Date: March 24, 2009

Podcaster: Nik Whitehead

Organization: None

Description: Nowadays we are all used to the idea that the Earth is nothing particularly special, just another planet orbiting a fairly average star in a pretty normal galaxy. This worldview is thanks to the work of Nicholas Copernicus, who published his heliocentric model of the universe in 1514. Prior to this all educated men and women believed that the universe was structured after the model of Ptolemy with the Earth at the centre of the universe, orbited by the sun, the planets and the stars. This model may seem strange to us today but for two thousand years it was the accepted view of the cosmos.

Bio: Nik is a lecturer in computer science at the University of Akureyri in northern Iceland… but computer science is not her passion. She has a Bachelors degree in astronomy and astrophysics then took her Masters and Doctoral degrees in computer science when she realised that there are not enough jobs in astronomy to go around. What she’d really like to be when she grows up is either the navigator of the starship Enterprise or maybe a space traffic controller. She is also an active member of the Society for Creative Anachronism, which gives her an excellent excuse to study the history of science.

Today’s Sponsor: This episode of “365 Days of Astronomy” is sponsored by Kevin Marvel.

Transcript: 

Hello! Welcome to the 365 Days of Astronomy podcast for March 24th. I’m Nik Whitehead, talking to you from Akureyri in Iceland, where I teach computer science at the university. Although computing is my day job, astronomy is my first love, and I’m delighted to be able to contribute to the International Year of Astronomy in some small manner.

You might be wondering what that music has to do with astronomy, a subject that’s often concerned with the latest high-tech equipment, or observations from fantastic machines in orbit around our planet or even on the surfaces of other worlds. But our knowledge of the universe is built on earlier theories, which were gradually rejected as scientists developed new models based upon the new observations of their day. Although these models are no longer used they make up the history of astronomy, which is a fascinating subject in its own right.

In 1514 Nicholas Copernicus put forward a radical new model of the universe, with the sun at the centre of the solar system orbited by the known planets. The model that this replaced was that of Claudius Ptolemaeus, a Greek astronomer also known later as Ptolemy. He described this model in his work The Amalgest in around 150 AD. This placed a spherical Earth at the centre of the universe, orbited by the sun, the planets and the stars. It was, in turn, based upon the cosmology of Aristotle. Contrary to the modern popular belief that everyone before Copernicus thought that the world was flat, this model was taught in universities and was accepted as true by all educated men and women

Within the model the Earth was spherical – partly because the sphere was the most perfect of all forms, and partly through logical reasoning. When you see a ship coming into the shore, you first see the mast, then the sails, and finally the hull. Aristotle reasoned that this could only be explained logically if the surface of the sea was curved. He backed this up with observations of the moon and stars showing, for instance, that the pole star is higher in the sky the further north you travel, an observation that also supported the idea that people were traveling on the surface of a sphere.

Beyond the Earth was the moon, and the orbit of the moon divided the universe into the sub-lunar and trans-lunar spheres. The sub-lunar sphere, below the orbit of the moon, was the area in which man and all other creatures could live and breathe. It was also the realm governed by the laws of physics. These laws contained two main features: all motion occurred in straight lines, and it was due to the influence of the four elements. These elements each ruled a concentric region of the sub-lunar sphere with Water at the centre. Above the Water sat the Earth and above it the Air. Finally, above all of these was the realm of Fire, reaching up to the moon.

All things, man included, were thought to be composed of these four elements in different proportions, and every object had a natural affinity with the region governed by the element which had the most influence on the object. Motion occurred because of the natural desire for each element to return to its rightful place. Things fell to the ground not because of gravity, bur rather because the Earth within a solid object was attracted to the Earth of the ground below it. Rain fell from the skies because the water in the atmosphere was attracted to the Waters beneath the Earth. The Aristotelian model provided answers to many common questions such as “Why does hot air rise?” The answer was that the heat in the air was a form of fire and so wanted to rise to join up with the fire above the air.

In this model all common objects traveling though the sub-lunar sphere moved in straight lines. No-one had yet described motion as having two components, horizontal and vertical, and as a result, the paths of moving objects could not be easily calculated. During the early Renaissance the group of people who knew most about the motion of objects were the gunners who worked with the artillery of the period. They used ‘gunnery tables’ to calculate the necessary angle of fire for a given weight of projectile and target distance, and these angles all had to be determined experimentally rather than by calculation. It was only at the end of the sixteenth century when Galileo published his works on motion and on mechanics that people could begin to calculate projectile paths. Even then it was not until Isaac Newton published his laws of motion in 1687 that these paths could be calculated accurately.

The trans-lunar sphere, beyond the orbit of the moon was the abode of the planets. Here the sun and the five other ‘wanderers’ – Mercury, Venus, Mars, Jupiter and Saturn in that order – orbited in perfect circular orbits with the sun orbiting between Venus and Mars. The orbits had to be circular, because the circle is the two-dimensional equivalent of the perfect sphere.

This caused a problem, as it was obvious to anyone watching the paths of the planets across the night sky that they speeded up, slowed down and even moved backwards at times. Fortunately there was a good circular work-around for this – Ptolemy’s model had the planets performing small circular orbits around a point in space that itself orbited the Earth in a perfectly circular orbit. By using these epicycles, as they were known, it was possible to use the Ptolemeic model to predict the motions of the planets to a high degree of accuracy.

Beyond the planets were the fixed stars, each of which was attached to the firmament, the perfect sphere that contained the entire universe. This marked the boundary of what Ptolemy called ‘the world’, and on the far side of the firmament was heaven itself.

This worldview had lasted for over a thousand years, but Copernicus’ heliocentric model marked a major turning point in the way that scientists viewed the universe. His model still maintained a number of features of the Ptolemaic model; in particular it still used circular orbits with epicycles in order to calculate the movements of the planets. What it did do, though, was to remove the Earth from the centre of the universe and helped scientists realise that Earth is just a small part of a far larger and more exciting cosmos.

[MUSIC: A Coye Joye by John Dowland played by Paul O’Dette]

End of podcast:

365 Days of Astronomy
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The 365 Days of Astronomy Podcast is produced by the New Media Working Group of the International Year of Astronomy 2009. Audio post-production by Preston Gibson. Bandwidth donated by libsyn.com and wizzard media. Web design by Clockwork Active Media Systems. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at 365DaysOfAstronomy.org or email us at info@365DaysOfAstronomy.org. Until tomorrow…goodbye.