Sky and Telescope - September 2018 - 53

N

Lunar Libration

G R A PHS: G REGG DINDER M A N / S&T

mostly because the Moon's sidereal
orbital period (as gauged by its position
among the stars) is 27.3 days, while
its synodic period (new Moon to new
Moon) is 29.5 days. They're not in sync.
The librations in longitude and
latitude aren't even in sync with each
other. Their relationship evolves in a
weird, 6-year-long cycle before starting to repeat. Every point around the
Moon's limb, at one time or another,
becomes the region most favorably
tipped to our view.

20

(Sept. 2016)

N

22

28

24

18

Sept. 2 30
4

26

Lunar Libration

26

(Sept. 2018)

24
22
6

28

16

W

E

Sept. 2

W

18

8

4

2°

12

16

10

6
10

E

20

30

14

2°

12

8

14

S

S

p The combined effects of libration in longitude and latitude can vary dramatically from month to

Observing at the Edge

month - as evidenced by these plots showing the lunar limb's tilt toward Earth in September of
2016 and 2018.

So what does all this bobbing and
weaving bring into view? What rare
bits of luna incognita await the libration-savvy observer?
First, you need to pay attention to
the Moon's phase. Suppose you get ready
for a really favorable libration along a
particular spot along the eastern lunar
limb - a section best seen when the
Moon is near first quarter - but then
you realize that right now the Moon is
near last quarter and your target zone is
in darkness. Better luck next time!
One no-hassle way to view good
libration targets is to flip to page 42 of
this issue. At upper left is an image of
the Moon with dots labeled 1, 13, 16,
and 26. At lower right are four lunar
features - the craters Galvani, Peires-

cius, Hamilton, and Xenophanes -
which you can glimpse along the lunar
limb on their respective dates. In every
issue, on that same page, you'll find
targets with favorable librations chosen
by S&T's editors. We've been offering
these since 1987.
The libration zones offer plenty of
interesting sights. The most spectacular
farside feature that you can hope to
see - and probably the most elusive - is
Mare Orientale, a sizable "sea" of lava
sitting in middle of a dramatic impact
basin roughly 900 km (600 miles)
across. It's a bit south and west of the
lava-floored crater Grimaldi. Although
the exact age of Orientale isn't known,
it's the Moon's youngest basin-forming

impact - as evidenced by a nearly perfect bull's-eye of three concentric rings
of uplifted mountains that surround
the lava plain at their center.
Every now and then, when libration
swings them into view along the western limb, you can glimpse the basin's
two outer rings: Montes Cordillera and
the outer Montes Rook. Get a little
luckier, and you can spot Lacus Veris,
a dark "finger" of pooled lava lying
just inside the ridge of Rook peaks. To
Views from Earth
Sunset

Midnight

Sunrise

Libration in Latitude
Moon

Earth

View from Earth

View from Earth

Diurnal
Libration
Moon

t Libration in longitude
occurs because the Moon's
orbital speed isn't constant.
It allows you to view beyond
the usual east and west
edges of the lunar disk.

p Libration in latitude,
which can create a tilt of
up to 6½°, permits you
to observe more of the
Moon's polar regions.

u Diurnal libration occurs
every day, as our planet's
rotation carries you from
one side of Earth to the
other and changes your
lunar perspective slightly.

Midnight

Sunrise

Earth

Sunset

sk yandtelescope.com * S E P TE M B E R 2 018

53

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

Contents
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