L46 Regiomontanus
A worn highland crater whose central peak stands unusually tall and sharp for its size — steep enough that some observers have long suspected a volcanic vent rather than simple impact rebound.
Source: IAU/Gazetteer & Sky & Telescope Lunar 100 (Wood, 2004)
L46 Regiomontanus Central Peak
Southern Highlands · Debunked “Volcanic” Peak📉 Vital Statistics
🔭 Field Notes
Regiomontanus itself is just the setting — a large, badly worn, distinctly oval highland crater whose southern and western rim structure has been heavily degraded and obscured. What you’re actually hunting for is a single small crater sitting near the summit of its off-center central peak, a target so small it barely registers next to the 126 km crater surrounding it.
- ▶ A Distorted Giant: Regiomontanus is squeezed against the southern rim of Purbach and sits just northwest of Walther, part of a chain of three overlapping highland craters. That crowding, plus billions of years of subsequent erosion, is why its outline reads as irregular and oval rather than the clean circle a fresh crater would show.
- ▶ The Volcano That Wasn’t: The small crater capping the central peak — now designated Regiomontanus A — was cited well into the 20th century as evidence that lunar craters could form volcanically, the peak itself read as a vent that later erupted at its tip. Higher-resolution imagery eventually settled the question: it is an ordinary impact crater, just offset slightly from the peak’s true summit.
- ▶ A Suspiciously Good Shot: A stray impactor landing squarely on a central peak looks like too neat a coincidence to be chance — which is exactly why the volcanic explanation held appeal for so long. But surveys have since identified numerous small impact craters superposed on central peaks and uplifted floor structures elsewhere on the Moon, showing that a summit strike, while uncommon, is well within the odds for billions of years of bombardment.
📍 Nearby L100 Targets
- L6 Tycho Crater: The Moon’s brilliant young ray crater, roughly 500 km to the south. Its 1.6 km central peak is unambiguously the product of crustal rebound from a fresh, violent impact — a useful contrast to Regiomontanus’s own much older, more ambiguous peak, and a reminder of what an undisputed complex-crater peak actually looks like.
- L15 Straight Wall (Rupes Recta): The Moon’s most famous linear fault, roughly 220 km to the north-northwest on the floor of Mare Nubium. Both targets once fed 19th-century debates about the Moon’s interior — one as supposed proof of volcanism, the other as evidence of ongoing tectonic activity — though only the fault’s origin as a structural feature has held up.
- L47 Alphonsus Dark Spots: The eleven dark-halo pyroclastic vents lining the floor fractures of Alphonsus, roughly 300 km to the north. Where Regiomontanus’s “volcano” turned out to be an ordinary impact crater in disguise, Alphonsus’s dark spots are the real thing — genuine, if small, sites of explosive lunar volcanism confirmed by spacecraft data.
🚀 Mission Log
Target Acquisition — L46 Regiomontanus Central Peak
Find the middle crater of a three-crater chain
Regiomontanus sits in the southern highlands southeast of Mare Nubium, the middle member of a north-south chain of three overlapping craters — Purbach pressing against its northern rim, Walther just to its south. The crater itself is just the setting: a large, badly worn, distinctly oval highland crater. What you’re actually hunting for is a single small crater sitting near the summit of its off-center central peak — tiny next to the 126 km crater surrounding it.
One lighting window does double duty
Around first or last quarter, with the terminator nearby, low sun angle brings out the crater’s irregular, oval outline and its heavily degraded southern and western walls. That same window is when the off-center central ridge throws its clearest shadow — a good-sized amateur scope under steady seeing can pick out the tiny summit crater riding on top of it.
Trace the collision history, then hunt the summit pit
Trace the outline of Regiomontanus against Purbach and Walther, and see if you can tell which crater came first from how their rims cut into one another — that same collision history is why the roughly kilometer-tall central peak sits well off toward the northwest rather than at the floor’s middle. At the very top of the ridge, look for Regiomontanus A, a small summit crater a few kilometers across — see if you can make out that it isn’t perfectly centered on the peak, a small detail that eventually undid a long-standing theory about it.
Pair it with two very different central-peak stories
Tycho (L6), roughly 500 km south, has an unambiguous 1.6 km central peak built by crustal rebound from a fresh, violent impact — a useful reminder of what an undisputed complex-crater peak actually looks like next to Regiomontanus’s own older, more ambiguous one. Alphonsus (L47), roughly 300 km north, is the real thing — genuine, if small, sites of explosive lunar volcanism confirmed by spacecraft data, in contrast to Regiomontanus’s “volcano” that turned out to be an ordinary impact crater in disguise.
