CREDIT52 JUNE 2020 • SKY & TELESCOPE
F
or observers, the Moon is like a
circular billboard in the sky. We
never see its entire back side from
Earth, but with the subtle effects of
the Moon’s orbit known as libration
(S&T: Sep. 2018, p. 52), we can peek a
few degrees around its edge, allowing
us to view nearly 60% of the Moon’s
surface. Within that just-around-the-
edge zone, during favorable librations
we can observe all of Mare Orientale,
even though the mare itself lies beyond
the western limb. Similarly, we can
glimpse the limbward halves of Mare
Marginis and Mare Smythii, both of
which extend beyond 90°E.
Direct observation beyond the 6°
to 7° width of the libration zone is
impossible, but there have been several
attempts to infer farside features
before the Space Age. Nathaniel Shaler,
an American geologist who studied the
Moon, speculated in 1903 that Montes
Cordillera, which appear as bumps on
the Moon’s western edge, could be the
rim of a giant crater. Shaler was right.
Lunar Orbiter 4 photographs revealed
the full extent of the three-ringed Ori-
entale impact basin that extends 10° to
15° beyond what Earthbound observers
can view. Orientale is a very large basin
indeed, with a small lake of dark mare
lava in the middle – Mare Orientale.
In 1624, Giovanni Domenico Cas-
sini observed very tall peaks on the
limb near the lunar south pole. The
indefatigable observer Johann Hierony-
mous Schröter rediscovered the peaks
at the turn of the 19th century and
named them the Leibnitz Mountains,
—a designation they retained until- Schröter didn’t suspect that they
might be the rim of a giant farside cra-
52 JUNE 2020 • SKY & TELESCOPE
Peering over the Limb
Spot evidence of these hidden farside features during
favorable librations.JUNE 2020 OBSERVING
Exploring the Moon by Charles Wood
ter, but William Hartmann and Gerard
Kuiper did. In their groundbreaking
1962 paper, “Concentric Structures
Surrounding Lunar Basins,” the pair
recognized that nearside circular maria
were mountain-edged impact basins.
The Leibnitz Mountains are among the
tallest on the lunar surface, so it’s not
surprising that the basin they partially
enclose is the Moon’s biggest impact
feature, the South Pole–Aitken (SPA)
Basin. Spanning around 2,500 kilome-
ters, the entire SPA lies on the farside.
Schröter also discovered the Doerfel
Mountains, west of the Leibnitz peaks.
The Doerfels are another limb-
hugging, tall range whose name is no
longer offi cially recognized. Schröter
and other observers attempted to
carefully map the range, but none
suggested they are the rim of a large
crater. In 2018, amateur astronomer
Jim Mosher defi nitively matched the
highest peaks with high spots near the
craters Drygalski and Hausen, situated
right along the SPA rim. Along with the
Leibnitz mountains, the Doerfels are
the only parts of the giant SPA Basin
that libration makes visible.
Shaler also initiated a separate
attempt to identify features on the
unobservable lunar farside by exam-
ining faint crater rays appearing
to radiate from sources beyond the
limb. Later, in the 1950s, Hugh Percy
Wilkins and Patrick Moore repeated
the exercise and proposed the exis-
tence of 9 farside ray craters. Each
lay beyond the western limb and was
situated in a long arc spanning from
near the north pole to Mare Orientale
and down to Bailly near the south
pole. One proposed crater supposedlyhad rays converging near the nearside
craters Bailly and Inghirami.
Using NASA’s Lunar Reconnais-
sance Orbiter Hapke-Normalized
Color QuickMap overlay (quickmap.
lroc.asu.edu), I don’t see any of the
rays Wilkins and Moore reported.
However, I can fi nd other farside cra-
ters whose rays extend onto the near-
side. The best example are three rays
from Ohm that extend 1,130 km across
farside highland material and continue
through the middle of Russell, as wellpThis image of Mare Orientale seen on an
evening of strong libration reveals the western
rim of the Rook Mountains (Montes Rook)
residing far beyond the limb.SEAN WALKER /S&T