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Classical astronomy followed principles established by Aristotle. Aristotle accepted the idea that there were four physical elements — earth, water, air, and fire. He put the earth in the center of the universe and contended that these elements were below the moon, which was the closest celestial body. There were seven planets, or wandering stars, because they had a course through the zodiac in addition to traveling around the earth: the moon, Mercury, Venus, the sun, Mars, Jupiter. Beyond that were the fixed stars. The physical elements, according to Aristotle moved vertically, depending on their ‘heaviness’ or ‘gravity’; the celestial bodies were not physical but a ‘fifth element’ or ‘quintessence’ whose nature was to move in perfect circles around the earth, making a daily rotation. Aristotle envisioned the earth as the true center of all the circles or ‘orbs’ carrying the heavenly bodies around it and all motion as ‘uniform,’ that is, unchanging.
But observers realized that the heavenly bodies did not move as Aristotle postulated. The earth was not the true center of the orbits and the motion was not uniform. The most obvious problem was that the outer planets seemed to stop, move backwards in ‘retrograde’ motion for a while, and then continue forwards. By the second century, when Ptolemy compiled his Almagest (this common name of Ptolemy's Syntaxis was derived from its Arabic title), astronomers had developed the concept that the orbit moves in ‘epicycles’ around a ‘deferrent,’ that is, they move like a flat heliacal coil around a circle around the earth. The earth was also off-center, on an ‘eccentric,’ as the heavenly bodies moved around a central point. Ptolemy added a point on a straight line opposite the eccentric, which is called the ‘equalizing point’ or the ‘equant,’ and around this point the heavenly bodies moved uniformly. Moreover, unlike the Aristotelian model, Ptolemy's Almagest did not describe a unified universe. The ancient astronomers who followed Ptolemy, however, were not concerned if his system did not describe the ‘true’ motions of the heavenly bodies; their concern was to ‘save the phenomena,’ that is, give a close approximation of where the heavenly bodies would be at a given point in time. And in an age without professional astronomers, let alone the telescope, Ptolemy did a good job plotting the courses of the heavenly bodies.
Originally posted by arpgme
There are about 300 sextillion stars in the universe (300,000,000,000,000,000,000,000). That is 3 followed by 23 zeros . Yes, this is just stars, it's not even counting all of the planets going around each and every one of them. Among all of those stars there are billions of planets which are able to be lived on . By livable, yes, I do mean Earth-like planets... Also, remember there is evolution which allows a species to adapt to a new environment. So what are the chances that there is absolutely NO life in the universe but us? How can anyone possibly think this?
Originally posted by Argyll
reply to post by arpgme
I think most people would agree with you! (I know I do)......but are these "aliens" visiting earth?....now that is another question, and one that a lot of people (myself included) struggle with.
Originally posted by arpgme
So what are the chances that there is absolutely NO life in the universe but us? How can anyone possibly think this?
Originally posted by samlf3rd
Everything on our planet is completely covered in bacteria and life-yet all the other planets have 0? C'mon...
Originally posted by disfugured
I don't believe IN Aliens, but I do believe they exist.
Believing them sounds too much like a religion.
Originally posted by Blue Shift
We're back to a bunch of dead chemicals somehow either RANDOMLY or MAGICALLY forming itself into a living thing? And we're back to the odds thing again. How likely do you think that is? If you take complete strands of DNA molecules -- give yourself a head start, after all -- and put them in a beaker and shake them, how long will it take to form a single, living procreating bacteria? If you took a bunch of transistors and wires and screws and sheets of metal and shook them in a bag, how long would it take for all those pieces to fall together into a complete, working computer?