Look up at the stars on a clear night and it isn’t difficult to imagine how our ancestors would have viewed the dome of heaven moving slowly over the earth in a spectacular display of lights. Early farmers began to recognise there was a link between cyclic movements they could see in the sky and different seasons. By watching the progress of the star patterns, or constellations, they noticed the huge dome overhead made one complete turn, corresponding closely to the seasons.
The Universe has always fascinated mankind and innate in man is his curiosity: his desire to look for meanings and explanations. Prior to the work of Galileo, Copernicus and Kepler, three great astrologer/astronomers of the sixteenth century, there was much confusion and disagreement about the actual shape of the earth. Many people thought that the world was flat and set out to prove it.
Claudius Ptolemy (c AD 100-c170) a Greek astronomer and mathematician, developed a theory based on earlier philosophers that the Earth lay at the centre of the universe with the Sun, Moon and known planets of that time revolving around the Earth in different circular orbits.
Because this theory seemed to make sense as it could predict the paths of the known planets in the skies with reasonable accuracy, it was widely accepted until Nicolas Copernicus put forward his Heliocentric Theory.
Nicolas Copernicus, a Polish astronomer, born 1473, deduced that the Sun was at the centre of the Solar System and the Earth and planets revolved around it. This revolutionary theory negated the long-established Ptolemic system and changed the face of astronomy.
The word ‘helios’ means Sun in Greek and heliocentric puts the Sun at the centre. To elucidate on the exact movements of the planets, Copernicus added more spheres along which the planets moved. He observed that all planets apart from the Sun had the same annual movement which explained the yearly movement of the Earth around the Sun. This theory was strengthened by the Sun’s movement providing light and heat to all other planets, thus it seemed to be a logical conclusion that the Sun was at the centre of the planetary system.
As Copernicus placed the Sun as the fixed point at the centre of the Universe, he also proposed that as well as orbiting the Sun annually, the Earth turned slowly on its own axis and that the long-term consequences of this accounted for the precession of the equinoxes. Copernicus called this the ‘third motion of the earth.’
Copernicus’ model of the heavens, called the Heliocentric or Sun-centred system had a significant impact on later great thinkers like Galileo, Kepler and Descartes. For as incredible and as revolutionary as his theory, Copernicus still kept the old ideas of circular motions of the planets and uniform velocities.
Johann Kepler, a German astronomer born in 1571 firmly believed in the Heliocentric system but there was still some confusion about planetary motions. Kepler finally came to the conclusion that the orbits of planets were not circles as Copernicus suggested but were instead the flattened circles called ellipses. This was the final piece of the puzzle and now having verified Copernicus’ Heliocentric Theory, Kepler drew up his three Laws of Planetary Motion:
1. ~ The planets move in elliptical orbits around the Sun. 2. ~ The imaginary line joining the centres of a planet and the Sun describes equal areas of space in equal times so a planet moves fastest when nearest the Sun and slowest when further away. And 3 ~ The square of the time taken to complete an orbit is proportion ot the cube of the planet’s mean distance from the Sun
And so it was not until the later addition of Kepler’s Laws of Planetary Motion that Copernicus’ Heliocentric Model of the Universe fell neatly into place.
Sources
http://csep10.phys.utk.edu/astr161/lect/history/kepler.html
http://www-groups.dcs.st-and.ac.uk/~history/Biographies/Copernicus.html
http://csep10.phys.utk.edu/astr161/lect/retrograde/copernican.html
http://projects.astro.illinois.edu/data/KeplersLaws/
http://2012wiki.com/index.php?title=Precession_of_the_equinoxes
