10Great Features for Moon Watchers

Sinus Aestuum is a lava pond hemming the Imbrium debris. Mare Orientale is another of the Moon’s large impact basins, Beginning observing On its eastern edge, dark volcanic material erupted explosively and possibly the youngest. Lunar scientists think it formed 170 along a rille. Although this region at first appears featureless, million years after Mare Imbrium. And although “Mare Orien- observe it at several different lunar phases and you’ll see the tale” translates to “Eastern Sea,” in 1961, the International dark area grow more apparent as the Sun climbs higher. Astronomical Union changed the way astronomers denote great features for Occupying a region below and a bit left of the Moon’s dead lunar directions. The result is that Mare Orientale now sits on center, Mare Nubium lies far from many lunar showpiece sites. the Moon’s western limb. From Earth we never see most of it. Look for it as the dark region above magnificent Tycho Crater. When you observe the Cauchy Domes, you’ll be looking at Yet this small region, where lava plains meet highlands, con- shield volcanoes that erupted from lunar vents. The lava cooled Moon watchers tains a variety of interesting geologic features — impact craters, slowly, so it had a chance to spread and form gentle slopes. 10Our natural satellite offers plenty of targets you can spot through any size telescope. lava-flooded plains, tectonic faulting, and debris from distant In a geologic sense, our Moon is now quiet. The only events by Michael E. Bakich impacts — that are great for telescopic exploring. interrupting its long sleep are occasional meteoroid impacts. You’ll note that Mare Nubium does not fill an obvious, cir- Luckily for observers, our satellite’s face visibly changes nightly his story explores 10 lunar features that show you a bit rugged Montes Alpes, the lunar Alps. Unlike Earth’s tectoni- cular basin like Mare Imbrium. A low-power view reveals Mare as sunlight falls on its features from an ever-changing variety of about lunar geology. Five are craters (or pairs of craters). cally created peaks, however, these Alps form part of a gigantic Nubium’s curved edges, hinting that it formed by the merging angles. TTheir geology is simple: A meteoroid falls on the lunar crater rim: the boundary of the Imbrium Basin, which formed of a few immense impacts. surface, forming a crater. Other features formed differently. when an asteroid struck the Moon 3.85 billion years ago. 8 An evening or two past First Quarter — or the same time You’ll find downloadable PDF files of Bakich’s two previous Moon observing stories at www.Astronomy.com/toc. before Last Quarter — sunlight falls at a low angle across the Michael E. Bakich is an Astronomy senior editor and veteran Moon watcher. 2 1 Rimae Atlas Hercules G Rima Grimaldi 9 Vallis Schröteri Atlas Cajal Cauchy Cauchy E Plotted here 3 are the loca- Rupes Cauchy 5 Sinas E Crüger tions of this Herodotus story’s 10 Aristarchus Williams features. τ ω Just match Mare Orientale the number Grove with the pic- 7 ture, point Aristarchus Crater in the Moon’s north- The craters Atlas (54 miles wide) and The Cauchy Domes are subtle features Most of Mare Orientale (Eastern Sea) sits your tele- 10 1 west quadrant has a diameter of 25 miles 2 Hercules (43 miles wide) are full of fea- 3 located near 7.7-mile-wide Cauchy Crater. 7 on the Moon’s farside, so it takes a favor- scope at that and a depth of 9,850 feet. Try spotting this tures observable through a 4-inch or larger tele- Two of the domes have labels — Tau (τ) Cauchy able libration in longitude to show it at our sat- spot, and you’re extraordinarily bright crater without optical aid scope. On the floor of Atlas, look for a winding and Omega (ω) Cauchy — in this photograph. ellite’s western edge. This 600-mile-wide basin on your way to 6 in the dark section when the Moon is a thin system of clefts known as Rimae Atlas. To the The crater itself is circular and appears bright at formed from the impact of a small asteroid. Also observing 10 hot crescent in the western evening sky. Nearby west of Atlas, Hercules features a dark floor and Full Moon. To the south of Cauchy Crater, the note two craters with dark floors to the north- targets on the Moon. Herodotus Crater spans 21.8 miles. Vallis craterlets G (8.7 miles wide) and E (5.6 miles striking, 75-mile-long fault Rupes Cauchy east of Mare Orientale — Crüger (29 miles wide) North is up in this image. 4 Schröteri, the largest sinuous valley on the wide). Try to detect the ruined (partially sub- changes into a rille at its northern extreme. Only and Grimaldi (138 miles wide). Lacus Aestatis Moon, starts 15.5 miles north of Herodotus and merged by lava floods) crater Williams to the part of Rima Cauchy appears here, but it runs for (Summer Lake) consists of the two elongated Lick Observatory runs nearly 100 miles. Paolo Lazzarotti south of the large pair. Paolo Lazzarotti 130 miles. Richard Bosman dark patches north of Crüger. Paolo Lazzarotti

