Explore 12 great lunar targets

Small-scope wonders
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Explore 12 great lunar targets
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Sharpen your observing skills on the Moon’s craters, lava flows, and an elusive letter X.
by Michael E. Bakich
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T
he Moon offers something for every observer. It has a
face that’s always changing. Following it telescopically
through a lunar month can be fascinating.
Ironically, when the Moon is brightest (Full Moon) is the
worst time to view it. From our perspective, the Sun is shining
on the Moon from a point directly behind us, minimizing shadows and thus revealing scant detail.
The best lunar viewing times are from when the thin crescent becomes visible after New Moon until about 2 days after
First Quarter (evening sky) and from about 2 days before Last
Quarter to almost New Moon (morning sky). Shadows are longer then, and features stand out in sharp relief.
This is especially true along the Moon’s shadow line — called
the terminator — which divides the light and dark portions.
Before Full Moon, the terminator shows where sunrise is occurring; after Full Moon, it marks the sunset line.
Along the terminator, you’ll see mountaintops protruding
high enough to catch sunlight while the dark lower terrain surrounds them. On large crater floors, you can follow “wall shadows” cast by sides of craters hundreds of feet high. All these
features seem to change in real time, and the differences you
can see in one night are striking.
The Moon orbits Earth approximately every 27.3 days.
Because Earth also orbits the Sun, the Moon and Sun line up
roughly every 29.5 days. The changing positions of the Moon
with respect to the Sun cause the Moon, as seen from Earth, to
cycle through a series of phases. One complete set of phases is a
lunar month. By definition, the lunar month begins at New
Moon. We can’t see New Moon from Earth because, at that
time, the Moon’s lit side faces the Sun. The Moon then is also
quite near the Sun in the sky.
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Archimedes Crater lies at 30° north latitude centered between the
eastern and western limbs. This 52-mile-wide (83 kilometers)
impact crater lies just northwest of the Moon’s largest mountain range,
the Montes Apenninus. Through an 8-inch or larger telescope, scan Archimedes’ floor for a large number of craterlets. For the best view of this crater, observe around First Quarter. North is up in all images. Alan Friedman
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Plotted here are the locations of this story’s 12 features.
Just match the number with the
picture, point your telescope at that
spot, and you’re on your way to observing
the Moon. North is up in this image. Lick Observatory
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Observing tips
The Moon is a brilliant object through a telescope. Many
observers employ either neutral density filters or variable polarizing ones to reduce the light. I prefer the latter because you can
change the amount of light filtered.
My late observing friend, Jeff Medkeff, introduced me to a
better way, however. Turn on a white light when observing the
Moon between First Quarter and Full. The addition of light
suppresses the eyes’ tendency to dark adapt and causes the eye
to use normal (scotopic) vision, which is of much higher quality
than dark-adapted (photopic) vision.
Lunar observers usually don’t use color filters. However, I do
have some friends who have noted a darkening of the lunar
basalt when they use a red filter. A red filter also can help
Michael E. Bakich is a senior editor of Astronomy.
Sinus Iridum, the Bay of Rainbows, on the Moon’s northwestern
2 edge, spans a whopping 162 miles (260 km). Flanking Sinus Iridum to the north is the C-shaped mountain range Montes Jura.
Numerous craterlets dot Sinus Iridum’s remarkably flat surface. The
largest is Laplace A, named for Laplace Promontorium, the bright
point that terminates the top of the “C.” Alan Friedman
© 2010 Kalmbach Publishing Co. This material may not be reproduced
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in any
Clavius Crater ranks as the third-largest crater on the Moon’s nearside. It’s visible to the naked eye and spans 140 miles (225 km). But
it’s what’s in Clavius that I suggest you observe. Look for the crater chain
of decreasing size that begins at Clavius’ eastern wall. Oblong Rutherfurd
Crater measures 33.5 miles by 30 miles (54 km by 48 km). Following it are
Clavius D (17 miles [28 km]), C (13 miles [21 km]), N (8 miles [13 km]), J (7.5
miles [12 km]), and JA (5 miles [8 km]). In addition to those, numerous craterlets that will test any size telescope dot Clavius’ floor. David Tyler
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Copernicus Crater is one of the Moon’s most famous formations.
This impact marks the center of a system of bright rays that extends
for up to 500 miles (800 km). Copernicus measures 58 miles (93 km) wide.
