A Brief History of Time - Stephen Hawking | Page 2

smile before replying, “W\
hat is the tortoise standing on.” “You’re very
clever, young man, very clever,” said the old lady. “But it’s t\
urtles all the way down!”
Most people would find the picture of our universe as an infinite tower \
of tortoises rather ridiculous, but why do
we think we know better? What do we know about the universe, and how do \
we know it? Where did the
universe come from, and where is it going? Did the universe have a begin\
ning, and if so, what happened before
then? What is the nature of time? Will it ever come to an end? Can we go\
back in time? Recent breakthroughs
in physics, made possible in part by fantastic new technologies, suggest\
answers to some of these
longstanding questions. Someday these answers may seem as obvious to us \
as the earth orbiting the sun – or
perhaps as ridiculous as a tower of tortoises. Only time (whatever that\
may be) will tell.
As long ago as 340 BC the Greek philosopher Aristotle, in his book On the Heavens, was able to put forward
two good arguments for believing that the earth was a round sphere rathe\
r than a Hat plate. First, he realized
that eclipses of the moon were caused by the earth coming between the su\
n and the moon. The earth’s
shadow on the moon was always round, which would be true only if the ear\
th was spherical. If the earth had
been a flat disk, the shadow would have been elongated and elliptical, u\
nless the eclipse always occurred at a
time when the sun was directly under the center of the disk. Second, the\
Greeks knew from their travels that
the North Star appeared lower in the sky when viewed in the south than i\
t did in more northerly regions. (Since
the North Star lies over the North Pole, it appears to be directly above\
an observer at the North Pole, but to
someone looking from the equator, it appears to lie just at the horizon.\
From the difference in the apparent
position of the North Star in Egypt and Greece, Aristotle even quoted an\
estimate that the distance around the
earth was 400,000 stadia. It is not known exactly what length a stadium \
was, but it may have been about 200
yards, which would make Aristotle’s estimate about twice the currentl\
y accepted figure. The Greeks even had a
third argument that the earth must be round, for why else does one first\
see the sails of a ship coming over the
horizon, and only later see the hull?
Aristotle thought the earth was stationary and that the sun, the moon, t\
he planets, and the stars moved in
circular orbits about the earth. He believed this because he felt, for m\
ystical reasons, that the earth was the
center of the universe, and that circular motion was the most perfect. T\
his idea was elaborated by Ptolemy in
the second century AD into a complete cosmological model. The earth stoo\
d at the center, surrounded by eight
spheres that carried the moon, the sun, the stars, and the five planets \
known at the time, Mercury, Venus,
Mars, Jupiter, and Saturn.
A Brief History of Time - Stephen Hawking... Chapter 1
file:///C|/WINDOWS/Desktop/blahh/Stephen Hawking - A brief history of ti\
me/n.html (1 of 7) [2/20/2001 3:14:06 AM]

Figure 1:1
The planets themselves moved on smaller circles attached to their respec\
tive spheres in order to account for
their rather complicated observed paths in the sky. The outermost sphere\
carried the so-called fixed stars,
which always stay in the same positions relative to each other but which\
rotate together across the sky. What
lay beyond the last sphere was never made very clear, but it certainly w\
as not part of mankind’s observable
universe.
Ptolemy’s model provided a reasonably accurate system for predicting \
the positions of heavenly bodies in the
sky. But in order to predict these positions correctly, Ptolemy had to m\
ake an assumption that the moon
followed a path that sometimes brought it twice as close to the earth as\
at other times. And that meant that the
moon ought sometimes to appear twice as big as at other times! Ptolemy r\
ecognized this flaw, but nevertheless
his model was generally, although not universally, accepted. It was adop\
ted by the Christian church as the
picture of the universe that was in accordance with Scripture, for it ha\
d the great advantage that it left lots of
room outside the sphere of fixed stars for heaven and hell.
A simpler model, however, was proposed in 1514 by a Polish priest, Nicho\
las Copernicus. (At first, perhaps for
fear of being branded a heretic by his church, Copernicus circulated his\
model anonymously.) His idea was that
the sun was stationary at the center and that the earth and the planets \
moved in circular orbits