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Explore 12 Great Lunar Targets Sharpen Your Observing Skills on the Moon’S Craters, Lava Flows, and an Elusive Letter X

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Small-scope wonders 11 7 Explore 12 great lunar targets Sharpen your observing skills on the Moon’s craters, lava flows, and an elusive letter X. 2 1 by Michael E. Bakich he Moon offers something for every observer. It has a face that’s always changing. Following it telescopically T 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. 4 The best lunar viewing times are from when the thin cres- 5 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. 8 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- 12 ring; after Full Moon, it marks the sunset line. 6 10 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 1 Archimedes Crater lies at 30° north latitude centered between the features seem to change in real time, and the differences you eastern and western limbs. This 52-mile-wide (83 kilometers) impact crater lies just northwest of the Moon’s largest mountain range, can see in one night are striking. the Montes Apenninus. Through an 8-inch or larger telescope, scan Archi- The Moon orbits Earth approximately every 27.3 days. medes’ floor for a large number of craterlets. For the best view of this cra- Plotted here are the loca- tions of this story’s 12 features. Because Earth also orbits the Sun, the Moon and Sun line up ter, observe around First Quarter. North is up in all images. Alan Friedman 3 9 Just match the number with the roughly every 29.5 days. The changing positions of the Moon picture, point your telescope at that with respect to the Sun cause the Moon, as seen from Earth, to spot, and you’re on your way to observing cycle through a series of phases. One complete set of phases is a the Moon. North is up in this image. Lick Observatory 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 tips The 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 3 Clavius Crater ranks as the third-largest crater on the Moon’s near- than dark-adapted (photopic) vision. side. It’s visible to the naked eye and spans 140 miles (225 km). But 4 Copernicus Crater is one of the Moon’s most famous formations. Lunar observers usually don’t use color filters. However, I do 2 Sinus Iridum, the Bay of Rainbows, on the Moon’s northwestern it’s what’s in Clavius that I suggest you observe. Look for the crater chain This impact marks the center of a system of bright rays that extends have some friends who have noted a darkening of the lunar edge, spans a whopping 162 miles (260 km). Flanking Sinus Iri- of decreasing size that begins at Clavius’ eastern wall. Oblong Rutherfurd for up to 500 miles (800 km). Copernicus measures 58 miles (93 km) wide. dum to the north is the C-shaped mountain range Montes Jura. Crater measures 33.5 miles by 30 miles (54 km by 48 km). Following it are Because of its great depth — 12,300 feet (3,750 meters) — sunrise and basalt when they use a red filter. A red filter also can help Numerous craterlets dot Sinus Iridum’s remarkably flat surface. The Clavius D (17 miles [28 km]), C (13 miles [21 km]), N (8 miles [13 km]), J (7.5 sunset shadows here create dramatic relief. The central peak rises 3,940 largest is Laplace A, named for Laplace Promontorium, the bright miles [12 km]), and JA (5 miles [8 km]). In addition to those, numerous cra- feet (1,200m) above the crater’s floor. Copernicus’ outer wall gives it a Michael E. Bakich is a senior editor of Astronomy. point that terminates the top of the “C.” Alan Friedman terlets that will test any size telescope dot Clavius’ floor. David Tyler peculiar hexagonal shape. Paolo Lazzarotti © 2010 Kalmbach Publishing Co. This material may not be reproduced • 60 Astronomy Octoberin any 09 form without permission from the publisher. www.Astronomy.com www.Astronomy.com 61 Pitatus Crater, which spans 60 miles (97 km), contains features Torricelli Crater appears pear-shaped at first glance because its west- Gassendi Crater, whose long axis measures 68 miles (110 km) Moretus Crater sits in a heavily impacted region near the Moon’s 10 strewn about its wide floor. A low central peak sits just to the north- 5 ern wall is open and connects to a smaller crater. Both structures lie in 6 across, is a spot that will hold your attention. Numerous clefts, hills, 9 south pole. When the Sun angle is low here, you’ll easily spot the cen- west of the crater’s center. Through an 8-inch telescope, look for the thin the upper part of a low-contrast circular formation named Torricelli R. The and central mountains interrupt its floor. To the north, the crater desig- tral peak that rises 1.3 miles (2.1 km) above the surrounding floor. Moretus grooves called Rimae Pitatus on the western floor. More than 20 lettered prominent crater to the east is 6.8-mile-wide (11 km) Torricelli A. Torricelli nated Gassendi A has broken its wall. Together, both craters give the measures 71 miles (114 km) wide. Note that Cysatus Crater immediately to (catalogued) craterlets surround Pitatus. Also be sure to observe the Crater measures 14.3 miles (23 km) across. Paolo Lazzarotti appearance of a diamond ring, especially at low magnification. Alan Friedman Moretus’ north is deeper, so it still lies in shadow in this image. David Tyler double-walled crater Hesiodus A directly to the west of Pitatus. Alan Friedman Plato Crater lies at the Moon’s top center for observers. Plato spans The Lunar X, also known as the Purbach or Werner Cross, appears at Messier and Messier A are two small craters that sit on the Moon’s 11 63 miles (101 km) and has one of the darkest crater floors on the 12 First Quarter near the terminator between the craters La Caille, Pur- Lacus Mortis, the Lake of Death, spans 93 miles (150 km) and lies in 8 eastern side only 2° south of its equator. Messier is an oblong crater Moon. Polish astronomer Johannes Hevelius (1611–1687) called Plato the bach, and Blanchinus. It sits 25° south of the Moon’s equator and dead- 7 the Moon’s northeastern quadrant. It contains the 25-mile-wide (40 measuring 5.6 miles by 6.8 miles (9 km by 11 km). Messier A spans 8.1 miles Greater Black Lake. One of the features to observe within this crater is its center left to right. When the Lunar X is visible, sunrise is occurring over km) crater Bürg. Try to spot the rilles to the west (left in this image) of by 6.8 miles (13 km by 11 km). Two linear rays extend westward from Mess- slumped inner wall, especially on the western (left) end. Even a small tele- this region. The X remains visible for only 4 hours, so look carefully! Ohio Bürg, which run for some 60 miles (100 km). Lunar cartographers desig- ier A for more than 60 miles (100 km). If your sky is steady, look for the thin scope at a magnification of about 100x will reveal the largest area, a trian- amateur astronomer Dana Thompson captured this image of the Lunar X nated these collectively as Rimae Bürg. Damian Peach rille Rima Messier, which lies to the northwest of the craters. Anthony Ayiomamitis gular section that caved in millions of years ago. Alan Friedman January 25, 2007, at 7:33 p.m. EST. Dana Thompson improve the view when the atmosphere is unsteady. It reduces maria are lower in altitude than the highlands. The dark mate- crater they can see or how many small craters in a given area Dust off your scope the Moon’s brightness as well. rial inside the maria is solidified basaltic lava from periods of they can observe through a particular telescope. One of the best ways to familiarize yourself with the Moon is to Two other methods can reduce the Moon’s brightness: high volcanism up to about a billion years after the Moon formed.
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