Sky and Telescope - September 2018 - 52

SEPTEMBER 2018 OBSERVING
Exploring the Solar System by J. Kelly Beatty
t A snapshot from a passing spacecraft

A Nod to
Lunar Libration
Every month, the Moon's face wobbles enough
to give you a peek at some of its hidden farside.

ave you ever settled in with your
telescope for a little lunar observing, only to sense that something about
the Moon was "off"? Maybe you suspect
that some of its landmark features
aren't quite where they should be. Or
maybe you spot something along the
limb that doesn't look familiar.
This subtle shifting happens all the
time - every month, in fact - due to
a periodic nodding of the lunar disk
called libration. And rather than being
an annoying distraction, libration is
actually a good thing. It lets you view
portions of the lunar farside that are
typically hidden from view.
We all learn in school that the Moon
shows us the same half of its globe all
the time - and keeps the other half
constantly out of view - thanks to a
permanent lock between its rotation
rate and orbital period. But what your
grade-school teacher likely didn't tell
you is that it's not a perfectly 50:50
proposition. Thanks to libration, any
portion of the lunar limb can become
shifted toward Earth by as much as 8°
or 9°. Over time, and when your timing's right, you'll be able to glimpse 59%
of the Moon's surface - including some
tantalizing features that periodically
pop into view along the limb.

the eastern or western limb. It arises
because the Moon's orbit is distinctly
not circular, with an eccentricity of
about 0.06. This causes the Moon to
move faster than average when near
perigee and slower than average when
near apogee - even though its rotation
rate remains constant. So sometimes the
Moon's orbital position can be either a
little behind or ahead of its spin, which
manifests as a back-and-forth nod of the
disk. This can be as much as 8° in longitude or 240 km (as big as the prominent
nearside crater Clavius).
Next is libration in latitude, an effect
that's nearly as pronounced. The plane
of the Moon's equator is tipped 6.7°
with respect to the plane of its orbit. So

Libration in Longitude

the lunar north pole is tipped in our
direction for half of each orbit and
tthe south pole is tipped toward us for
the other half. These shifts in latitude
th
allow
allo you to peer deeply into the farside polar regions.
Diurnal libration, the third component, has nothing to do with the
Moon itself. Instead, it's created by our
changing vantage point on a rotating
Earth. As the diagram on the next page
shows, we're on one side of Earth when
we view the Moon rising in the east but
on the opposite side of our planet when
it sets in the west.
The total amount of this swing is
about 9,000 km at the latitude of the
contiguous U.S. states, Europe, and
Australia, enough to experience an
additional ±0.7° of libration in longitude. You can use this to your advantage by remembering that it favors the
eastern lunar limb (celestial west) at
moonrise and the western lunar limb
(celestial east) at moonset.
You might think that librations in
longitude and latitude should occur at
the same part of each lunar phase cycle
month after month. But they don't,

Views from Earth

A
Moon

Farside

B
B
Farside

Nearside

D
Earth

A Geometric Trifecta
Three different motions contribute to
the nodding of the lunar disk observable
on any given night.
The largest component, libration in
longitude, is evident as a shift along
52

S E P TE M B E R 2 018 * SK Y & TELESCOPE

t
bi
or
s
'
on
Mo
Diagrams not to scale

M OON: N ASA / USGS; DIAG R A MS: G REGG DINDER M A N / S&T

shows the side of the Moon visible from
Earth (at right) and a portion of the unseen
farside (at left). The giant impact basin
Orientale is just below center.

Sky and Telescope - September 2018

Table of Contents for the Digital Edition of Sky and Telescope - September 2018