Come along for a 7-night tour of some of the Moon's most compelling features visible with small telescopes.
The Moon is waxing this week. Some skywatchers might take that as a cue to catch up on sleep. Not us. We'll use the opportunity to focus on favorite lunar features on each of the upcoming seven nights.
The terminator is the moving line that separates the sunny from the night side of the Moon. During the waxing phases, it slowly rolls back like a curtain in a play to reveal new scenes and characters. If you own a telescope, fresh features come into view every night: magnificent craters like Copernicus and Gassendi, riverine rills, and ancient volcanic domes.
Because of lunar libration and the Moon's ever-changing phase, the same viewing conditions rarely repeat. You can observe our satellite for years and still see things you never noticed before or catch a favorite feature under just the right light.
The accompanying maps, all made using Patrick Chevalley's and Christian Legrand's excellent Virtual Moon Atlas, show the Moon's appearance for the coming week at 10 p.m. CDT (11 p.m. EDT, 9 p.m. MDT, and 8 p.m. PDT). Most of the features are big enough to see in any telescope, with only a few requiring at most a 6-inch. Except where noted, compass directions are lunar not celestial, i.e. the terminator moves from east to west across the Moon.
April 25th / Moon's age: 10 Days
A great night to begin! The terminator still slices through heavily cratered terrain in the southern highlands as it pushes west toward the vast mare Oceanus Procellarum (Ocean of Storms). Copernicus crater and its sizable nimbus of hazy rays are now in full view. The crater itself is 93 km across, but its radial rays extend some six Copernicus-widths in all directions, making it relatively easy to identify the Copernicus region with the naked eye.
The crater boasts amphitheater-like, slumped walls and a relatively smooth floor paved in impact melt and dotted with several small peaks that rise up to 1.2 km above the floor.
Not far to the southwest of the crater, look for the15-km bowl of Hortensius. Just to its north you'll spy up to six lunar domes, the remains of shield volcanoes that oozed lava billions of years ago. They range in size from 6–15 km wide. Several have tiny craterlets atop their summits that served as vents for the lava source below. If the atmosphere is serene, use magnifications of 200× and higher.
To the north you can't miss the crisp semi-circle of light along the northwestern border of Mare Imbrium that forms the "shoreline" of Sinus Iridum, the Bay of Rainbows. Under a low Sun, it's one of the Moon's most eye-catching features. The rim is all that remains of a large crater that later flooded with lava to form a plain or bay. Look for delicate, parallel "wrinkle ridges" in the bay, where basaltic lavas cooled and contracted, resulting in folding and faulting of the lunar crust.
Next, we head south to the concentric ring crater Hesiodus A, located just west of the much larger crater Pitatus. The double-ringed symmetry of this small crater is absolutely captivating. There are few craters like it. A variety of hypotheses have been proposed to explain the origin of concentric craters' unique inner rings, everything from simultaneous impacts to mass wasting (rock and soil moving downslope under the force of gravity) to impacts in dual-layered domes.
Our final stop is the crater Clavius located a short distance south of Tycho, which is just now "growing" into its rays as full Moon approaches. Despite its location smack dab in the Moon's crater-rich southern highlands, 231-km-wide Clavius dominates the region through sheer size. An astronaut standing in its center would see only a broad, rimless plain in every direction. Clavius is big enough to have its own craters, including an attractive arc of impact holes that bullet across its center.
April 26th / Moon's age: 11 days
Two prominent craters come into view tonight: Kepler, a sharp, 32-km-wide bowl at the center of a splashy ray pattern visible with the naked eye, especially at full Moon. The rays extend more than 300 km and overlap with the feathery outliers of Copernicus just to its east.
Gassendi is an ancient, battered crater 110 km across with low walls, a floor wormed with cracks called rills and three central peaks. At its south end, the rim was breached by mare lavas that flooded and smoothed over the area. The resulting dark moat stands in stark contrast to the crusty appearance of the rest of the crater.
Lava not only breached the wall but welled up from underneath, uplifting and fracturing Gassendi's interior to create the fascinating complex of rills. I've always enjoyed testing my visual acuity and patience by trying to ferret out these delicate cracks when the air steadies and the view suddenly sharpens. The easiest to discern are the three widest south of the central peaks. Use high magnification.
April 27th / Moon's age: 12 days
The ancient Greek astronomer Aristarchus had a brilliant idea placing the Sun at the center of the solar system. He's recognized for that achievement with one of the brightest craters on the Moon. In spite of its modest 40-km size and ray system, Aristarchus is easily visible with the naked eye as a small, bright spot in the northeast (celestial direction) corner of the Moon, especially around full.
The crater is located along the edge of the Aristarchus Plateau, a 200-km-wide block of lunar crust that rises 2 km above the local mare landscape. While much of the Moon appears decidedly black-and-white, this region has a distinctive brownish-green color to my eye. I'd be curious to know what you see.
