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SkipNChurch, March 9, 2015 in The Arena
Hello and welcome to this tutorial about cosmology - the science of understanding the universe.
It's my hope that this opening post and this thread in general will be of help to anyone wishing to understand more about the origin and workings of the cosmos. Bhim, the Roguescholar and myself will be happy to field your questions. If I've been remiss and left anything out I'm sure they'll spot it and fill in the gaps or correct me accordingly.
Today we will begin with the one of the most basic and fundamental rules of cosmology - the Copernican principle.
Before we can properly understand what this principle is and how it's used, we first need to see why the Copernican Revolution occurred. This means going back in European history to a time of intellectual and religious crisis.
During the Renaissance period (14th to 17th centuries) there was a growing crisis in astronomical science.A crisis that needed resolving. A crisis of disagreement between theory and observation. Between what was believed to be true and what actually appeared to be true. Something in the skies was very, very wrong!
Since ancient times it had been accepted that the Earth was fixed, static and immobile, while the Sun, the Moon and the planets all revolved around it. This prevailing geocentric (Earth-centered) cosmology was accepted by the Christian church and had become part of official dogma. Up to this time what was known about the universe could be (more or less) easily reconciled with the Bible and with Christian theology. But increasingly precise measurements of the orbits of the planets by astronomers were casting doubt on the truth of the geocentric system. What was being observed did not agree with the accepted explanation, causing a crisis that needed a resolution.So what were the planets doing that was causing so much trouble?Well, it's not what they were actually doing, it's what they appeared to do that was the problem. Instead of smoothly proceeding in their orbits around the Earth, they seemed to reverse their motion and travel backwards in a loop, before resuming their original direction of travel. The worst offender was the planet Mars. This Wiki page animation helps illustrate the problem.http://en.wikipedia....ican_RevolutionOn the left is the heliocentric (Sun-centered) view of the solar system, with the Earth (blue) and Mars (red) orbiting the Sun (yellow). On the right is the geocentric (Earth-centered) view, with the Sun and Mars orbiting the Earth. In the geocentric system, astronomers on Earth will see Mars appears to make a loop once every orbit. Like this... http://en.wikipedia....trograde_motionNow, please note that Mars doesn't actually travel in vast loops across the sky. It only appears to, when seen from the Earth. This is a vast optical illusion caused of the difference in orbital speeds between our planet and Mars. We occupy a smaller and faster orbit then Mars, taking about 365 days to go round the Sun, as opposed to 686 days for the red planet. Therefore our greater speed allows us to overtake it as we go round. From our p.o.v. Mars appears to slow down in it's orbit, stop, go backwards and then go forwards again. This is nicely illustrated, here...To see for yourself how this works, trace the blue lines from Earth to Mars, going from A thru G.Everything seems ok until C, when Mars seems to stop moving forward. In D it's moving backwards, in E it comes to a stop again and in F, it's going along properly once more, just as if nothing was wrong. The problem of Mars' loopy orbit was solved by moving the Earth from the center of the solar system and putting the Sun there. Suddenly everything fell into place. The orbits of the planets no longer required untidy loops and the behavior of Mars was explained as nothing more than an optical illusion. Please compare the above heliocentric (Sun-centered) diagram in the above, 'Retrograde Motion' diagram to the Wiki animation at top left.
THEY ARE THE SAME.
THIS MEANS THAT THE CORRECT SOLUTION IS ALSO... THE TRUTH.
THIS MEANS THAT THE INCORRECT BELIEF PROMOTED BY THE CHURCH... COULD NOT BE TRUE.
The problem wasn't being caused by Mars or anything else in the sky.The problem was being caused by the unquestioned assumption that everything went around the Earth. By dropping that assumption astronomers could make sense of how the universe worked. The fault didn't lie with their observations and measurements, the fault lay in the prevailing view that the Earth was at the center of all things.This realization now presented the astronomers with a difficult question.Which body of information correctly described how the universe worked? The new Sun-centered system or the old Earth-centered system? Should they continue to put their confidence in the traditional, Church-endorsed view - which failed to explain what they were seeing? Or should they abandon it in favor of the new paradigm - which did explain what they were seeing? Which one of these two options was true? The one guided by faith or the one guided by observation? This difficult question was not easily or quickly answered.But a key technological advance provided further observational evidence that the Sun-centered solar system was the correct (and therefore true) option. Galilieo Galilei's use of the telescope opened up the night skies to a new level of observation and scrutiny. Before then ALL astronomy, was strictly naked eye astronomy. If something couldn't be seen by eye then the human race had absolutely no knowledge of it whatsoever. This explains why, before the invention of the telescope, nobody knew of the moons of Mars, the rings of Saturn or of the planet Neptune. None of these things could be seen without the magnifying power of the telescope.
