Beginning observing
10
great features for
Moon watchers
Our natural satellite offers plenty of targets you can spot through any size telescope.
by Michael E. Bakich
T
his story explores 10 lunar features that show you a bit
about lunar geology. Five are craters (or pairs of craters).
Their geology is simple: A meteoroid falls on the lunar
surface, forming a crater. Other features formed differently.
An evening or two past First Quarter — or the same time
before Last Quarter — sunlight falls at a low angle across the
rugged Montes Alpes, the lunar Alps. Unlike Earth’s tectonically created peaks, however, these Alps form part of a gigantic
crater rim: the boundary of the Imbrium Basin, which formed
when an asteroid struck the Moon 3.85 billion years ago.
Sinus Aestuum is a lava pond hemming the Imbrium debris.
On its eastern edge, dark volcanic material erupted explosively
along a rille. Although this region at first appears featureless,
observe it at several different lunar phases and you’ll see the
dark area grow more apparent as the Sun climbs higher.
Occupying a region below and a bit left of the Moon’s dead
center, Mare Nubium lies far from many lunar showpiece sites.
Look for it as the dark region above magnificent Tycho Crater.
Yet this small region, where lava plains meet highlands, contains a variety of interesting geologic features — impact craters,
lava-flooded plains, tectonic faulting, and debris from distant
impacts — that are great for telescopic exploring.
You’ll note that Mare Nubium does not fill an obvious, circular basin like Mare Imbrium. A low-power view reveals Mare
Nubium’s curved edges, hinting that it formed by the merging
of a few immense impacts.
Mare Orientale is another of the Moon’s large impact basins,
and possibly the youngest. Lunar scientists think it formed 170
million years after Mare Imbrium. And although “Mare Orientale” translates to “Eastern Sea,” in 1961, the International
Astronomical Union changed the way astronomers denote
lunar directions. The result is that Mare Orientale now sits on
the Moon’s western limb. From Earth we never see most of it.
When you observe the Cauchy Domes, you’ll be looking at
shield volcanoes that erupted from lunar vents. The lava cooled
slowly, so it had a chance to spread and form gentle slopes.
In a geologic sense, our Moon is now quiet. The only events
interrupting its long sleep are occasional meteoroid impacts.
Luckily for observers, our satellite’s face visibly changes nightly
as sunlight falls on its features from an ever-changing variety of
angles.
8
You’ll find downloadable PDF files of Bakich’s two previous
Moon observing stories at www.Astronomy.com/toc.
Michael E. Bakich is an Astronomy senior editor and veteran Moon watcher.
2
1
Rimae Atlas
Hercules
G
Vallis Schröteri
Cajal
Atlas
E
Sinas E
Grimaldi
Rima
Cauchy Cauchy
Rup
es C
auch
y
Herodotus
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Plotted here
are the locations of this
story’s 10
features.
Just match
the number
with the picture, point
your telescope at that
spot, and you’re
on your way to
observing 10 hot
targets on the Moon.
North is up in this image.
Crüger
Williams
Aristarchus
τ
ω
Mare Orientale
Aristarchus Crater in the Moon’s northwest quadrant has a diameter of 25 miles
and a depth of 9,850 feet. Try spotting this
extraordinarily bright crater without optical aid
in the dark section when the Moon is a thin
crescent in the western evening sky. Nearby
Herodotus Crater spans 21.8 miles. Vallis
Schröteri, the largest sinuous valley on the
Moon, starts 15.5 miles north of Herodotus and
runs nearly 100 miles. Paolo Lazzarotti
The craters Atlas (54 miles wide) and
Hercules (43 miles wide) are full of features observable through a 4-inch or larger telescope. On the floor of Atlas, look for a winding
system of clefts known as Rimae Atlas. To the
west of Atlas, Hercules features a dark floor and
craterlets G (8.7 miles wide) and E (5.6 miles
wide). Try to detect the ruined (partially submerged by lava floods) crater Williams to the
south of the large pair. Paolo Lazzarotti
2
The Cauchy Domes are subtle features
located near 7.7-mile-wide Cauchy Crater.
Two of the domes have labels — Tau (τ) Cauchy
and Omega (ω) Cauchy — in this photograph.
