The Sleepwalkers (31 page)

To
get
this
extraordinary
phenomenon
into
a
proper
perspective,
one
must
not
only
be
on
one's
guard
against
the
wisdom
of
hindsight,
but
also
against
the
opposite
attitude,
that
kind
of
benevolent
condescension
which
regards
the
past
follies
of
Science
as
the
unavoidable
consequences
of
ignorance
or
superstition:
"our
forebears
just
did
not
know
better".
The
point
I
shall
try
to
make
is
that
they
did
know
better;
and
that
to
explain
the
extraordinary
cul
de
sac
into
which
cosmology
had
manoeuvred
itself,
we
must
look
for
more
specific
causes.

In
the
first
place,
the
Alexandrian
astronomers
can
hardly
be
accused
of
ignorance.
They
had
more
precise
instruments
for
observing
the
stars
than
Copernicus
had.
Copernicus
himself,
as
we
shall
see,
hardly
bothered
with
star-gazing;
he
relied
on
the
observations
of
Hipparchus
and
Ptolemy.
He
knew
no
more
about
the
actual
motions
in
the
sky
than
they
did.
Hipparchus'
Catalogue
of
the
fixed
stars,
and
Ptolemy's
Tables
for
calculating
planetary
motions,
were
so
reliable
and
precise
that
they
served,
with
some
insignificant
corrections,
as
navigational
guides
to
Columbus
and
Vasco
da
Gama.
Eratosthenes,
another
Alexandrian,
computed
the
diameter
of
the
earth
as
7,850
miles,
with
an
error
of
only
1/2
per
cent;
⁵
Hipparchus
calculated
the
distance
of
the
moon
as
301/4
earth
diameters
–
with
an
error
of
only
0.3
per
cent.
⁶

Thus,
insofar
as
factual
knowledge
is
concerned,
Copernicus
was
no
better
off,
and
in
some
respects
worse
off,
than
the
Greek
astronomers
of
Alexandria
who
lived
at
the
time
of
Jesus
Christ.
They
had
the
same
observational
data,
the
same
instruments,
the
same
know-how
in
geometry,
as
he
did.
They
were
giants
of
"exact
science".
Yet
they
failed
to
see
what
Copernicus
saw
after,
and
Herakleides-Aristarchus
had
seen
before
them:
that
the
planets'
motions
were
obviously
governed
by
the
sun.

Now
I
have
said
before
that
we
must
beware
of
the
word
"obvious";
but
in
this
particular
case
its
use
is
legitimate.
For
Herakleides
and
the
Pythagoreans
had
not
been
led
to
the
heliocentric
hypothesis
by
a
lucky
guess,
but
by
the
observed
fact
that
the
inner
planets
behaved
like
satellites
of
the
sun,
and
that
the
outer
planets'
retrogressions
and
changes
in
earth-distance
were
equally
governed
by
the
sun.
Thus,
by
the
end
of
the
second
century
B.C.,
the
Greeks
had
all
the
major
elements
of
the
puzzle
in
their
hands,
⁷
and
yet
failed
to
put
them
together;
or
rather,
having
put
them
together,
they
took
them
to
pieces
again.
They
knew
that
the
orbits,
periods
and
velocities
of
the
five
planets
were
connected
with,
and
dependent
on,
the
sun
–
yet
in
the
system
of
the
universe
which
they
bequeathed
to
the
world,
they
managed
to
ignore
completely
this
all-important
fact.

This
mental
snow-blindness
is
all
the
more
remarkable
as,
qua
philosophers
,
they
were
aware
of
the
dominant
part
played
by
the
sun
which,
qua
astronomers
,
thy
nevertheless
denied
.

A
few
quotations
will
illustrate
this
paradox.
Cicero,
for
instance,
whose
knowledge
of
astronomy
is,
of
course,
entirely
based
on
Greek
sources,
writes
in
The
Republic
:
"The
sun
...
ruler,
prince
and
leader
of
the
other
stars,
sole
and
ordering
principle
of
the
universe
(is)
so
large
that
its
light
brightens
and
fills
the
all...
The
orbits
of
Mercury
and
Venus
follow
him
as
his
companions"
⁸

Pliny
writes
a
century
later:
"The
sun
is
carried
around
in
the
midst
of
the
planets,
directing
not
only
the
calendar
and
the
earth
but
also
the
stars
themselves
and
the
sky."
⁹

Plutarch
speaks
in
a
similar
vein
in
On
the
Face
in
the
Moon
Disc
:

"But
in
general
how
can
we
say:
the
earth
is
in
the
centre
–
in
the
centre
of
what?
The
universe
is
infinite;
and
the
infinite,
which
has
neither
beginning
nor
end,
has
no
centre
either...
The
universe
does
not
assign
any
fixed
centre
to
the
earth,
which
drifts
homelessly
and
unsteadily
through
the
infinite
emptiness
without
a
proper
goal..."
¹⁰