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.

H

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

A
Moon

Farside

B
B
Farside

Nearside

D
Earth

C

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

C

t
bi
or
s
'
on
Mo
Diagrams not to scale

D

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.



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

Contents
Sky and Telescope - September 2018 - Cover1
Sky and Telescope - September 2018 - Cover2
Sky and Telescope - September 2018 - 1
Sky and Telescope - September 2018 - Contents
Sky and Telescope - September 2018 - 3
Sky and Telescope - September 2018 - 4
Sky and Telescope - September 2018 - 5
Sky and Telescope - September 2018 - 6
Sky and Telescope - September 2018 - 7
Sky and Telescope - September 2018 - 8
Sky and Telescope - September 2018 - 9
Sky and Telescope - September 2018 - 10
Sky and Telescope - September 2018 - 11
Sky and Telescope - September 2018 - 12
Sky and Telescope - September 2018 - 13
Sky and Telescope - September 2018 - 14
Sky and Telescope - September 2018 - 15
Sky and Telescope - September 2018 - 16
Sky and Telescope - September 2018 - 17
Sky and Telescope - September 2018 - 18
Sky and Telescope - September 2018 - 19
Sky and Telescope - September 2018 - 20
Sky and Telescope - September 2018 - 21
Sky and Telescope - September 2018 - 22
Sky and Telescope - September 2018 - 23
Sky and Telescope - September 2018 - 24
Sky and Telescope - September 2018 - 25
Sky and Telescope - September 2018 - 26
Sky and Telescope - September 2018 - 27
Sky and Telescope - September 2018 - 28
Sky and Telescope - September 2018 - 29
Sky and Telescope - September 2018 - 30
Sky and Telescope - September 2018 - 31
Sky and Telescope - September 2018 - 32
Sky and Telescope - September 2018 - 33
Sky and Telescope - September 2018 - 34
Sky and Telescope - September 2018 - 35
Sky and Telescope - September 2018 - 36
Sky and Telescope - September 2018 - 37
Sky and Telescope - September 2018 - 38
Sky and Telescope - September 2018 - 39
Sky and Telescope - September 2018 - 40
Sky and Telescope - September 2018 - 41
Sky and Telescope - September 2018 - 42
Sky and Telescope - September 2018 - 43
Sky and Telescope - September 2018 - 44
Sky and Telescope - September 2018 - 45
Sky and Telescope - September 2018 - 46
Sky and Telescope - September 2018 - 47
Sky and Telescope - September 2018 - 48
Sky and Telescope - September 2018 - 49
Sky and Telescope - September 2018 - 50
Sky and Telescope - September 2018 - 51
Sky and Telescope - September 2018 - 52
Sky and Telescope - September 2018 - 53
Sky and Telescope - September 2018 - 54
Sky and Telescope - September 2018 - 55
Sky and Telescope - September 2018 - 56
Sky and Telescope - September 2018 - 57
Sky and Telescope - September 2018 - 58
Sky and Telescope - September 2018 - 59
Sky and Telescope - September 2018 - 60
Sky and Telescope - September 2018 - 61
Sky and Telescope - September 2018 - 62
Sky and Telescope - September 2018 - 63
Sky and Telescope - September 2018 - 64
Sky and Telescope - September 2018 - 65
Sky and Telescope - September 2018 - 66
Sky and Telescope - September 2018 - 67
Sky and Telescope - September 2018 - 68
Sky and Telescope - September 2018 - 69
Sky and Telescope - September 2018 - 70