📝 Observation Log — L46 Regiomontanus Central Peak
0/4 CompleteIs Regiomontanus’ central peak visible tonight?
Regiomontanus sits just north of Walter and south of Purbach, at 28.0°S, 0.6°W — almost exactly on the central meridian, which makes it one of the more forgiving targets on this list. Optimal viewing falls around First Quarter (roughly Day 7–8), when low-angle sunlight throws a long shadow from the unusually tall, sharp central peak that has long fueled speculation about a volcanic origin. Its near-central longitude means libration plays only a minor role here, mostly nudging the peak’s shadow geometry rather than its visibility.
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When to Observe the Regiomontanus Central Peak
Regiomontanus is a battered, oddly-shaped old crater whose real interest lies in a single small pit sitting on its central peak — a feature that was once treated as clinching evidence for lunar volcanoes, and now serves as a lesson in how easily that evidence can mislead.
- For Orientation: Regiomontanus sits in the southern highlands southeast of Mare Nubium, the middle member of a north-south chain of three craters — Purbach to its north, Walther to its south.
- For the Crater’s Shape: Around first or last quarter, when the terminator is nearby, low sun angle brings out the crater’s irregular, oval outline and its heavily degraded southern and western walls.
- For the Peak Itself: That same low-sun window is also when the central ridge throws its clearest shadow, and a good-sized amateur scope under steady seeing can pick out the tiny summit crater riding on top of it.
What to Look For
Regiomontanus is roughly 125–130 km across, its outline distorted into an irregular oval rather than a clean circle. Part of that distortion comes from its neighbor Purbach, which formed later against Regiomontanus’s northern rim, combined with billions of years of subsequent degradation, wall collapse, and overlapping smaller impacts that have left the whole crater looking crowded and asymmetric. The southern and western walls are so worn down by subsequent impacts that they’re barely traceable today.
Most complex craters have a central peak roughly at the middle of the floor. Regiomontanus’s peak sits well off toward the northwest, close to the crater’s border with Purbach — a legacy of the same collision history that distorted the rim. The ridge rises a little over a kilometer above the relatively level, lava-resurfaced floor around it.
Right at the top of the central peak sits a small crater a few kilometers across, cataloged as Regiomontanus A. It’s slightly offset from the very summit of the ridge rather than sitting dead center on it.
The floor of Regiomontanus is relatively level and resurfaced by old lava, peppered with small impact craters of various ages, giving it a busier, more textured look than a simple bowl-shaped crater. Together with the off-center peak and worn rim, it’s a floor that tells a long history of overlapping impacts rather than a single clean event.
The Science: A Volcano That Wasn’t
Regiomontanus earns its spot on the Lunar 100 as a cautionary tale as much as a scenic one — a reminder of how the Moon’s early observers built, and later dismantled, a whole theory of lunar volcanism around features like this.
The Case That Once Seemed Closed
For much of the 19th and early 20th centuries, many lunar observers argued that craters were volcanic calderas rather than impact scars, and a small pit sitting squarely on a mountain summit looked like exactly the kind of vent that theory predicted. Regiomontanus A, perched on the crater’s central peak, was cited as one of the more persuasive examples.
Why It Doesn’t Hold Up
Modern crater studies instead treat Regiomontanus A as an ordinary impact crater that happens to have landed on the peak — recognizable by its bowl shape, raised rim, and the fact that it isn’t perfectly centered on the ridge crest, exactly as an unrelated later impact wouldn’t be. Once central peaks were understood to form routinely in complex craters of this size, a stray impact landing on one was recognized as a matter of geometry and odds rather than a sign of eruption.
What the Case Still Illustrates
Regiomontanus A isn’t unique — similar summit pits on other lunar central peaks were floated as volcanic evidence in the same era, and were reassessed the same way. The broader debate over whether any lunar craters are volcanic in origin was largely settled by mid-20th-century impact studies and early spacecraft observations, and was then decisively confirmed by Apollo-era sample returns and orbital mapping.
Where Alphonsus shows genuine, if ancient, lunar volcanism, Regiomontanus shows how a single well-placed impact crater can look exactly like one — and why lunar geology had to learn to tell the difference.