Volcanism may have occurred during the plateau's formation as there are numerous sinuous rills in the area, including one of the Moon's best, Schroeter's Valley. The small asteroid that blasted out Aristarchus arrived relatively recently, only 450 million years ago, the reason the crater looks so bright. Too little time has passed for space weathering to darken it.
South of Aristarchus look for a similar size crater with a smooth floor called Marius. Immediately west and north of Marius you'll see a pimply landscape of volcanic domes called the Marius Hills. The domes are up to a few kilometers across and rise from 200–500 meters high. As with any low profile lunar feature, they're best seen when the terminator is nearby or around the time of lunar sunrise (or sunset). Tonight's the night! A magnification of 150× should snap them into view.
April 28th / Moon's age: 13 days
We're now a day before full Moon, and all the features we see along the terminator appear increasingly foreshortened because we view them nearly edge-on. That turns the 227-km-wide crater Schickard into giant bathtub. As big as Clavius, Schickard's uplifted and lava-flooded interior hosts fewer prominent craters, but it does have one curious feature — a wide swath of lighter material draped over the darker interior.
According to Charles Wood in his book The Modern Moon: A Personal View, Schickard has a compelling backstory. After its formation more than 3.8 billion years ago, lava seeped through cracks in the crater's floor and spread across its breadth. Millions of years later, the giant impact that gouged out the giant Mare Orientale bulls-eye to the west, lofted billions of tons of pale lunar highland rock skyward, blanketing the crater with debris when it rained back down.
Later, new lavas bubbled up at the north and south ends of the crater and partially covered the ejecta, leaving the central apron intact. Amazing how much history can stare back at you through the eyepiece.
To the north of Schickard look for a pair overlapping, foreshortened craters, Sirsalis and its older, more eroded companion, Sirsalis A. Extending from the heavily cratered highlands south of the pair north to the dark shore of Oceanus Procellarum, you'll spy a lengthy crack called the Sirsalis Rill. At 386 km long × 3 km wide, it's one of the longest on the Moon. Unlike the winding, river-like sinuous variety, which once channeled lava, Sirsalis is a straight rill or graben — a section of crust that sank between two parallel faults. Parts of its can be challenging, one of the reasons I like to return here often. The easier sections include a piece near the twin craters and the large arc at its south end.
April 29th / Moon's age: 14 days
Full Moon is the best time to make faces on an extraterrestrial canvas. The man-in-the-Moon is the most familiar, but there's also a rabbit and a woman. I've outlined the woman, who looks a lot like my grandma with the 1940s hair and pearl necklace.
Lucky for us, the nearside has a good mix of dark maria — impact basins punched out by asteroids that later filled with dark, titanium-rich lavas — and lighter-toned highland crust rich in aluminum and calcium. The contrast between the two makes it easy to picture faces and other patterns. Were the crater-rich, mare-poor farside aimed our way, the task would be more challenging!
April 30th / Moon's age: 15 days
This is an extra special night. Besides our highlights, the Moon will occult the 4th-magnitude star Gamma (γ) Librae around 11:30 EDT (12:30 CDT, 10:30 MDT, and 9:30 PDT) for observers across much of the United States, western Canada, and northern South America. Click here for a map and table of cities with the times of disappearance and reappearance.
Tycho and its spider web of rays captivate around the time of full Moon, but we'll sidestep this celebrity and visit the crater Moretus in the densely cratered highlands to the south. With a favorable libration tipping the southern hemisphere into good view, we get to relish this deep crater and its prominent central peak that rises 2.1 km above the 5-km-deep floor.
Central peaks are rebound features that expose deep layers of the lunar crust that would otherwise be inaccessible to future astronaut geologists. They'll be key places to visit and sample by both humans and robots in the (hopefully) near future.
Way up in the northeast near the dark oval of Mare Crisium, look for two patchy, side-by-side maria, Mare Undarum and Mare Spumans, the sea of waves and foaming sea, respectively. I think a lot of us ignore them because they they're small seas sliced and diced by highlands material and often "crushed" by foreshortening even during the best librations. Dark-floored craters in the vicinity give the entire region the appearance of "lake country." Float your boat here a few minutes and take in the sights.
May 1st / Moon's age: 16 days
With the Moon rising around 10 o'clock local time, it's starting to get late, making tonight a good time to wrap up. We can't do better than the showy arrangement of similar-sized craters equally spaced in a gentle arc along the southeastern limb. Their mellifluous names bear repeating out loud: Langrenus, Vendelinus, Petavius, Furnerius.
Vendelinus and Furnerius are both beaten down and weathered, while the other two have sharper walls and prominent central peaks. If any crater is the highlight in the group, it's 177-km-wide Petavius with its double-rim, domed floor warped by subsurface lava flows and neatly sliced by a wide rill that stands out in even a 3-inch scope.
I hope you enjoyed these lunar excursions. By seeing the ubiquitous evidence of impact and volcanism we can all better appreciate how vibrant and "alive" the Moon once was.