Sky & Telescope - February 2024 - 52

FEBRUARY 2024 OBSERVING
Exploring the Solar System by Charles A. Wood
Grove
Ghost
crater
Lacus Somniorum
Daniell
Hall
Posidonius
G. Bond
A Slumbering Lunar Lake
Y
ou might say that lunar observers
have been asleep when it comes
to Lacus Somniorum, the Lake of
Dreams. This poorly defined patch of
mare lavas resides between the crater
Posidonius and another lunar lake,
Lacus Mortis. There have been very few
reports about Lacus Somniorum. But
it's worth disturbing your slumber on a
cloudless night to visit its dark shores.
Observers with an 8-inch or larger
telescope can explore two fascinating
rille systems, an unexpectedly complex
rille-floored crater, and the ghost of a
giant, ancient crater.
Lacus Somniorum is located in the
Moon's northeastern quadrant and
appears as a mostly flat and featureless
oval area about 450 kilometers (280
miles) wide east to west, and half that
dimension north to south. There's no
evidence from topographic, morpho52
FEBRUARY 2024 * SKY & TELESCOPE
logic, or gravitational data to indicate
that its lavas fill a buried impact basin.
The east-west edges are where the mare
stops, meeting basin ejecta from the
Imbrium basin on the west and tattered,
old, eroded craters at the east. Daniell
crater is near the middle of Somniorum
and is a good starting point to explore
this overlooked lava plain.
Daniell itself is a strange crater.
This elliptical feature spans about
28 km north to south, but only 23
km east to west. A close examination
shows the longer direction is due to
massive rim slumps that expanded the
crater's dimensions. But it's the floor
of the crater that's most unusual. It's
surfaced by smooth lava, and, like
nearby Posidonius, Daniell's floor is
tilted, with the southern side some 60
m higher than the northern section.
Two rilles cross the floor parallel to the
A neglected corner of the Moon contains interesting surprises under the right lighting.
crater's long axis, and another rings
the floor, concentric with the crater's
walls. The area between the two
parallel rilles is 100 m lower than the
surfaces on either side of the parallel
rilles, having subsided like a large graben
between two rille-bounded, higher
surfaces. Daniell is considered a floorfractured
crater - one whose floor is
uplifted and fractured by an intrusion
of magma entering from below. One
wonders while examining the feature
if the parallel rilles are also faults that
accommodated the rise of the outer
parts of the floor.
About two crater diameters to the
west-northwest of Daniell is the most
conspicuous section of Rimae Daniell.
This very straight linear feature appears
as a thin, bright line five days after new
moon. A more dramatic view occurs
near sunset in the region 14 days later,
Rimae G. Bond
NASA / GSFC / ARIZONA STATE UNIVERSITY
Rimae Daniell

Sky & Telescope - February 2024

Table of Contents for the Digital Edition of Sky & Telescope - February 2024

Contents
Sky & Telescope - February 2024 - Cover1
Sky & Telescope - February 2024 - Cover2
Sky & Telescope - February 2024 - 1
Sky & Telescope - February 2024 - Contents
Sky & Telescope - February 2024 - 3
Sky & Telescope - February 2024 - 4
Sky & Telescope - February 2024 - 5
Sky & Telescope - February 2024 - 6
Sky & Telescope - February 2024 - 7
Sky & Telescope - February 2024 - 8
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Sky & Telescope - February 2024 - Cover3
Sky & Telescope - February 2024 - Cover4
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