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VACETS Regular Technical Column

"Science for Everyone"

"Science for Everyone" was a technical column posted regularly on the VACETS forum. The author of the following articles is Dr. Vo Ta Duc. For more publications produced by other VACETS  members, please visit the VACETS Member Publications page or Technical Columns page.

The VACETS Technical Column is contributed by various members , especially those of the VACETS Technical Affairs Committe. Articles are posted regulary on vacets@peak.org forum. Please send questions, comments and suggestions to vacets-ta@vacets.org

Mon, 16 Jan 1995

Cosmology: A Journey Through Time

Cosmology, what is it? It is a study that deals with the origin, structure, and space-time relationships of the universe. Our understanding of the cosmos today is far different from that of our ancestors some thousands, hundreds, or even tens of years ago. And, perhaps, our perceptions and assumptions about our universe today may be dramatically changed and viewed as myths by our children. It is of interest to start the series by an article that brings us back to the distant past, to the ancient myths about the creation of the universe, and from there, we will gradually travel back to our present time, to a universe of unimaginable size that itself has no center at all.

Nearly all the ancient cosmologies have several features in common. First, the earth is seen as having a center, a mysterious place that connects the earth and its inhabitants with the cosmos. It is often represented as a mountain, a tree, or some sort of axis that unites heaven and earth. For the Indians it was Mount Meru, for the Sumerians, Mount Sumer. The Hebrews identified the earth's sacred center with Mount Zion. The Muslims saw it as the Holy Rock of Jerusalem. The ancient Egyptians identified the sacred center with the Mountain of the Moon, the legendary source of the life-giving Nile River... These prescientific peoples also considered the earth to be the physical center of the cosmos. That was patently obvious to anyone who spent any time watching the way the sun, moon, and stars moved about the earth.

Another common aspect of these cosmologies is the importance of directions. Each of the four directions (north, east, south, and west) has spiritual and cosmological significance and is often represented by different colors, plant and animal spirits, and powers.

Finally, the universe is often depicted as having a kind of layered structure. The ancient Chinese believed in the existence of an "inner earth" within the one we know, a mirror image of the world above; Indian mythology speaks of seven heavens; the Aztec cosmology has thirteen layers of heaven and nine underworld levels...

In addition to these basic features, early cosmologies often assigned different meanings of roles to various plants and animals, to terrestrial phenomena such as the wind, storms, lightning, and clouds, and to astronomical objects such as the sun, the moon, the stars, the planets... and the band of light that flows across the night sky, the Milky Way. Almost every ancient culture had some myths about the Creation and the Milky Way. The myth most familiar to most of us is the "Ngu+u Lang Chu+'c Nu+~" story and it is told at the end of this article.

The myths that dominated and influenced the Western world for thousands of years was those of the ancient Greeks. The Greeks used a series of moving celestial spheres to explain the observed movements of the sun, the moon, the stars, and the planets. Celestial objects that did not fit into this regular scheme, such as meteors and comets, were considered messengers from the gods. For centuries, the Greeks viewed the earth as a flat, circular disk surrounded by a world ocean. The heavenly realm lies above the earth, and its underworld counterpart lies below. The sky is a roof above the earth, supported by great pillars. The major change in Greek cosmology was made by Pythagoras in the sixth century B.C.. His cosmology was a combination of numerological mysticism with some of the earlier ideas. At the center of the universe was the immobile earth, and earth was a sphere. Circling the earth were the sun, moon, and five planets, each on its perfect circular path. The stars were fixed in a distant crystalline sphere. Other philosophers who lived after the time of Pythagoras came up with different visions of the cosmos. Leucippus thought the earth was shaped like a drum. Heracleitus thought it was bowl-shaped and the fundamental element was fire. Democritus considered the earth to be hollow. He also postulated that all things were made of tiny indivisible entities, which he called atoms. The constant motion of atoms created the universe. Heavier atoms clustered together to make the earth, while lighter ones came together to make the heavenly bodies.