Sky and Telescope - September 2018 - 71
Sky and Telescope - September 2018 - 72
Sky and Telescope - September 2018 - 73
Sky and Telescope - September 2018 - 74
Sky and Telescope - September 2018 - 75
Sky and Telescope - September 2018 - 76
Sky and Telescope - September 2018 - 77
Sky and Telescope - September 2018 - 78
Sky and Telescope - September 2018 - 79
Sky and Telescope - September 2018 - 80
Sky and Telescope - September 2018 - 81
Sky and Telescope - September 2018 - 82
Sky and Telescope - September 2018 - 83
Sky and Telescope - September 2018 - 84
Sky and Telescope - September 2018 - Cover3
Sky and Telescope - September 2018 - Cover4
http://www.nxtbook.com/nxtbooks/newtrack/st_201811
http://www.nxtbook.com/nxtbooks/newtrack/st_201810
http://www.nxtbook.com/nxtbooks/newtrack/st_201809
http://www.nxtbook.com/nxtbooks/newtrack/st_201808
http://www.nxtbook.com/nxtbooks/newtrack/st_201807
http://www.nxtbook.com/nxtbooks/newtrack/st_201806
http://www.nxtbook.com/nxtbooks/newtrack/st_201805
http://www.nxtbook.com/nxtbooks/newtrack/st_201804
http://www.nxtbook.com/nxtbooks/newtrack/st_201803
http://www.nxtbook.com/nxtbooks/newtrack/st_201802
http://www.nxtbook.com/nxtbooks/newtrack/st_201801
http://www.nxtbook.com/nxtbooks/newtrack/st_201712
http://www.nxtbook.com/nxtbooks/newtrack/st_201711
http://www.nxtbook.com/nxtbooks/newtrack/st_201710
http://www.nxtbook.com/nxtbooks/newtrack/st_201709
http://www.nxtbook.com/nxtbooks/newtrack/st_201708
http://www.nxtbook.com/nxtbooks/newtrack/st_201707
http://www.nxtbook.com/nxtbooks/newtrack/st_201706
http://www.nxtbook.com/nxtbooks/newtrack/st_201705
http://www.nxtbook.com/nxtbooks/newtrack/st_201704
http://www.nxtbook.com/nxtbooks/newtrack/st_201703
http://www.nxtbook.com/nxtbooks/newtrack/st_201702
http://www.nxtbook.com/nxtbooks/newtrack/st_201701
http://www.nxtbook.com/nxtbooks/newtrack/st_201612
http://www.nxtbook.com/nxtbooks/newtrack/st_201611
http://www.nxtbook.com/nxtbooks/newtrack/st_201610
http://www.nxtbook.com/nxtbooks/newtrack/st_201609
http://www.nxtbook.com/nxtbooks/newtrack/st_201608
http://www.nxtbook.com/nxtbooks/newtrack/st_201607
http://www.nxtbook.com/nxtbooks/newtrack/st_201606
http://www.nxtbook.com/nxtbooks/newtrack/st_201605
http://www.nxtbook.com/nxtbooks/newtrack/st_201604
http://www.nxtbook.com/nxtbooks/newtrack/st_201603
http://www.nxtbook.com/nxtbooks/newtrack/st_201602
http://www.nxtbook.com/nxtbooks/newtrack/st_201601
http://www.nxtbook.com/nxtbooks/newtrack/st_201512
http://www.nxtbook.com/nxtbooks/newtrack/st_201511
http://www.nxtbook.com/nxtbooks/newtrack/st_201510
http://www.nxtbook.com/nxtbooks/newtrack/st_201509
http://www.nxtbook.com/nxtbooks/newtrack/st_201508
http://www.nxtbook.com/nxtbooks/newtrack/st_201507
http://www.nxtbook.com/nxtbooks/newtrack/st_201506
http://www.nxtbook.com/nxtbooks/newtrack/st_201505
http://www.nxtbook.com/nxtbooks/newtrack/st_201504
http://www.nxtbook.com/nxtbooks/newtrack/st_201503
http://www.nxtbook.com/nxtbooks/newtrack/st_201502
http://www.nxtbook.com/nxtbooks/newtrack/st_201501
http://www.nxtbook.com/nxtbooks/newtrack/st_201412
http://www.nxtbook.com/nxtbooks/newtrack/st_201411
http://www.nxtbook.com/nxtbooks/newtrack/st_201410
http://www.nxtbook.com/nxtbooks/newtrack/st_201409
