The Sleepwalkers (119 page)

His
next
attempt
to
remedy
the
disagreement
between
his
dream
and
the
observed
facts
concerned
the
moon.
Should
her
orbit
be
included
into
the
thickness
of
the
earth's
sphere,
or
not?
He
explained
frankly
to
his
dear
readers
that
he
would
choose
the
hypothesis
which
best
fits
his
plan;
he
will
tuck
the
moon
into
the
earth's
shell,
or
banish
her
into
the
outer
darkness,
or
let
her
orbit
stick
half-way
out,
for
there
are
no
a
priori
reasons
in
favour
of
either
solution.
(
Kepler's
a
priori
proofs
were
mostly
found
a
posteriori
.)
But
fiddling
with
the
moon
did
not
help
either,
so
young
Kepler
proceeded
to
a
frontal
attack
against
the
Copernican
data.
He
declared
them
with
admirable
impertinence
to
be
so
unreliable
that
Kepler's
own
figures
would
be
strongly
suspect
if
they
agreed
with
Copernicus'.
Not
only
were
the
tables
unreliable;
not
only
was
Copernicus
inexact
in
his
observations,
as
reported
by
Rheticus
(from
whom
Kepler
quotes
long,
damning
passages);
but
the
old
Canon
also
cheated:

"How
human
Copernicus
himself
was
in
adopting
figures
which
within
certain
limits
accorded
with
his
wishes
and
served
his
purpose;
this
the
diligent
reader
of
Copernicus
may
test
by
himself...
He
selects
observations
from
Ptolemy,
Walter
and
others
with
a
view
to
making
his
computations
easier,
and
he
does
not
scruple
to
neglect
or
to
alter
occasional
hours
in
observed
time
and
quarter
degrees
of
angle."
⁹

Twenty-five
years
later,
Kepler
himself
ammusedly
commented
on
his
first
challenge
of
Copernicus:

"After
all,
one
approves
of
a
toddler
of
three
who
decides
that
he
will
fight
a
giant."
¹⁰

So
far,
in
the
first
twenty
chapters
of
his
book,
Kepler
had
been
concerned
with
finding
reasons
for
the
number
and
spatial
distribution
of
the
planets.
Having
satisfied
himself
(if
not
his
readers)
that
the
five
solids
provided
all
the
answers,
and
that
existing
discrepancies
were
due
to
Copernicus'
faulty
figures,
he
now
turned
to
a
different,
and
more
promising
problem,
which
no
astronomer
before
him
had
raised.
He
began
to
look
for
a
mathematical
relation
between
a
planet's
distance
from
the
sun,
and
the
length
of
its
"year"
–
that
is,
the
time
it
needed
for
a
complete
revolution.

These
periods
were,
of
course,
known
since
antiquity
with
considerable
precision.
In
round
figures,
Mercury
needs
three
months
to
complete
a
revolution,
Venus
seven
and
a
half
months,
the
earth
a
year,
Mars
two
years,
Jupiter
twelve
years,
and
Saturn
thirty
years.
Thus
the
greater
the
planet's
distance
from
the
sun,
the
longer
it
takes
to
complete
a
revolution,
but
this
is
only
roughly
true:
an
exact
mathematical
ratio
was
lacking.
Saturn,
for
instance,
is
twice
as
far
out
in
space
as
Jupiter,
and
should
therefore
take
twice
as
long
to
complete
a
circuit,
that
is
twenty-four
years;
but
Saturn
in
fact
takes
thirty.
The
same
is
true
of
the
other
planets.
As
we
travel
from
the
sun
outward
into
space,
the
motion
of
the
planets
along
their
orbits
gets
slower
and
slower.
(To
get
the
point
quite
clear:
they
not
only
have
a
longer
way
to
travel
to
complete
a
circuit,
but
they
also
travel
at
a
slower
rate
along
it.
If
they
travelled
at
the
same
rate,
Saturn,
with
a
circuit
twice
as
long
as
Jupiter's,
would
take
twice
as
long
to
complete
it;
but
it
takes
two
and
a
half
times
as
long.)
Nobody
before
Kepler
had
asked
the
question
why
this
should
be
so,
as
nobody
before
him
had
asked
why
there
are
just
six
planets.
As
it
happens,
the
latter
question
proved
scientifically
sterile,
^*
the
former
immensely
fertile.
Kepler's
answer
was,
that
there
must
be
a
force
emanating
from
the
sun
which
drives
the
planets
round
their
orbits.
The
outer
planets
move
slower
because
this
driving
force
diminishes
in
ratio
to
distance
"as
does
the
force
of
light".

____________________