Sky and Telescope - September 2018 - 54

SEPTEMBER 2018 OBSERVING
Exploring the Solar System

see Mare Orientale itself requires lots
of patience, because it peeks into view
clearly only every few years. My suggestion is to find a calendar for 2020
and circle October 10-11. Those will be
especially good nights for spotting Mare
Orientale on a last-quarter Moon.
Another challenging target is the
crater Lorentz. It's a whopping 371 km
across, larger than any crater on the
Moon's nearside. But it's also ancient,
and over the eons its rim has become
indistinct. Fortunately, you'll have
an especially good chance to glimpse
Lorentz on the nights of August 29th
to September 1st. To track it down,
draw an imaginary line across Oceanus
Procellarum from Aristarchus through
the small but sharp crater Lichtenberg
until you reach the limb.
Four lesser-known lunar seas slip
into view relatively often on the opposite side of the disk. Start by locating
Mare Crisium, the distinctly oval dark
lava plain that's always in view (when

sunlit) along the eastern limb. Use it as
a judge of whether the libration in that
region is favorable: If it's very close to
the edge, don't bother. But if it's well
separated from the limb, keep looking!
To Crisium's immediate right is the
dark, irregular circle called (rather obviously) Mare Marginis. Trail a bit southward along the limb, past the large but
strongly foreshortened crater Neper, to
reach Mare Smythii. Then keep sliding
southward until you reach the clutch of
dark smudges marking Mare Australe.
September's target craters Peirescius
and Hamilton are there, as is a prominent, lava-filled saucer named Lyot. The
fourth "marginal" sea along the Moon's
eastern limb is Mare Humboldtianum,
a dark and compact spot well above
Crisium at a latitude of 60° north.

Libration Resources
I'll close with some helpful aids to
expand your libration-assisted observ-

ing. For a WYSIWYG view of what's
viewable along the lunar limb on any
given night, you can't beat Virtual
Moon Atlas. A labor of love by veteran
French observers Christian Legrand and
Patrick Chevalley, it's available as a free
download at ap-i.net/avl/en/download.
Tip: Once you've installed it, open the
Configuration menu, make sure the
Topocentric button is selected, and then
enter your site's latitude and longitude
(the default location is in France).
Fred Espenak, though justifiably
famous for his eclipse-chasing skills,
also provides daily data on lunar libration in tabular form (and much, much
more) at his astropixels.com website.
A shortcut to the lunar ephemerides is
https://is.gd/librations.
Finally, an essential aid to knowing what you're seeing along the lunar
limb is Sky & Telescope's Field Map of
the Moon, which utilizes an excellent
portrayal of nearside surface features by
the late Antonín Rükl. The map's outer
limb extends more than 8° onto the farside, thus showing and identifying any
libration-aided feature you'd want to see.
It's available from shopatsky.com.
So why settle for viewing only 50%
of the Moon's surface? Thanks to libration, you can squeeze in another 9%
with just a little advance planning.
Happy hunting!
¢ Senior Editor KELLY BEATTY has been
exploring the Moon's nooks and crannies
since the early 1960s.

Hamilton
Oken
MARE
CRISIUM
MARE
HUMBOLDTIANUM

MARE
MARGINIS
Neper

Peirescius

Lyot
MARE
AUSTRALE

Endymion
Atlas
MARE
SMYTHII
Hercules

MARE
FECUNDITATIS

Langrenus

54

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

When libration of the
Moon's eastern limb is
favorable, look for these four
dark-hued maria and some of the
prominent craters near them.

SPACE VIE W: N ASA / NORTHE AST REGION A L PL A NE TA RY DATA CEN TER; E A RTH
VIE W: SE A N WA LK ER; VIE WS OF M A RIA: G A RY SERONIK (3)

t Orientale
is a dramatic,
bull's-eyeshaped basin
that lurks just
beyond the
nearside limb.
But you can
glimpse parts
of it during especially favorable librations
of the Moon's
southwestern
terrain.


http://www.ap-i.net/avl/en/download http://www.astropixels.com https://www.is.gd/librations http://www.shopatsky.com

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