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Are We the Center of the Universe? The question can be answered on a…

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Are We the Center of the Universe?
The question can be answered on a philosophical level or on a more literal physical level. I will not address the philosophical question. The roots of the physical question come from the ancients. I will concentrate on the Greeks and western civilization as the paradigm for the dangers of such thinking.
Geo-Centric versus Helio-Centric Theories
We know all about the Greeks who developed intricate models of the Solar System (Universe) (e.g., Ptolemy) based on their ideas of the physical laws which governed motion (Aristotle). The Greeks developed Earth-centered models for the Solar System, i.e., geo-centric models.

These notions were not put to rest until the 1600's when a string of workers (Copernicus, Galileo, Kepler, Brahe, and Newton) finally showed that the Sun was the center of the Solar System, i.e., helio-centric models. The change took many years, however, because of scientific reasons:

* the observational data was poor which meant that it was impossible to choose between different models and the helio-centric models did not work any better than the geo-centric models
* the physics of the day suggested that geo-centric models were more physical than helio-centric models

It was left to Brahe to develop a database which was accurate enough to differentiate between competing models, to Kepler to develop an accurate model for planetary motions, and to Newton to develop a physical theory which explained the laws of Kepler.
Are We at the Center of the Milky Way Galaxy?
As if we hadn't learned our lesson, there was also a debate in the early part of this century concerning the location of the Sun in the Milky Way galaxy. We know that the Milky Way galaxy is a disk-type galaxy known as a spiral galaxy composed of ~ 100 billion stars and that the Sun is roughly 2/3 of the way out in the disk in a spiral arm (see figure). But, how do we know such things (about the shape of our galaxy and our position in the galaxy)?

* Herschel (and later, Kapteyn) performed a simple exercise. They simply counted stars in various directions in the Milky Way galaxy. Their results led to the idea that the Sun was the center of the Milky Way! Hmmmmm.
* Shapley and Trumpler resolved the issue. Essentially the problem is is that dust obscures distant stars. Trumpler came up with this idea and Shapley demonstrated that it was true. How?

Are We at the Center of the Universe?
Well this issue comes up again concerning the expansion of the Universe. However, since we learned our lesson, the notion that we were at the center of the Universe was never seriously considered. But let us consider why this could be an issue and why we rule it out.

The upshot of several thousand years of work is that we are not in a preferred position in the Universe. We are in orbit around an averagish sort of star, which moves around in an averagish spiral galaxy which moves around in a small cluster of galaxies which moves around in an averagish cluster of clusters of galaxies and so on. Apparently, we are just a rather pedestrian member of the vast enterprise known as the Universe.

Source: http://zebu.uoregon.edu/~imamura/209/mar31/nico.html

Some other things:
Our Milky Way Galaxy and its companions
ASTRO 322, Galaxies in the Universe, Winter Semester, 2006 <--I'm guessing this is for a class for a class in Alberta,Canada? Either way,it is informative and nicely done!

Happy night, ya'll!
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On November 12th, 2007 03:54 pm (UTC), endogenesis commented:
Huzzah for average-ish!
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