The crater itself is circular and appears bright at
Full Moon. To the south of Cauchy Crater, the
striking, 75-mile-long fault Rupes Cauchy
changes into a rille at its northern extreme. Only
part of Rima Cauchy appears here, but it runs for
130 miles. Richard Bosman
3
5
7
Grove
1
3
Most of Mare Orientale (Eastern Sea) sits
on the Moon’s farside, so it takes a favorable libration in longitude to show it at our satellite’s western edge. This 600-mile-wide basin
formed from the impact of a small asteroid. Also
note two craters with dark floors to the northeast of Mare Orientale — Crüger (29 miles wide)
and Grimaldi (138 miles wide). Lacus Aestatis
(Summer Lake) consists of the two elongated
dark patches north of Crüger. Paolo Lazzarotti
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10
6
4
Lick Observatory
Montes Apenninus
C
Rima Birt
Kepler
F
B
Rimae Plato
A
N
Kepler Crater lies between Oceanus Procellarum to the west and Mare Insularum
to the east. Kepler measures only 20 miles
across, but it dominates this region of lunar
landscape. When you view it, note its uneven
floor and the bright rays emanating from the
crater. Kepler’s associated craterlets (A, B, C, D,
and F) are easy to see. View similarly sized Encke
Crater, and look for 2.2-mile-wide Encke N,
which lies on its western wall. Paolo Lazzarotti
5
© 2012 Kalmbach Publishing Co. This material may not be reproduced in any form
www.Astronomy.com
• October 2011
58 Astronomy
without
permission from the publisher.
A
Montes Alpes
Vall
is A
lpes
Kunowsky
The eastern part of Mare Nubium (the
Sea of Clouds) contains the craters Nicollet
(9.4 miles wide) and Birt (10.6 miles wide). To
the west of Birt, a rille named Rima Birt runs to
the north-northwest from the crater. Mare
Nubium also features Rupes Recta — the
Straight Wall — a 68-mile-long, 1,000-foot-high
fault that slopes about 7°. Mare Nubium spans
445 miles. Look closely at its floor for surface
folds called wrinkle ridges. Alan Friedman
6
Montes Alpes lies between Mare Imbrium
and Mare Frigoris. The 60-mile-wide crater
Plato dominates this area. Move south of it to
the lunar Alps, and pay attention to Vallis Alpes,
a wide cleft that stretches 112 miles and measures 6 miles across at its widest point. This feature divides the northwestern third of the range
from the rest. Lava flooded its floor long ago.
Crank up the magnification and look for the
slender rille that bisects the valley. Alan Friedman
8
Theophilus
K
Sinus Aestuum
Wrinkle ridges
Drygalski
Observe Drygalski Crater, and learn a bit
about lunar libration. This 101-mile-wide
ring mountain lies at the Moon’s southern edge.
Most of the time, we get an oblique view of it.
Occasionally, we see more of the crater’s floor
because of libration, which is a small rocking
motion the Moon undergoes. At any time, half
of its surface is visible. What part we view
changes because of libration. In total, we see 59
percent of the lunar surface. Damian Peach
B
Eratosthenes
Mare
Imbrium
Birt
Encke
4
Ibn Rushd
Mare Frigoris
Rupes Recta
Nicollet
A
D
Plato
Cyrillus
Stadius
Sinus Aestuum (the Bay of Billows) is the
flat area that covers most of this image.
Eratosthenes Crater (36 miles wide) has large
terraced walls and central peaks. When the Sun
angle is low, Eratosthenes stands out strongly
because it’s 2.2 miles deep. In contrast, at Full
Moon (when the Sun is overhead there), the
crater seems almost to disappear. Compare it at
both times to Stadius Crater, which has incomplete, low walls. Paolo Lazzarotti
9
Theophilus and Cyrillus are two craters
lunar scientists classify as ring mountains.
Each has a diameter of 60 miles, but while
Theophilus has walls that rise 0.75 mile high,
Cyrillus has disintegrated walls, indicating it’s
much older. Theophilus’ central peaks rise 0.9
mile above the surrounding floor. The crater Ibn
Rushd formerly carried the designation Cyrillus
B, but the International Astronomical Union
renamed it in 1976. Paolo Lazzarotti
10
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