state-of-the-art new observatory for Tycho.  Johannes Kepler was employed as Tycho’s assistant, who later used Tycho’s more accurate observational data for his own astronomical calculations.  These more precise measurements clearly showed that the stars lacked parallax, thus confirming that either the Earth was stationary or the stars were a vast distance from the Earth.

Tycho proposed a hybrid ‘geo-heliocentric’ system in which the Sun and Moon orbited the Earth, while the other planets orbited the Sun (known as the Tychonic system). This system provided a safe position for astronomers who were dissatisfied with the Ptolemaic model but were reluctant to endorse the Copernican model.  The Tychonic system became more popular after 1615 when Rome decided officially that the heliocentric model was contrary to both philosophy and scripture, and could be discussed only as a computational convenience that had no connection to the truth.

Galileo Galilei (1564- 1642 CE)

Galileo Galilei was a physicist, mathematician, engineer, astronomer, and philosopher who arguably contributed more than anybody to both the second scientific revolution and the astronomical renaissance.  He was born 1564 in Pisa, Italy, and educated in the Camaldolese Monastery at Vallombrosa, 35 km southeast of Florence.  He enrolled at the University of Pisa for a medical degree, but switched to mathematics and natural philosophy.

Galileo Galilei

Galileo’s contributions to observational astronomy include the telescopic confirmation of the phases of Venus, the discovery of the four largest satellites of Jupiter (named the Galilean moons in his honour), the roughness of the Moon’s surface, and the observation and analysis of sunspots. He also made contributions to physics, including the science of dynamics, leading to Newton’s laws of motion later on.

He championed Copernicus’ heliocentrism when it was still controversial – most astronomers at this time subscribed to either geocentrism or the Tychonic system.  They doubted heliocentrism due to the absence of an observed stellar parallax, without appreciating the enormous distances involved.

When confronted with this absence of stellar parallax, Galileo attempted an ad hoc modification to the Copernican model by incorrectly claiming that the tides are caused by the earth’s rotation combined with its orbit around the Sun.  This is despite the ancient Greek philosopher Seleucus some 1600 years earlier having correctly theorized that tides were caused by the gravitational effect of the Moon’s orbit around the Earth.  .

Galileo showing his telescope to the Doge of Venice

After the Roman Inquisition of 1615, works advocating the Copernican system were placed on the index of banned books and Galileo was forbidden from advocating heliocentrism.  This resulted in heated correspondence between Galileo and the Vatican.  Unfortunately, Galileo’s aggressive manner alienated not only the Pope but also the Jesuits, both of whom had tolerated him up until this point. He was tried by the Holy Office and found ‘vehemently suspect of heresy’, then forced to recant, and spent the rest of his life under house arrest.

At least Galileo did not suffer the cruel fate of the philosopher and cosmologist Giordano Bruno, who in 1600 was burned at the stake for heresy in Rome’s Campo de’ Fiori (a market square where there is now a statue of him).  Bruno had gone even further than the Copernican model, correctly proposing that the stars were just distant suns surrounded by their own exoplanets.  He suggested the possibility that these planets could even foster life of their own (a philosophical position known as cosmic pluralism).  Bruno also believed that the Universe is in fact infinite, thus having no celestial body at its ‘center’.

Johannes Kepler (1571- 1630 CE)

Johannes Kepler was a German mathematician, astronomer and astrologer (before these areas of study separated). A key figure in the second scientific revolution, he is best known for his three laws of planetary motion, which endure today. These laws also provided one of the foundations for Isaac Newton’s theory of universal gravitation.

Kepler was born in 1571 in Stuttgart area of Germany. At age six, he observed the Great Comet of 1577, and at age nine, the lunar eclipse of 1580.  These events inspired him to study philosophy, theology mathematics and astronomy at the University of Tübingen.  Here he learned both the Ptolemaic system and the Copernican system of planetary motion.  He later observed a bright supernova (exploding star) of 1604, which became known as Kepler’s Supernova.

After graduation, Kepler became a mathematics teacher at a seminary school in Graz, Austria. Later he became an assistant to astronomer Tycho Brahe, and eventually the imperial mathematician to Emperor Rudolf II and his two successors Matthias and Ferdinand II. He was also a mathematics teacher in Linz, Austria, and an adviser to General Wallenstein. Additionally, he did fundamental work in the field of optics, invented an improved version of the refracting telescope (the Keplerian Telescope).

Frontispiece to the Rudolphine Tables consisting of a star catalogue and planetary tables published by Johannes Kepler in 1627, using observational data collected by Tycho Brahe

Kepler then set about calculating the entire orbit of Mars, using the geometrical rate law and assuming an egg-shaped ovoid orbit. After many failed attempts, in early 1605 he at last hit upon the idea of an ellipse, which he had previously assumed to be too simple a solution for earlier astronomers to have overlooked. Finding that an elliptical orbit fitted the Mars data, he immediately concluded that all planets move in ellipses, with the sun at one focus — which became Kepler’s first law of planetary motion.

He then formulated two more laws of planetary motion.  These are firstly, that a line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time; and secondly, that the square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit.

Because of his religious beliefs, Kepler became convinced that God had created the universe according to perfectly harmonious geometrical shapes and patterns. He began by exploring regular polygons and regular solids, including the figures that would come to be known as Kepler’s solids.  Unfortunately, Kepler wasted a lot of his time fruitlessly searching for this underlying ‘harmony of the spheres’, drawing all sorts of weird and wonderful diagrams.  He even (unsuccessfully) tried to relate these geometric shapes to musical harmonies.

Kepler’s solids

Concluding remarks

The great physicist Isaac Newton was later able to build upon the pioneering work of Galileo and Kepler, leading him to make his famous quotation ‘If I have seen further it is only by standing on the shoulders of giants’.

In contrast, it is perplexing to observe two great human failures. Firstly, how science was repeatedly led astray by fruitless searches for perfection in ‘God’s design of the cosmos’.  Secondly, that astronomical knowledge not only progressed very little during the 1400 years between Ptolemy and Copernicus, but in some areas it actually regressed.  The ancient Greeks had not only proposed a heliocentric model of the cosmos, but they had also calculated the diameters of the Sun and Moon, as well as their distances from the Earth.  They also knew that the tides were caused the gravitational effect of the Moon’s orbit around the Earth.  This valuable knowledge was either forgotten or rejected until the astronomical renaissance some 1800 years later. So much for the notion of inevitable human progress.

References:

Koestler, A (1959) The Sleepwalkers. London: Hutchinson.

Kuhn, T.S. (1962) The Structure of Scientific Revolutions 3rd ed. Chicago: University of Chicago Press.

Toulmin, S. and Goodfield, J. (1961) The Fabric of the Heavens.  London: Hutchinson.

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