Montes Apenninus C Rima Birt Mare Frigoris Ibn Rushd Kepler Plato B

F Nicollet Rupes Recta B Rimae Plato D A Eratosthenes Birt Mare Imbrium K Theophilus A Encke A Montes Alpes N Sinus Aestuum Wrinkle ridges Cyrillus Drygalski Vallis Alpes Kunowsky Stadius

Observe Drygalski Crater, and learn a bit Kepler Crater lies between Oceanus Pro- The eastern part of Mare Nubium (the Montes Alpes lies between Mare Imbrium Sinus Aestuum (the Bay of Billows) is the Theophilus and Cyrillus are two craters 4 about lunar libration. This 101-mile-wide 5 cellarum to the west and Mare Insularum 6 Sea of Clouds) contains the craters Nicollet 8 and Mare Frigoris. The 60-mile-wide crater 9 flat area that covers most of this image. 10 lunar scientists classify as ring mountains. ring mountain lies at the Moon’s southern edge. to the east. Kepler measures only 20 miles (9.4 miles wide) and Birt (10.6 miles wide). To Plato dominates this area. Move south of it to Eratosthenes Crater (36 miles wide) has large Each has a diameter of 60 miles, but while Most of the time, we get an oblique view of it. across, but it dominates this region of lunar the west of Birt, a rille named Rima Birt runs to the lunar Alps, and pay attention to Vallis Alpes, terraced walls and central peaks. When the Sun Theophilus has walls that rise 0.75 mile high, Occasionally, we see more of the crater’s floor landscape. When you view it, note its uneven the north-northwest from the crater. Mare a wide cleft that stretches 112 miles and mea- angle is low, Eratosthenes stands out strongly Cyrillus has disintegrated walls, indicating it’s because of libration, which is a small rocking floor and the bright rays emanating from the Nubium also features Rupes Recta — the sures 6 miles across at its widest point. This fea- because it’s 2.2 miles deep. In contrast, at Full much older. Theophilus’ central peaks rise 0.9 motion the Moon undergoes. At any time, half crater. Kepler’s associated craterlets (A, B, C, D, Straight Wall — a 68-mile-long, 1,000-foot-high ture divides the northwestern third of the range Moon (when the Sun is overhead there), the mile above the surrounding floor. The crater Ibn of its surface is visible. What part we view and F) are easy to see. View similarly sized Encke fault that slopes about 7°. Mare Nubium spans from the rest. Lava flooded its floor long ago. crater seems almost to disappear. Compare it at Rushd formerly carried the designation Cyrillus changes because of libration. In total, we see 59 Crater, and look for 2.2-mile-wide Encke N, 445 miles. Look closely at its floor for surface Crank up the magnification and look for the both times to Stadius Crater, which has incom- B, but the International Astronomical Union percent of the lunar surface. Damian Peach which lies on its western wall. Paolo Lazzarotti folds called wrinkle ridges. Alan Friedman slender rille that bisects the valley. Alan Friedman plete, low walls. Paolo Lazzarotti renamed it in 1976. Paolo Lazzarotti