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!!!
