Seven Nights of Enticing Lunar Sights

Come along for a 7-night tour of some of the Moon's most compelling features visible with small telescopes.

Expanding act

As the Moon's terminator opens to celestial east this week, new features come into view each night.
Frank Barrett with additions by the author

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

Big and Small Sights

Lots of great features make April 25 a great start to a week of lunar observing. North is up in this and all photos.
Virtual Moon Atlas with additions by author

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.

Volcanism at Play

The "shore" of Sinus Iridum is formed by the arc of the Jura Mountains. Wrinkle ridges are faintly visible across the bay. At right, a half-dozen domes, several topped with pits, stand out in slanted sunlight north of Hortensius. See also the Kepler photo (below) for help in locating Hortensius.
Wikipedia CC BY-SA 3.0 (left) / Mike Wirths

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.

Get a Ringer

The 15-kilometer-wide concentric ring crater, Hesiodus A, offers Moon watchers a unique treat.
ACT-REACT Quickmap / LRO / NASA

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

Closeup on two craters

Two very different craters come into view on Thursday evening: Kepler with his crown of rays and the Gassendi, its floor riddled with cracks.
Virtual Moon Atlas with additions by author

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.

Do-Ray-Me

Kepler pals up with Copernicus to brighten the dark mare moonscape with cloaks of rays.
ACT-REACT Quickmap / NASA / LRO

Cracked and crumbled

Gassendi looks flatter than many craters because its floor was uplifted from below and its southern rim breached by lava. North is at upper right. Gassendi was one of three landing sites selected for Apollo 17, but was later discarded in favor of the Taurus-Littrow Valley. 
Damian Peach

April 27th / Moon's age: 12 days

Dome-town and a crater 'night light'

On Friday evening, we meet one of the brightest features on the Moon, the crater Aristarchus. To its south is smaller Marius and a neighboring region thick with domes called the Marius Hills.
Virtual Moon Atlas with additions by author

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.

Field of a Moon Observer's Dreams

The vast plains of Oceanus Procellarum are host to several remarkable ancient volcanic provinces and the brightest large crater on the Moon. The Hills don't show well in this light, but here's a better view taken at sunrise by Jim Phillips.
ACT-REACT Quickmap / NASA / LRO

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.

Technicolor plateau

I colorized part of this photo to match the color I see in the Aristarchus Plateau. How does it look through your scope?
NASA / LRO

April 28th / Moon's age: 13 days

Edgewise lunar sunrise

Features appear foreshortened along the terminator near full Moon, but there's still much to see including one of the longest rills on the Moon and giant crater Schickard crater.
Virtual Moon Atlas with additions by author

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.

 

Giant Crater with a Curious Neighbor

Schickard is a circular crater, but you'd never know from seeing it scrunched up near the lunar limb. Its next-door neighbor is Wargentin, a fascinating crater in its own right. Lava filled the crater to the brim, creating a circular plateau.
Wikipedia / CC BY-SA 3.0

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.

Really Long Rill

The Sirsalis Rill has both challenging and easy sections. See how far you can traverse this crack before it disappears in the rough terrain.
ACT-REACT Quickmap / NASA / LRO

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

Goddess of the regolith

During full Moon Sunday night, we can catch a colorful moonrise while imagining patterns in the maria and highlands including this likeness of a woman. The Moon has traditionally embodied female deities or spirits.
Virtual Moon Atlas with additions by author

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

Dappled seas

A day past full we drop in on two lesser observed maria before they're covered by the encroaching terminator, which now assumes a new role as the advancing line of lunar sunset.
Virtual Moon Atlas with additions by author

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.

Waning Sights

Moretus's central peak (top) rises from the crater's relatively smooth floor, while the "spotted maria" south of Mare Crisium have a unique broken appearance.
Damian Peach (left) / ACT-REACT Quickmap / NASA / LRO

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

Chain gang

A striking arrangement of similar-sized craters are etched in sunset light along the waning Moon's southeastern limb.
Virtual Moon Atlas with additions by author

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.

10 thoughts on “Seven Nights of Enticing Lunar Sights

  1. bwanabwana

    Great summary of what’s coming up on the Moon.

    I’ve been testing an ATIK Color Horizon camera with a Sky-Watcher 190 Mak-Newt for the past few days and what better target than the Moon!? I used a STC Duo-Spectra filter (10nm Oiii and 10nm Ha) to keep the exposure within reasonable limits.

