Alpine Valley ... Altai Scarp ... Apennines** ... Archimedes** ... Ariadaeus Rille ... Aristarchus** ... Cauchy ... Clavius** ... Copernicus* ... Crisium* ... Earthshine* ... Fracastorius** ... Full Moon* ... Frigoris* ... Gassendi** ... Grimaldi* ... Humorum* ... Hyginus Rille ... Kepler* ... Langrenus ... Lunar Surface* ... Messier ... Nectaris* ... New Moon ... Orientale ... Petavius** ... Plato* ... Posidonius** ... Proclus ... Rupes Recta ... Sacrobosco ... Schickard** ... Schroter's Valley ... Serenitatis* ... Sinus Iridum* ... Theophilus, Cyrillus, Catharina** ... Tycho* ... Vaporum* ... Wargentin.
Tycho
(Wood #6)
Despite being smack amidst the tortured
topography
of the southern lunar highlands, crater Tycho is perhaps the most
obvious
feature on the moon. As a matter of fact, it is one of
Cherrington's
ten most notable lunar landmarks. And tonight, the 56 mile (90
km)
wide and 14,000 feet (4300 meters) deep impact crater stands
magnificently
bathed in either a lunar sunrise (lunar day 9) or a lunar sunset (lunar
day 22). But its famous prominent rays of debris, hurled radially
outwards from a long-ago impact, are better seen during lunar days 14
through
16, when the moon is full. Tycho is "the most extensive rayed
crater"
on the moon (Rukl) and Wood adds "... an epicenter ... with
impact
melts." It is thus Wood's #6 on his list of 100 notable lunar
features.
Cherrington believes that Tycho is very young, "no
more than 50 million years old."
A wondrous 1967 close-up picture of Tycho by Lunar
Orbiter 5 highlights a fine article by Wood in Sky & Telescope,
August
1999; in which he discusses crater Tycho in depth, including its
impactor,
the rays, and the melts.
Clavius
(Wood #9)
The enormous oval crater Clavius, some 152x132
miles
in size (243x211 km), is easily found on the Moon's southwest limb, not
far from its south pole. Cherrington points out that Clavius is
so
large that if one were to stand at its center then its 16,000 foot
perimeter
walls would not be visible due to the Moon's curvature and "the
observer
would have the impression of standing on a limitless, flat plain."
Described by Grego as a "superb walled plain
sometimes
naked-eye visible when near the terminator," Clavius is best observed
on
lunar day 9 (sunrise) or on lunar day 22 (sunset) when the sun angle is
low.
Wood points out that Clavius "lacks basin features
despite its size" and should therefore be considered a peculiar
geological
oddity. Note the lack of terraces and slumps on the inner side of
its perimeter walls.
A close-up picture of Clavius, "well seen on the
nights following the First Quarter Moon," is found in Sky &
Telescope,
August 2003, page 138.
Plato
Plato is a beautiful walled plain with a dark
flat
floor sunk into the Lunar Alpines (Montes Alpes) on the north shore of
the vast Mare Imbrium. Clearly circular, with a diameter of about
65 miles (104 km), the crater is considered by Cherrington as one of
the
ten most important landmark features on the Moon's near side.
Plato
is one of the most active sites for transient lunar phenomena (TLPs)
with
70 of 1,200 sightings having occurred here since 1783. Its dark
floor,
which "often mesmerizes observers" (Burnham, Astronomy, November 2004,
page 74), contrasts nicely with the surrounding light-colored
highlands.
Wood's #83 is four tiny craterlets in Plato's
floor.
They average a bit more than a mile (2 km) wide and are at the "limits
of detection" (Wood).
Rupes Recta
(Wood # 15)
Once the Moon had cooled after vast outpourings
of
lava during the early stage of its development, its surface began to
crack
(fault) in response to the lessened pressures of the cooling
process.
These "tension-release" faults, often called "normal" faults, are found
everywhere on the moon's surface and the fault known as Rupes Recta,
the
"Straight Fault" or the "Straight Wall," is considered the finest
example
of a lunar normal fault. A fine picture appears in Sky &
Telescope,
November 2004, page 138.
Rupes Recta, in southeastern Mare Nubium, is a very
prominent feature, quite straight and rather lengthy at some 110 km
(nearly
70 mi) long with a vertical height of 240-300 meters (800-1000 ft) and
an apparent width of 2.5 km (1.5 mi). Rupes Recta is most easily
seen when the sun angle is low, especially during the sunrise of lunar
day 9 or the sunset of lunar day 22.
The surface expression of a fault is known as a
scarp. A scarp can be quite steep (cliff-like) or sloping so
gradually
that it would be virtually invisible to anyone on the surface but
nevertheless
apparent from altitude.
Although it appears cliff-like to lunar observers,
the slope of Rupes Recta is quite moderate, rising only about one foot
vertically for every nine feet horizontally (1:9) or about seven
degrees,
which is about the steepest gradient usually encountered on the U.S.
Interstate
Highway system.
Can you determine which side of the fault is lower?
Miscellany
Note that Pickering's 6th naked-eye feature,
which
he described as "The notch between Serenitatis and Tranquillitatis," is
obvious between Promontory Archerusia and Mount Argaeus. The
notch
is indeed visible to the naked eye.
Mons Pico (Wood #23), apparently a mountain
peak poking through the flat terrain of northeastern Mare Imbrium, is
called
a basin ring fragment by Wood. There are many such isolated
mountains,
known as "monadnocks" to geologists or "nunataks" to the Eskimos of
Greenland,
scattered about the lunar surface; but unlike the monadnocks found on
Earth,
which are erosional remnants, these lunar features cannot be erosional
remnants but rather the protruding peaks of
buried mountains. Mons Pico seems to be one of the largest.
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