Sky & Telescope - October 2022 - 52

OCTOBER 2022 OBSERVING
Exploring the Solar System by Charles A. Wood
Within Ancient Thebit
The Straight Wall isn't a single, continuous feature.
I
s there an amateur astronomer who
hasn't enjoyed looking at Rupes
Recta? Commonly known as the
Straight Wall, as the Latin name
implies, the 110-km-long (68-mile) feature
is relatively straight, and its shadow
reveals its nature as a wall, cleft, or
scarp. Even small telescopes can show
the Straight Wall's shadow gradually
diminishing as the Sun rises over the
feature. From full Moon on, the Wall
reflects the Sun's rays and appears as a
bright line etched on the lunar surface.
Measurements reveal that the eastern
side is about 400 meters higher than
the mare-covered western side.
Although correctly interpreted as
a fault scarp for many decades, the
Straight Wall escaped modern analysis
until 2015, when Amanda Nahm (then
at the Lunar and Planetary Institute)
and Richard A. Schultz (formerly of
ConocoPhillips) applied modern techniques
developed to study terrestrial
faults. Their analysis characterizes the
fault orientation, depth, and material
pGrazing illumination at sunrise reveals the
point west of the Straight Wall where the ancient
crater's floor abruptly becomes level.
strength to help determine the forces
and processes that created the feature.
Nahm and Schultz began by noting
A
B
C
+
Birt
Thebit
D
Ancient
Thebit
that the Straight Wall isn't a single fault
but rather five segments, each slightly
concave in the direction of the west
side. Where the segments meet, there
are minor knicks or offsets. Observers
with 8-inch or larger telescopes
using high magnification can detect
these small offsets. Nahm and Schultz
labelled them A, B, C, D, from north to
south. Lunar Reconnaissance Orbiter
Camera images provide closeup views,
with offset D being especially interesting.
The segment to the north of D
ramps down to the base of the fault
just where the next segment begins.
The ramp would be the easiest place for
astronauts to travel from the elevated
east side, down 400 m to the west with
its access to Mare Nubium.
Craters and large, displaced masses
of rock at the segment boundaries
along the Straight Wall show only
vertical movement, so the scarp is
what's known as a normal fault. This
vertical displacement is verified with
readily available height measurements,
and the length of the fault requires
only a ruler and a scaled photograph
to measure. The depth of faulting was
obtained by comparing changes in
topography across the fault and the
 The large, flooded crater seen here is informally
known as Ancient Thebit.
52 OCTOBER 2022 * SKY & TELESCOPE
LABELLED IMAGE: NASA / GSFC / ARIZONA STATE
UNIVERSITY; LOW LIGHTING ANGLE: JIM PHILLIPS
Rille
Rima Birt
Rupes Recta

Sky & Telescope - October 2022

Table of Contents for the Digital Edition of Sky & Telescope - October 2022

Contents
Sky & Telescope - October 2022 - Cover1
Sky & Telescope - October 2022 - Cover2
Sky & Telescope - October 2022 - 1
Sky & Telescope - October 2022 - Contents
Sky & Telescope - October 2022 - 3
Sky & Telescope - October 2022 - 4
Sky & Telescope - October 2022 - 5
Sky & Telescope - October 2022 - 6
Sky & Telescope - October 2022 - 7
Sky & Telescope - October 2022 - 8
Sky & Telescope - October 2022 - 9
Sky & Telescope - October 2022 - 10
Sky & Telescope - October 2022 - 11
Sky & Telescope - October 2022 - 12
Sky & Telescope - October 2022 - 13
Sky & Telescope - October 2022 - 14
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Sky & Telescope - October 2022 - 84
Sky & Telescope - October 2022 - Cover3
Sky & Telescope - October 2022 - Cover4
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