    Image from April 25th at: https://www.flickr.com/photos/58664881@N02/40843070045/

    I’ll be following your guided tour list whenever the sky cooperates…

    Brian Allan

  2. Fabrice MoratFabrice Morat

    Hello Bob,

    Your “short letters” are increasingly lengthy ! I’ve observed recently the moon with my 600mm scope :
    On april 25th, Milichius PI dome was in my eyepiece but i find the summit crater quite difficult (seeing adequate but no more)
    On april 26th, Gassendi of course but i like to observe the crater opposite in Mare humorum, named Vitello with its fantastic C-shaped gap. The same night, the horseshoe-shaped Prinz with its numerous sinuous rilles, parallel, thiner and thiner. Rimae aristarchus were in the same direction at the limit of the terminator.
    Tonight (27th), seeing is variable, Aristarchus is always whitish, as marked with chalk. Cobra head is not my best sight because of the mediocre libration… The moon seems ill with its spotty surface : Marius hills are however well placed !

    Good luck for your new guide book,

    Fabrice M.

    1. Bob KingBob King Post author

      Fabrice,
      Vitello’s a fascinating crater with a serpent-shaped rill. Thanks for bring it up. I’ll have to include it in a future installment.

  3. Anthony BarreiroAnthony Barreiro

    I’m out of my league. I’ve been enjoying the Moon through 10×42 binoculars! And through clouds, and, as she heads south in her orbit, through my neighbor’s tree. Last night I walked up Bernal Hill to watch the full Moon rise through the fog bank over the east bay hills. I stuck around for 45 minutes to watch Jupiter follow the Moon, then I walked around to the other side of the hill to watch Venus and Aldebaran set into the fog bank over the Pacific. A lovely low-magnification, small aperture evening.

  4. EricV

    You made me curious now and interested to become an astronomist ( at age 59) , I liked a lot
    your patiënt and professional response to reactions on another topic, of what what visible
    on the moon from the earth . My first question is:
    cab you recommend which ( budget) camera and telescope I can start to make pictures or time lapse recordings of the moon and observe the moon and the sky. My second question relates to the retroreflectors they left behind on the moon and which they claim they can calculate the distance by the reflected particles.
    I read on a site that the distance that was calculated between the centre of the earth and the moon is 385000,6 km. And then lasers on earth are pointed to those reflectors . The size of the beams would be 4 miles at arrival at the moon, and a few lightparts would be reflected back per minute. I dont understand this: one the equator the earth would move with a speed of about 1500km per hour, so the laser situated their would have this speed. If the station is above or beyond, the speed would be lower. But the moon is circling around the earth also , and if the moon would be circling with in a circle with a radius of 385000,6km , then
    the speed would be 2419030.11317532 km/month
    and if the month is 30 days , the speed of the moon is 3359,76 km/hour
    How can they follow the reflector that has this speed , while the laser has also a speed of maybe 700-1500km/hour
    I know it is a technical question , but I think you would know the answer to this question as astronomer . Thanks in advance .

    1. Bob KingBob King Post author

      Hi Eric,

      Quite a good question. Let me try to answer as best as I can. The laser light makes the round trip from the Earth and back in 2.6 seconds. During that time, an observer at the equator, where the Earth’s rotational speed is about 1000 mph, travels 0.27 miles. The beam is already a few miles wide when it reaches the moon, and the retroreflector spreads it out even more. By the time it returns to Earth, the beam is 9 miles across. During the round trip, the observing station has only moved about 1/4 mile. Earth’s rotational velocity (tangent to the moon) causes the beam to shift an additional ~ 1.2 miles. Even with these shifts, the observing station is still well within the 9-mile beam width, allowing it to pick up a few of the returning photons. As for the moon’s orbital speed, I don’t believe it’s significant here because the distance the moon travels during the extremely brief time the beam touches and reflects off the retroreflector is negligible. I hope this helps in answering your question. For more details, you might enjoy this paper on the topic: https://arxiv.org/pdf/1309.6294.pdf

      1. EricV

        Dear Bob, I can’t imagine how only a few of the billion light particles are reflected and can then be traced back to the original beam, I understand that the particles have a certain signature, and must be
        then from that same beam. But my question is, will not random particles be reflected back as there are water, ice and metal parts in the atmosphere .
        The more important question I had is this one:
        You say that the movement of the moon ( 3359,76 km per hour) is not so important because the time the beam reach it and is bounced is a millionth of a second or less, and what does the moon move with that speed ? Maybe 1mm!! But the great mystery : how do you point a beam to touch the reflector that goes with a speed of 3359,76km/hour
        while your sending equipment travels with 1500km/hour ?
        In the video I saw from mythbusters they do not speak about any speed of the moon, they aim the beam , they find the spot , and then the return particles prove the fact that the reflector is there , without touching the aiming question .
        So how they aim?

        1. Bob KingBob King Post author

          Eric,

          Another great question. Remember that the telescope is motorized and tracks the Moon precisely: through the eyepiece the moon does not move. That lets the astronomer aim the beam exactly at the spot where the retroreflector is located. The beam also spreads out as we discussed earlier, so as long as part of it reaches the reflector, that’s enough to intercept some of the returning photons.

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