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The 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 shad-ows and thus revealing scant detail.

The best lunar viewing times are from when the thin cres-cent 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 lon-ger 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 occur-ring; 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 sur-rounds them. On large crater floors, you can follow “wall shad-ows” 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.

Observing tipsThe Moon is a brilliant object through a telescope. Many observers employ either neutral density filters or variable polar-izing 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

Sharpen your observing skills on the Moon’s craters, lava flows, and an elusive letter X.by Michael E. Bakich

Small-scope wonders

Michael E. Bakich is a senior editor of Astronomy.

Explore 12 great lunar targets

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 Archi-medes’ floor for a large number of craterlets. For the best view of this cra-ter, observe around First Quarter. North is up in all images. AlanFriedman

Sinus Iridum, the Bay of Rainbows, on the Moon’s northwestern edge, spans a whopping 162 miles (260 km). Flanking Sinus Iri-

dum 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.” AlanFriedman

Clavius Crater ranks as the third-largest crater on the Moon’s near-side. 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 cra-terlets that will test any size telescope dot Clavius’ floor. DavidTyler

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. PaoloLazzarotti

Plotted here are the loca-tions 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. LickObservatory

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62 Astronomy • October 09 www.Astronomy.com 63

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 cra-ters). 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 scopeOne 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 individu-ally or through an astronomy club. For online details, see www.astroleague.org/al/obsclubs/lunar/lunar1.html.

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 mate-rial 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 high-land. 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 clas-sify 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

For the times when the Lunar X will be visible in 2010, go to www.Astronomy.com/toc.

Torricelli Crater appears pear-shaped at first glance because its west-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. PaoloLazzarotti

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 desig-nated these collectively as Rimae Bürg. DamianPeach

Gassendi Crater, whose long axis measures 68 miles (110 km) across, is a spot that will hold your attention. Numerous clefts, hills,

and central mountains interrupt its floor. To the north, the crater desig-nated Gassendi A has broken its wall. Together, both craters give the appearance of a diamond ring, especially at low magnification. AlanFriedman

Messier and Messier A are two small craters that sit on the Moon’s 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 Mess-ier 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. AnthonyAyiomamitis

Moretus Crater sits in a heavily impacted region near the Moon’s south pole. When the Sun angle is low here, you’ll easily spot the cen-

tral 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. DavidTyler

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 tele-scope at a magnification of about 100x will reveal the largest area, a trian-gular section that caved in millions of years ago. AlanFriedman

Pitatus Crater, which spans 60 miles (97 km), contains features strewn about its wide floor. A low central peak sits just to the north-

west 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. AlanFriedman

The Lunar X, also known as the Purbach or Werner Cross, appears at First Quarter near the terminator between the craters La Caille, Pur-

bach, and Blanchinus. It sits 25° south of the Moon’s equator and dead-center 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. DanaThompson

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