http://www.nxtbook.com/nxtbooks/newtrack/st_201408
http://www.nxtbook.com/nxtbooks/newtrack/st_mars
http://www.nxtbook.com/nxtbooks/newtrack/st_201407
http://www.nxtbook.com/nxtbooks/newtrack/st_201406
http://www.nxtbook.com/nxtbooks/newtrack/st_201405
http://www.nxtbook.com/nxtbooks/newtrack/st_201404
http://www.nxtbook.com/nxtbooks/newtrack/st_201403
http://www.nxtbook.com/nxtbooks/newtrack/st_201402
http://www.nxtbook.com/nxtbooks/newtrack/st_201401
http://www.nxtbook.com/nxtbooks/newtrack/st_201312
http://www.nxtbook.com/nxtbooks/newtrack/st_201311
http://www.nxtbook.com/nxtbooks/newtrack/st_201310
http://www.nxtbook.com/nxtbooks/newtrack/st_201309
http://www.nxtbook.com/nxtbooks/newtrack/st_201308
http://www.nxtbook.com/nxtbooks/newtrack/st_201307
http://www.nxtbook.com/nxtbooks/newtrack/st_201306
http://www.nxtbook.com/nxtbooks/newtrack/st_201305
http://www.nxtbook.com/nxtbooks/newtrack/st_201304
http://www.nxtbook.com/nxtbooks/newtrack/st_201303
http://www.nxtbook.com/nxtbooks/newtrack/st_201302
http://www.nxtbook.com/nxtbooks/newtrack/st_201301
http://www.nxtbook.com/nxtbooks/newtrack/st_201212
http://www.nxtbook.com/nxtbooks/newtrack/st_201211
http://www.nxtbook.com/nxtbooks/newtrack/st_201210
http://www.nxtbook.com/nxtbooks/newtrack/st_201209
http://www.nxtbook.com/nxtbooks/newtrack/st_201208
http://www.nxtbook.com/nxtbooks/newtrack/st_201207
http://www.nxtbook.com/nxtbooks/newtrack/st_201206
http://www.nxtbook.com/nxtbooks/newtrack/st_201205
http://www.nxtbook.com/nxtbooks/newtrack/st_201204
http://www.nxtbook.com/nxtbooks/newtrack/st_201203
http://www.nxtbook.com/nxtbooks/newtrack/st_201202
http://www.nxtbook.com/nxtbooks/newtrack/st_201201
http://www.nxtbook.com/nxtbooks/newtrack/st_201112
http://www.nxtbook.com/nxtbooks/newtrack/st_201111
http://www.nxtbook.com/nxtbooks/newtrack/st_201110
http://www.nxtbook.com/nxtbooks/newtrack/st_201109
http://www.nxtbook.com/nxtbooks/newtrack/st_201108
http://www.nxtbook.com/nxtbooks/newtrack/st_201107
http://www.nxtbook.com/nxtbooks/newtrack/st_201106
http://www.nxtbook.com/nxtbooks/newtrack/st_201105
http://www.nxtbook.com/nxtbooks/newtrack/st_201104
http://www.nxtbook.com/nxtbooks/newtrack/st_201103
http://www.nxtbook.com/nxtbooks/newtrack/st_201102
http://www.nxtbook.com/nxtbooks/newtrack/st_201101
http://www.nxtbook.com/nxtbooks/newtrack/st_201012
http://www.nxtbook.com/nxtbooks/newtrack/st_201011
http://www.nxtbook.com/nxtbooks/newtrack/st_201010
http://www.nxtbook.com/nxtbooks/newtrack/st_201009
http://www.nxtbook.com/nxtbooks/newtrack/newtrack_st_201009
http://www.nxtbook.com/nxtbooks/newtrack/st_201008_v2
http://www.nxtbook.com/nxtbooks/newtrack/st_201008
http://www.nxtbook.com/nxtbooks/newtrack/st_201007
http://www.nxtbook.com/nxtbooks/newtrack/st_201005_v2
http://www.nxtbook.com/nxtbooks/newtrack/st_201006
http://www.nxtbook.com/nxtbooks/newtrack/st_201005
http://www.nxtbook.com/nxtbooks/newtrack/st_201004
http://www.nxtbook.com/nxtbooks/newtrack/st_201003v4
http://www.nxtbook.com/nxtbooks/newtrack/st_201003v3
http://www.nxtbook.com/nxtbooks/newtrack/st_201003v2
http://www.nxtbook.com/nxtbooks/newtrack/st_201003
http://www.nxtbook.com/nxtbooks/newtrack/st_201002
http://www.nxtbook.com/nxtbooks/newtrack/st_201001
http://www.nxtbookMEDIA.com