CHEMICAL IMPACT
HUMANS HAVE ALWAYS been fascinated by
the heavens, by the behavior of the sun
by day and the stars by night. Although
more accurately measured now because
of precise instruments, the basic observations of these events have remained the
same over the past 4000 years. However, our interpretations of the events have
changed dramatically. For example,
about 2000 B.C. the Egyptians postulated that the sun was a boat inhabited
by the god Ra, who daily sailed across
the sky.
Over the years, patterns in the
changes in the heavens were recognized
and, through marvelous devices such as
Stonehenge in England, were connected
to the seasons of the year. People also
noted that seven objects seemed to move
against the background of “fixed stars.”
These objects, actually the sun, the moon,
and the planets Mercury, Venus, Mars,
Jupiter, and Saturn, were called the “wanderers.” The planets appeared to move
from west to east, except Mars, which
seemed to slow down and even move
backwards for a few weeks.
One of the first explanations for these
observations came from Eudoxus, born in
400 B.C. He imagined the earth as fixed,
with the planets attached to a nested set
of transparent spheres that moved at different rates around the earth. The stars
were attached to the outermost sphere.
This model, although clever, still did not
account for the strange behavior of Mars.
Five hundred years later, Ptolemy, a
Greek scholar, worked out a plan more
complex than that of Eudoxus, in which
the planets were attached to the edges
of spheres that “rolled around” the
spheres of Eudoxus (see time line). This
model accounted for the behavior of all
the planets, including the apparent reversals in the motion of Mars.
Because of human prejudice that the
earth should be the center of the universe,
Ptolemy’s model was assumed to be cor-
Observations, Theories, and the Planets
rect for more than a thousand years, and
its wide acceptance actually inhibited
the advancement of astronomy. Finally, in
1543, a Polish cleric, Nicholas Copernicus, postulated that the earth was only
one of the planets, all of which revolved
around the sun. This “demotion” of the
earth’s status produced violent opposition
to the new model, and in fact, Copernicus’s writings were “corrected” by religious
officials before scholars were allowed to
use them.
The Copernican theory persisted and
was finally given a solid mathematical
base by Johannes Kepler. Kepler postulated elliptical rather than circular orbits
for the planets in order to account more
completely for their observed motions.
Kepler’s hypotheses were in turn further
refined 36 years after his death by Isaac
Newton, who recognized that the concept of gravitation could account for the
positions and motions of the planets.
However, even the brilliant models of
Newton were found incomplete by Albert
Einstein, who showed that Newton’s mechanics was a special case of a much
more general model.
Thus the same basic observations
were made for several thousand years,
but the explanations—the models—have
changed remarkably from the Egyptians’
boat of Ra to Einstein’s relativity.
Our models will inevitably change,
and we should expect them to do so.
They can help us make scientific progress,
or they can inhibit progress if we become
too attached to them. Although the fundamental facts of chemistry will remain the
same, the models in a chemistry text written a century from now will certainly be
quite different from the ones presented
here.
A time line showing how models of
the solar system changed over
4000 years.
2000 B.C.: Egyptians: sun is a boat sailed
by a god
1600 B.C.
1200 B.C.
800 B.C.
400 B.C.: Eudoxus: earth fixed; planets
roll on spheres
100 A.D.: Ptolemy: modified Eudoxus’s
model to account for apparent
direction reversals
400 A.D.
800 A.D.
1200 A.D.
Copernicus: sun-centered solar system
Kepler: elliptical planetary orbits
Galileo: confirmed Kepler’s laws
Newton: universal law of gravitation
Einstein: general relativity
2000 A.D.