NICOLAUS COPERNICUS (1473-1543)

1543 – Poland

‘The Sun is at the centre of the solar system, fixed and immobile, and planets orbit around it in perfect circles in the following order: Mercury, Venus, Earth with its moon, Mars, Jupiter and Saturn’

diagram of the heliocentric Copernican system

The heliocentric Copernican system

The Copernican system defied the dogma that the Earth stood still at the centre of the universe – a concept that dated back to ARISTOTLE, which had been given observational legitimacy by PTOLEMY and authority by Christendom – and set forth a new theory of a Sun centered universe. Why would God create a hugely complicated system of equants, epicycles and eccentrics, as Ptolemy had used to explain planetary motion around the Earth, when it would be much more simple and graceful to have them all revolving around the Sun?

“Eight hundred years before Copernicus, a model of the solar system was advanced with the Earth as a planet orbiting the Sun along with other planets”
A few centuries later this idea fell into disfavour with the early Christian Church, which placed mankind at the centre of the universe in a geo-centric model. The alternative teaching would be deemed heresy punishable by death and it would not be until the seventeenth century that the work of GALILEO, KEPLER and NEWTON gave credence to the ideas revitalized by Copernicus in 1543.

Not only did Copernicus place the Sun at the centre of the solar system, but he also gave detailed accounts of the motions of Earth, the Moon and those planets that were known at that time. Between 1510 and 1514 he drafted Commentariolus, his initial exposition of the theory. In order to have credence, the idea required that the Earth itself be not fixed in position. He said that the Earth revolves on its own axis once every twenty-four hours, which accounts for day and night and explains the apparent movement of the stars and Sun across the sky. Copernicus suggested in Commentariolus that the time taken for each planet to complete its cycle through the night sky might increase the further it is from the Sun.

Mercury’s cycle takes 88 days, which makes it the nearest planet to the Sun. Venus takes 225 days, Earth 1 year, Mars 1.9 years, Jupiter 12 years and Saturn 30 years. Thus Copernicus was able to work out the truth and attempted to establish the order of the planets.

He did not publish his findings because they were thought to contravene the teachings of the Catholic Church. Religious leaders of his time were against him. Martin Luther (founder of the Lutheran Church in Germany) denounced him as ‘a new astrologer…. the fool’ who wanted ‘to overturn the entire science of astronomy’. His book De Revolutionibus Orbium Coelestium (On the revolution of the celestial spheres) was published at the very end of his life, and a copy placed on his deathbed. Thus the greatest astronomer of his time died without seeing his book in print – the book as influential as Newton’s Principia and Darwin’s ‘On The Origin of Species’.

Portrait of Copernicus

The text was rejected by many academics; partially because the author had undermined the simplicity of his initial ideas by clinging to the Aristotelian belief that planetary motion took place in perfect circles. This meant Copernicus had been forced to introduce his own system of epicycles and other complex motions to fit in with observational evidence, thereby producing as equally complicated an explanation as the geocentric one he had initially rejected for its lack of simplicity.

It was not until Johannes Kepler offered the solution that the planets move in an elliptical, not circular, motion in 1609 that the simplicity that Copernicus had been seeking was offered and the rest of the model could be vindicated.

In fact, it was not until 1616 that the Church banned the text Copernicus eventually published for its ‘blasphemous’ content, although that sanction remained in place until 1835.

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JOHANNES KEPLER (1571-1630)

1609-19 – Germany

‘1600 – Kepler works in Prague with TYCHO BRAHE the imperial mathematician, under the patronage of Rudolph II
1601 – On Brahe’s death, Kepler inherits his position (and crucially, his astronomical notes)’

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KEPLER

  • First Law: The planets move in elliptical orbits with the Sun at one focus

  • Second Law: The straight line joining the Sun and any planet sweeps out equal areas in equal periods of time

  • Third Law: The squares of orbital periods of the planets are proportional to the cube of their mean distances from the Sun

Modern measurements of the planets show that they do not precisely follow these laws; however, their development is considered a major landmark in science.

Kepler’s ardent faith in the Copernican system – ‘The Sun not only stands at the centre of the universe, but is its moving spirit’, he asserted – brought him the disfavour of religious leaders. With his realisation that the planets do not rotate in perfect circles but in fact orbit in an ellipse, he provided the mathematical explanation for planetary motion, which had eluded Copernicus and Ptolemy.

The first two laws were published in 1609 ( Astronomia Nova – New Astronomy ) and the third in 1619 ( Harmonicses Mundi – Harmonics of the World ). Their publication put an end to PTOLEMY’s cycles & epicycles. His work provided the observational and arithmetical proof to support COPERNICUS‘ theories.

His second law states that an imaginary line between the Sun and the planets sweeps out an equal area in equal periods of time.

Stating that the planets ‘sweep’ or cover equal areas in equal amounts of time regardless of which location of their orbit they are in means that, as the Sun is only one of two centres of rotation in a planet’s orbit, a planet is nearer to the Sun at some times than at others. Thus the planet must speed up when it is nearer the Sun and slow down when it is further away.

His third law finds that the period (the time for one complete orbit – a year for the Earth, for instance) of a planet squared is the same as the distance from the planet to the Sun cubed (in astronomical units). This allows distances of planets to be worked out from observing their cycles alone.

Kepler was a versatile genius who, besides discovering these three laws, compiled tables of star positions ( Tabulae Rudolphinae – 1627 ) and developed the astronomical telescope.

Kepler also studied the anatomy of the human eye and founded the science of geometrical optics ( ‘Dioptrics’ – 1611 ), proposing the ray theory of light after ALHAZEN’s discussion in Opticae Thesaurus ; he described the eye in the same terms – as a pinhole camera, with light entering through the pupil and forming an image of the outside world on the retina at the back of the eye.

His credible solution to predicting planetary motion would act as the stimulus for questions that would lead to ISAAC NEWTON‘s theory of gravity.

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