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It is an intersting reflection on the general climate throught beforethr 20th century that no one had suggested that the universe was expanding or contracting.It was generally accepted that either the universe had existed forever in an unchanging state, or that it had been created at a finite tike in the past more or less as we observe it ioday .Even those who realised that Newton's theory of gavity showed that the universe could not be staticdidnot by making the graviational force repulsive at very large distances .This did not significiant this didnot significiantly affect their productions of the motions of the planets,but it allowed an infinite distribution of stars to remain in equilibrim -with the attractive forces between near by stars balanced by the repulsive forces from those that were farther away.However we now believe such an the attractive forces so that the stars would continue to fall toward each other.On the other hand,if the stars got a bit farther away from each other the repulsive forces would dominate and drive them farther apart.

The Stars

Newton realized that according to his theory of gravity,the stars should attract each other.so it seemed they could not remain essential ly motionless.would they now all fall together at some point?He argued that this would indeed happen if there were only a finit number of stars distributed over a finite region of space but reasoned that if.On the other hand,there were an infinite number of stars,distributed more or less uniformly over infinite space,this would not happen because there would not be any central point for them to fall to .
This argument is an instance of the pitfalls that you can encounter in talking about infinity.In an infinite universe every point can beregarded as the centerbecuase every point has an infinite number of stars on each side of it.The correct approachit was realised only much later,is to consider the finite situation in which the stars all fall in on each other ,and then how to ask how things change if one adds more stars roughly uniformly distriuted outside this region.According to Nwtons law the extra stars would make no difference at all to the original ones on average,so the stars would fall in hust as fast.we can add as many stars as we like,but they will stillalways collapse in on themselves .we now know it is impossible to have an infinite static model of the universe in which gravity is always attractive.

The sun

The sun was the stationary at the centre and that the earth and the planets moved in circular orbits around the sun.Nearly a centruy passed before this idea was takenseriously.Then two astronomers-the German,johannas kelper and the italian,Galileo Galiliei-started publicy to support the copernican theory despiter the fact that the orbits it predicted did now quite match the ones onserved .The death blow to the Atistrolien-ptolemaic theory came in 1609.In that year,Galileo started observing the night sky with a telescope which had just been invented.When he looked at the planet jupiter.Galileo found that it was accompinied by several small satellites or moons that orbited around it.This implied that every thing didnot have to orbit directly around the earth ,as Aristotle and ptolemy had thought.At the same time,johnanes leper had modified copernicus theory,suggesting that the planets moved not in circles but in ellipses.The preductions now finally matched the observations,but kelper could not reconcile them with his idea that the planets were made to orbit the sun by magnetic forces.An explanation was provided only much later,in 1687,when sir lsaac Newton published his philispher Naturalis principles Mathemaica in which he not only put forword a theory of how bodies move in space and time but aoso developed the complicated mathematics needed to analyyse thos motions .Newton postulated a law of universal graviation according to which each body in the universe was attracted toward every were to each other.It was this same force that caused objects to fall to the ground.Newton went on to show that according to his law,gravity the earth and the causes the earth and the planets to follow elliptical pathsaround the sun.

Aristotle

Aristotle thought that the earth was stationary and that the sun the moon the planets and that the sun,the moon ,the planets and the stars moved in circular orbits about the earth.Thisidea was elaboarted by ptolemy in the second century AD in to a complete cosmological model.The earth stood at the center surrounded by eight sphares that carried the moon the sun the stars, and the five planets known at the time.Mercury venus ,Mars,Jupiter and saturm.The planets thenselves moved on smaller circles attached to their respective sphares in order to account for their rather complicated observed paths in the sky.The outermost sphare carried the so-called fixed stars,which always stay in the positions relative to each other but which rotate together accross the sky.What lay beyond the last sphere was never made very clear,but it certainly was not part of mankinds observe universe.

Earth

As long ago as 340 BC the Greek philosophe Aristotle .On the heaven s was able to put forward two good arguments for beleiving that the eartjh was a round sphere rather than a flat place .First he realised that eclips of the moon were caused by the earth comimg between sun and the moon.The earth's shadow on the moon always round ,which would be true only if the earth was spherical.Second the Greeks knew ffrom their travels that the North star appeared lower in the sky when viewed in the south than it did in more northerly regions (since the North star lies over the north pole,it appears to
be directly above on the observer at the north pole but to some one looking from the equateor.it appears to lie just at the horizon.

The Universe

Most people find this picture of our universe rather radiculous ,but why do we think we know better?What to do we know about the Universe,and how do we know it?Where did the universe come from and where is it going ?Did the universe have begining,and if so,what happend before then?what is the nature time?will it ever come to an end?Recent break throughs in physics , made possible in part by fantastic new technologies suggest answeres to some of these long standing questions .Some day these answeres may seem as obvious to us as the earth obiting the sun-or perhaps as ridicious as a tower of tortoises.only time (what ever that may be)will tell.

Astronomy

Our picture of the universe:Long long ago it was belived that the world is really a flat plate support on the back of a gaint tortoise standing on an infinite tower of tortoise.