Sky & Telescope - February 2024 - 53

CLOSE-UP OF DANIELL RILLES: WES HIGGINS;
BOND RILLES: NASA / GSFC / ARIZONA STATE UNIVERSITY
Several sections of Rimae Daniell are visible
in this photo taken as the Sun set over the
western extent of Lacus Somniorum.
when a thin shadow is just visible. The
southwest side of the mare rises 140 m
over a 5-km span approaching the rille
and then precipitously drops 220 m to
the flat terrain on the northeast. This
section of Rimae Daniell isn't actually a
rille but rather a fault, spanning about
half the height and 60% of the length
of the better known Straight Wall in
Mare Nubium.
At about 70 km from its southern
end, the fault forks and transforms into
a thin, flat-floored rille segment that
continues northwest. A second part
bends west and cuts across mare lavas
for nearly 100 km before continuing
into hilly ejecta deposited during the
formation of the Imbrium or Serenitatis
basins. At least three similar rilles cut
through the ejecta and curve around
the Serenitatis basin, as do other
rilles along the basin's north and east
sides. These flat-floored rilles could be
fractures created when the terrain surrounding
Serenitatis bent downward as
the mare-laden basin subsided. The best
Earth-based image of the Daniell Rilles
that I know of, shown above right, isn't
from a spacecraft but rather taken by
Oklahoma amateur Wes Higgins. He
used his 18-inch Newtonian reflector to
capture the entire rille system under the
grazing light of a setting Sun.
Return to Daniell and then head
east-southeast to the 20-km-diameter
crater G. Bond. The flat-floored rille
Rimae G. Bond curves from south of
the crater and heads northward past
the disintegrated crater Hall for about
80 km until it's truncated by another
rille set at a right angle. This second
section of the G. Bond Rilles extends
east-northeast and west-southwest
from the main rille, with the longest
segment extending some 130 km to
the east end of Lacus Somniorum,
where it's barely visible as it crosses
the region's rough and ancient cratered
terrain. These details aren't well
plotted in most lunar atlases but will
reward careful observers using high
power under low illumination angles.
Finally, north of the Daniell Rilles is
a probable ancient crater immediately
west of 28-km-wide Grove crater. A distinct
rim fragment extends northwest
from Grove and curves west, perhaps
continuing as a short, curved mare
ridge to the south, defining (with a little
imagination) the incomplete outline of
a roughly 100-km-wide crater. Rubbly
terrain covers most of its southeast and
west segments, but the interior is 400
m deep and has a hint of a central peak,
adding evidence that it's a nearly-gone
ghost crater.
The flat-floored rilles of
Rimae G. Bond intersect
like a T north of Hall crater.
For nearly two daylong periods in
the lunar morning and evening, Lacus
Somniorum transforms from an easily
overlooked plain of bland lavas into
a fascinating region of subtle details
to challenge the most seasoned lunar
observers. The next time Posidonius
reveals its fractured floor under a low
Sun, be sure to cast your gaze to its
north. You won't be disappointed.
¢Contributing Editor CHUCK WOOD
never dozes when it comes to observing
the Moon.
sk yand tele scope .o r g * FEBRUARY 2024 53
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Sky & Telescope - February 2024

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