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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libido
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