DOC.
42
SPECIAL AND GENERAL RELATIVITY 359
Solution
of
Gravitation 115
planets
under
consideration.
Thus,
if
we
correct
the observed
motion of the
planets
for
these
two influences,
and if New-
ton's
theory
be
strictly
correct,
we
ought
to
obtain
for
the
orbit
of the
planet
an
ellipse,
which
is
fixed
with reference
to
the
fixed
stars.
This
deduction,
which
can
be tested with
great
accuracy,
has
been confirmed
for all
the
planets
save
one,
with
the
precision
that
is capable
of
being
obtained
by
the
delicacy
of observation attainable
at
the
present
time.
The
sole
excep-
tion
is Mercury,
the
planet
which lies
nearest
the
sun.
Since
the time of
Leverrier,
it
has
been known that the
ellipse
corresponding
to
the orbit
of
Mercury,
after it
has
been
cor-
rected
for
the
influences mentioned
above, is not stationary
with
respect
to
the fixed
stars,
but
that
it
rotates exceedingly
slowly
in the
plane
of the orbit and in the
sense
of the orbital
motion.
The
value obtained
for
this
rotary
movement
of the
orbital
ellipse was
43
seconds of
arc per century, an
amount
ensured
to
be
correct to
within
a
few seconds of
arc.
This
effect
can
be
explained by
means
of
classical
mechanics
only
on
the
assumption
of
hypotheses
which have little
probability,
and which
were
devised
solely
for
this
purpose.
On the basis of
the
general theory
of
relativity,
it is found
that the
ellipse
of
every planet
round
the
sun
must necessarily
rotate
in the
manner
indicated
above;
that
for all
the
planets,
with the
exception
of
Mercury,
this rotation
is
too
small
to
be
detected
with the
delicacy
of observation
possible at
the
present time;
but that
in
the
case
of
Mercury
it
must amount
to
43
seconds
of
arc
per century,
a
result which
is
strictly
in
agreement
with observation.
Apart
from this
one,
it
has
hitherto been
possible
to
make
[61]
[62]
[63]
[64]