The next philosopher to offer a significantly different cosmology was Anaxagoras (5th century B.C.). When he was about thirty-three, a meteorite the size of a wagon fell to earth and he claimed that it had fallen from the sun, which was neither fire nor a god but a glowing stone bigger than all of Greece itself. The stars, too, were glowing stones, only much more distant from earth than the sun. The earth, which was flat and supported by air, not water. The moon, in turn, was a flat disk like the earth, and shone not from its own light as many earlier philosophers claimed but from light reflected from the sun. Anaxagoras' pupil Socrates did not construct a cosmology, but his most famous pupil, the philosopher Plato (4th century B.C.) became the next major cosmological thinker among the Greeks. Plato believed that the earth was not flat but a sphere, since the sphere is a perfect shape and thus mirrors the perfection of the cosmos. The sky was also a sphere. So were the sun, the moon, the planets and stars. All these bodies moved about the earth in circular orbits. The occasional retrograde motion of the planets (planets like Mars, Jupiter, and Saturn periodically appear to reverse their regular counterclockwise motion across the night sky, and temporarily move backward) could be explained by a complex system of circular movements. Eudoxus was the first astronomer to produce an elaborate cosmology based on Plato's suggestions. He was also the first Greek astronomer to explain the movements of the heavenly bodies in a manner that might be called scientific. His description of celestial movements used a series of transparent concentric spheres. An example is his explanation of the movement of the moon around the earth. Eudoxus used three hollow spheres, connected and in motion at the same time. The outermost sphere rotates around the earth from east to west once every 24 hours with its axis running through the star Polaris. The next sphere has its axis tilted at an angle of 23.5 degrees to the outermost sphere and rotates from east to west with a period of 223 lunar month (a little bit more than 18 years). The innermost sphere's axis is inclined at five degrees to that of the second sphere and rotates from west to east at a rate of once every 27.2 days. The moon sits on this sphere and is exactly 90 degrees down from the axis of rotation. This model explains the monthly motions of the moon through its synodic, draconic, sidereal periods, and the longer 18-year lunar period. The movements of the sun, planets, and stars were also similarly constructed with different number of spheres. A total of 27 spheres were used for the sun, the moon, five planets, and all the stars in this system. Callipus later modified the Eudoxus's system a bit by adding few more spheres to increase the mathematical accuracy. Aristotle, Plato's student, recognized that more spheres were necessary for the scheme to be physically real, and he added them to his system. His scheme had a total of 56 spheres. Aristotle had many pupils, but his most famous was the young Macedonian king Alexander the Great. His armies swept and spread Greek thought across the known world. He built an Egyptian city, name in his own honor, Alexander. In that city, many of the later Greek astronomer/philosophers lived and carried out their work.

Aristarchus (3rd century B.C.), believed that the sun, not the earth, was the center of the universe. The earth, the moon, the planets, and all the stars traveled in circular paths around the central sun. The spherical earth also rotated on its axis once a day, thus explaining why the sun appears to rise and set and why the the stars seem to move across the night sky as if attached to a distant transparent sphere. The earth moved around the sun in a circular path once a year, and that explained the annual movement of the sun through the zodiac. A second important Alexandrian astronomer was Eratosthenes. He was the first person to make an accurate estimate of the size of the earth. He estimated the circumference of the earth to be 37,200 km, amazingly close to the modern measurement of 40,074 km. The third important astronomer of that time was Hipparchus (2nd century B.C.). Hipparchus based his model on the basic cosmology of Aristotle and displaced the earth slightly from the center of the cosmos. This model explained why the planets move more quickly through some parts of their paths than through others. Some 250 years later, Ptolemaeus adapted Hipparchus's system and turned it into the ruling astronomical paradigm. He invented a geometric device called the equant. It is a circle similar to an eccentric. The earth and a point called the equant point were situated in opposite directions and equal distance from the center of an equant. As a planet moves along the equant, the line between the equant point and the planet will move through equal angles in equal times. When finished, Ptolemy's model of the cosmos was fairly simple and for almost 15 centuries, it was accepted as the truth. It was accepted because it worked well. It allowed astronomers to predict the positions of the planets with the accuracy they wanted. It agreed with and supported the prevailing Greek-based system of philosophy. As Christianity rose and became the dominant religious system of the Western world, its leaders and teachers were able to use the Ptolemaic model to confirm and support their own vision of the cosmos, and of humanity's relation to God.

The Ptolemaic model of the cosmos came to fulfill the major tasks of a functioning mythology. It would not be overthrown until 15 centuries later. The revolution against the Ptolemaic system would be led by a Polish Catholic priest, a German who had once attended a Lutheran seminary, and an Italian who had the Pope as a friend.

(Next week: The journey from 16th century to present)

"Ngu+u Lang Chu+'c Nu+~"

It was a clear summer night many years ago in the homeland. The whole family just finished eating. I was sitting on my mom's lap listening to her talking to the neighbor in the yard. Looking up the sky, I saw a faint river of light stretching across the heavens. What was that? My mom began to tell me the story of the two lovers who were separated and could only see each other on a certain day of the year. It was a sad and beautiful story. Many of us probably have heard this story when we were little kids. It may be redundant to re-tell the story but I can not resist the temptation to tell it. So here it is.

The story is about the Weaver Girl (the star Vega) who was a shy young woman concerned with her work around the home of her father, the One Who Stands Outside Time. Her father arranged for Weaver Girl to marry the Herdsman (the constellation Eagle). The Herdsman tended cattle on the far side of the Milky Way. He came over to the near side, married the Weaver Girl, and they were very happy together. However, the Weaver Girl became so lost in her love for her husband that she began to neglect her work around her father's house.

That disturbed the father, so he arranged to change things. He called together a flock of magpies, who made a winged bridge over the river of the Milky Way. Then the father ordered the Herdsman to cross the bird bridge to the far side and resume herding the cattle. The Herdsman did so, and the magpies flew away as soon as he reached the far side of the Milky Way. The Weaver Girl returned to her homely duties, and the Herdsman resumed his herding in that dark land which is hidden from human eyes. Once a year, on the seventh day of the seventh month, it is possible for the two lovers to briefly visit together. People pray that the weather will be clear that night. Then, it is said, the magpies will re-form the winged bridge over the Milky Way, and the Weaver Girl can cross over the silver sky river to visit her husband the Herdsman.

The end!!!


Duc Ta Vo, Ph.D.
ducvo@lanl.gov

For discussion on this column, join vacets-tech@vacets.org


Copyright © 1996 by VACETS and Duc Ta Vo

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Spacetime-Travel and Relativity (Part 2)

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