Because of its great depth — 12,300 feet (3,750 meters) — sunrise and
sunset shadows here create dramatic relief. The central peak rises 3,940
feet (1,200m) above the crater’s floor. Copernicus’ outer wall gives it a
peculiar hexagonal shape. Paolo Lazzarotti
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Torricelli Crater appears pear-shaped at first glance because its west5 ern wall is open and connects to a smaller crater. Both structures lie in
the upper part of a low-contrast circular formation named Torricelli R. The
prominent crater to the east is 6.8-mile-wide (11 km) Torricelli A. Torricelli
Crater measures 14.3 miles (23 km) across. Paolo Lazzarotti
Gassendi Crater, whose long axis measures 68 miles (110 km)
6 across, is a spot that will hold your attention. Numerous clefts, hills,
and central mountains interrupt its floor. To the north, the crater designated Gassendi A has broken its wall. Together, both craters give the
appearance of a diamond ring, especially at low magnification. Alan Friedman
Moretus Crater sits in a heavily impacted region near the Moon’s
9 south pole. When the Sun angle is low here, you’ll easily spot the central peak that rises 1.3 miles (2.1 km) above the surrounding floor. Moretus
measures 71 miles (114 km) wide. Note that Cysatus Crater immediately to
Moretus’ north is deeper, so it still lies in shadow in this image. David Tyler
Pitatus Crater, which spans 60 miles (97 km), contains features
strewn about its wide floor. A low central peak sits just to the northwest of the crater’s center. Through an 8-inch telescope, look for the thin
grooves called Rimae Pitatus on the western floor. More than 20 lettered
(catalogued) craterlets surround Pitatus. Also be sure to observe the
double-walled crater Hesiodus A directly to the west of Pitatus. Alan Friedman
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Lacus Mortis, the Lake of Death, spans 93 miles (150 km) and lies in
the Moon’s northeastern quadrant. It contains the 25-mile-wide (40
km) crater Bürg. Try to spot the rilles to the west (left in this image) of
Bürg, which run for some 60 miles (100 km). Lunar cartographers designated these collectively as Rimae Bürg. Damian Peach
Messier and Messier A are two small craters that sit on the Moon’s
8 eastern side only 2° south of its equator. Messier is an oblong crater
measuring 5.6 miles by 6.8 miles (9 km by 11 km). Messier A spans 8.1 miles
by 6.8 miles (13 km by 11 km). Two linear rays extend westward from Messier A for more than 60 miles (100 km). If your sky is steady, look for the thin
rille Rima Messier, which lies to the northwest of the craters. Anthony Ayiomamitis
Plato Crater lies at the Moon’s top center for observers. Plato spans
63 miles (101 km) and has one of the darkest crater floors on the
Moon. Polish astronomer Johannes Hevelius (1611–1687) called Plato the
Greater Black Lake. One of the features to observe within this crater is its
slumped inner wall, especially on the western (left) end. Even a small telescope at a magnification of about 100x will reveal the largest area, a triangular section that caved in millions of years ago. Alan Friedman
The Lunar X, also known as the Purbach or Werner Cross, appears at
First Quarter near the terminator between the craters La Caille, Purbach, and Blanchinus. It sits 25° south of the Moon’s equator and deadcenter left to right. When the Lunar X is visible, sunrise is occurring over
this region. The X remains visible for only 4 hours, so look carefully! Ohio
amateur astronomer Dana Thompson captured this image of the Lunar X
January 25, 2007, at 7:33 p.m. EST. Dana Thompson
improve the view when the atmosphere is unsteady. It reduces
the Moon’s brightness as well.
Two other methods can reduce the Moon’s brightness: high
magnification and an aperture mask. The first restricts the field
of view to a small area of the lunar surface and reduces light
throughput. The second is a cardboard mask with a smaller
hole cut out of it. Cover the front of your telescope with it, and
it will transform your telescope into one of smaller aperture
(letting less light through), but with the same focal length.
Lunar cartographers differentiate features on the Moon as
lighter areas called “highlands” and darker features called
“maria” (the Latin word for “seas” [singular = mare]). The
maria are lower in altitude than the highlands. The dark material inside the maria is solidified basaltic lava from periods of
volcanism up to about a billion years after the Moon formed.
Essentially, anything not covered by lunar basalt is a highland. The highlands consist of ancient lunar surface rock and
materials thrown out during the explosive impacts that formed
the basins. The highlands are an observer’s treasure trove of
mountains and valleys, bright areas and shadows.
Of the 1,940 named features on the Moon, astronomers classify 1,545 (nearly 80 percent) as craters. Many more craters dot
the highlands than maria sites. Craters range in size, and some
observers challenge themselves by either noting how small a
crater they can see or how many small craters in a given area
they can observe through a particular telescope.
If you take the second challenge, choose a mare or a crater
with a large, flat bottom, and search for craterlets (small craters). For example, on the large crater Plato’s (#11) floor are four
craterlets, each about 1.25 miles (2 kilometers) across. You
should be able to view those features through an 8-inch scope.
When observing larger craters, note whether you can see
“rays” emanating from them. Those features formed when
crushed rock sprayed out after an impact. Rays appear as radial
streaks and can lie a great distance from the crater itself. For a
good example, point your scope toward Copernicus (#4).
Dust off your scope
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One of the best ways to familiarize yourself with the Moon is to
undertake an observing project. Completing a project involves
observing every item on its list, such as the 12 in this story. The
Astronomical League hosts another such project, the Lunar
Observing Club. Anyone can view the list, but to receive the
certificate you must be a member of the league, either individually or through an astronomy club. For online details, see www.
astroleague.org/al/obsclubs/lunar/lunar1.html.
For the times when the Lunar X will be visible in 2010, go to
www.Astronomy.com/toc.
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