Libration
Definition of Libration
Li`bra´tion
Pronunciation:
lî`brā´shŭn
n.
1.
The
act
or
state
of
librating
.
2.
(Astron.)
A
real
or
apparent
libratory
motion
,
like
that
of
a
balance
before
coming
to
rest
.
Libration of the moon
any
one
of
those
small
periodical
changes
in
the
position
of
the
moon
'
s
surface
relatively
to
the
earth
,
in
consequence
of
which
narrow
portions
at
opposite
limbs
become
visible
or
invisible
alternately
.
It
receives
different
names
according
to
the
manner
in
which
it
takes
place
;
as
:
(
a
)
Libration in longitude
,
that
which
,
depending
on
the
place
of
the
moon
in
its
elliptic
orbit
,
causes
small
portions
near
the
eastern
and
western
borders
alternately
to
appear
and
disappear
each
month
. (
b
)
Libration in latitude
,
that
which
depends
on
the
varying
position
of
the
moon
'
s
axis
in
respect
to
the
spectator
,
causing
the
alternate
appearance
and
disappearance
of
either
pole
. (
c
)
Diurnal
or
parallactic libration
,
that
which
brings
into
view
on
the
upper
limb
,
at
rising
and
setting
,
some
parts
not
in
the
average
visible
hemisphere
.
Related Words
astronomy
,
librate
,
oscillation
,
uranology
,
variation
Word:
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