Until 1610, when Galileo trained his telescope on the planet Venus it had only ever been a bright point of light in the sky. By careful observation Galileo discovered that Venus wasn't just a shining pinpoint of light, similar to a star. He found that as it moved thru the sky it showed phases, just like our Moon. It waxed and waned; from New, thru First Quarter, Full, last Quarter and back to New. He noted that Venus' phases were linked to it's position relative to the Sun. So there could only be only one possible conclusion that made sense. The Sun's light was falling on Venus, just as it fell on our Moon, causing it to wax and wane, just as our Moon does. It meant that Venus orbited the Sun, not the Earth!
The diagram below shows the phases of Venus, as would have been observed by Galilieo.
Further evidence against the Earth-centered view was discovered when Galileo observed the planet Jupiter.
Like Venus, for all of human history, Jupiter was just a star-like object in the night sky. But Galileo found that like Venus and like our Moon Jupiter displayed a visible disk. More significantly, he noticed that it was flanked by four bright objects that changed position relative to it, from night to night. By carefully recording these changes he was able to deduce that Jupiter was being orbited by four moons. This observation flew in the face of the received wisdom of that time. According to the Church-endorsed geocentric view, everything in the sky revolved around the Earth. The Sun, the Moon and the planets - ALL of them orbited us. Nothing else was supposed to orbit anything else but the Earth!
Yet here, with the moons of Jupiter, was clear evidence that this was not so.
The traditional certainties were crumbling in the face of new observations and new evidence - precipitating a crisis over what really constituted the truth. Was the truth of reality (the faith-based dogma of the Christian church) to be passively received and accepted without question? Or was the truth of reality open to question? Could it be discovered by observation, analysis and rational understanding? As more and more astronomers began using better and better telescopes to probe the skies, the weight of evidence shifted more and more away from the former and towards the latter.
This fundamental change in human thought is now known as the Copernican Revolution.
As the pace of scientific investigation quickened, more evidence flooded in to confirm the Copernican view of reality. By the middle of the 20th century telescopic observations and measurements of the skies revealed that our place in the universe is in no way anything special, privileged or out of the ordinary. We now know that stars like our Sun, planetary systems like ours and galaxies like the Milky Way are commonplace throughout the universe. That the Earth is made of the same chemical elements as other planets and stars and these elements are just the same, wherever you look in the universe. This overwhelming body of evidence has helped establish what is known in cosmology as the Copernican Principle.
Hopefully this post will have given you an adequate understanding of the origins of the Copernican principle.
But rather than just proceed on that assumption I'm now going to bring my post to a close and open this thread up to any questions arising from it or relating to it.
I'm pleased to report that I'm working on my next tutorial.
It will build on the first lesson about the Copernican Principle - this time going beyond the solar system, out into the stars and then beyond. It will deal with our place in the Milky Way galaxy, our galaxy's place among the other galaxies and our place within the observable universe. This will set the stage for a future tutorial about the Big Bang and the expansion of the universe - two very much misunderstood and misinterpreted aspects of cosmology.
I'm also pleased to report that Bhim will be helping us understand Einstein's theory of General Relativity.
He and I are working at our own speeds on these tutorials, so I cannot say who will be posting first.
This message is an update about where things stand in this thread.
Bhim and I have been in contact and have agreed on how to proceed. Even though he's a very busy man, he's making time to put together a tutorial on the subject of Einstein's theory of General Relativity. GR is fundamental to astronomy, astrophysics and cosmology. Gaining a better understanding of what it is and how it works will 'open up' the universe to you in new and exciting ways. I'm eagerly looking forward to what Bhim has to say and I hope you are too!
I'll be continuing with my exploration of the role of Copernican Principle in cosmology.
Having steadily widened the scope of my tutorials (from our solar system, out to the intergalactic scale) I'll go on to explain how the CP is applied in the context of an e-x-p-a-n-d-I-n-g universe. This will require a number of steps.
1. A brief overview of how the universe was discovered to be expanding.
2. An explanation of the kind of expansion we observe in the universe today.
3. What was discovered when scientists ran this expansion backwards in time.
4. An explanation of how the universe expanded in it's very first moments. (This is very different to # 2.)
5. Dealing with the inevitable misunderstandings and misconceptions about the Big Bang.
Please note that # 5 is NOT a disparagement or indictment of anyone!
Given the bizarre and counter-intuitive nature of Big Bang cosmology I think it's inevitable that the Peanut Gallery will be FLOODED with questions and requests for further explanation and clarification.
Bhim and I expect this, are reconciled to it and will do our best to help.
It's entirely possible that the forward progress of the tutorials will grind to a halt at this point - as puzzled and confused members get their queries answered at a pace that's good for them. That's not a problem. That's exactly what these tutorials are designed to do. To provide understanding to anyone who wants it.
Please maintain a holding pattern while Bhim and I put together our respective tutorials.
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