L55 Baco
A 65 km highland crater whose floor has been worn nearly featureless by billions of years of minor impacts — unusually smooth for a crater sitting in some of the most heavily battered terrain on the Moon.
Source: IAU/Gazetteer & Sky & Telescope Lunar 100 (Wood, 2004)
L55 Baco
Southern Highlands · Unusually Smooth Crater Floor📉 Vital Statistics
🔭 Field Notes
Baco sits in one of the most rugged, heavily overlapped stretches of the southern highlands, yet its own floor stands out precisely because it doesn’t look like its surroundings. Patrick Moore described it as having “lofty walls and a low central peak,” but billions of years of minor impacts have worn the original terracing smooth and dotted the rim with small craterlets. Inside, the floor is nearly flat, carrying only a few low ridges near its midpoint and conspicuously few small craters of its own — a genuinely calmer surface than almost anything else nearby, in a neighborhood otherwise defined by relentless overlapping bombardment.
- ▶ A Name Changed on Purpose: Baco is named for the medieval English natural philosopher Roger Bacon, but the German astronomer Johann von Mädler shortened the name to “Baco” in his 1837 map, following his preference for Latinized single-word crater names. The IAU formally adopted the name in 1935.
- ▶ Smoother Than It Has Any Right to Be: Unlike the densely overlapped terrain nearby at Maurolycus, Baco’s floor is nearly featureless — only minor ridges near the center and no significant small craters of its own. Charles Wood highlighted exactly this quality when he selected Baco for the Lunar 100: a smooth floor and surrounding plains are a genuine oddity for a crater this old sitting in such heavily battered highland terrain.
- ▶ Worn Walls, Buried Terraces: The rim and inner wall show clear signs of long-term erosion by countless minor impacts — the original terracing has been ground down to a smoother slope, and small craterlets now overlie the rim itself, evidence of just how much bombardment this stretch of highland has absorbed since Baco first formed.
📍 Nearby L100 Targets
- L45 Maurolycus: A 114 km complex crater roughly 300 km north-northwest, chosen for the Lunar 100 as a textbook example of an entire region at crater-saturation equilibrium. It’s a useful comparison in scale: Maurolycus and its crowded neighbors show what relentless overlapping bombardment looks like across a whole stretch of highland, while Baco shows what an individual crater’s floor can look like once erosion has smoothed it flat.
- L40 Janssen Rille: A rare highland graben roughly 455 km northeast, cutting across the floor of the ancient, battered walled plain Janssen. Both targets sit in old, unflooded highland crust, but where Rimae Janssen points to crustal extension, Baco’s smooth floor points instead to a long, quiet history of erosion by small impacts.
- L7 Altai Scarp: The Moon’s most prominent escarpment, roughly 820 km north-northeast, forming the best-preserved rim segment of the ancient Nectaris Basin. It’s a useful contrast in preservation styles: Altai Scarp survives as a sharp, still-legible cliff, while Baco’s original crater architecture has been softened almost entirely by long-term bombardment.
🚀 Mission Log
Target Acquisition
Find Maurolycus, then sweep south-southeast
Start at Maurolycus, a large, complex crater and one of the more recognizable landmarks in this crowded stretch of the southern highlands. From there, sweep roughly 300 km south-southeast across increasingly rugged, overlapped terrain until the ground briefly calms down — that unusually flat, quiet floor belongs to Baco, a 65 km crater that stands out mainly by not looking like everything around it.
Chase the terminator to see just how worn the rim really is
Baco’s original terracing has been ground down by countless minor impacts, so a low, raking terminator is essential for picking out what remains of its rim structure and the small craterlets dotting it. That same low light also throws the smooth floor into useful contrast against the surrounding highland — even under favorable lighting, expect the floor itself to stay largely featureless, since that flatness is the whole point of this target rather than a lighting problem to work around.
Work up in power to confirm just how empty the floor is
At 100x–150x, scan Baco’s interior for small craters the way you would in any ordinary highland crater — and come up mostly empty. Aside from a few low ridges near the midpoint, the floor is conspicuously bare of the small impact scars that saturate the terrain immediately outside the rim. That absence, more than any single feature, is what you’re really confirming here.
Pair it with the saturated highlands and the rille and scarp beyond
Back at your starting point, roughly 300 km north-northwest, Maurolycus (L45) shows what crater-saturation equilibrium looks like across an entire region — the crowded norm that makes Baco’s smooth floor stand out in the first place. Roughly 455 km northeast, Janssen Rille (L40) cuts across the floor of the ancient walled plain Janssen, a genuine tectonic feature rather than the erosional smoothness on display at Baco. And roughly 820 km north-northeast, Altai Scarp (L7), the best-preserved rim segment of the Nectaris Basin, offers a useful contrast in preservation styles — a sharp, still-legible cliff standing against Baco’s much softer, more thoroughly worn architecture.
📝 Observation Log — L55 Baco
0/4 CompleteIs Baco visible tonight?
Baco sits deep in the southern highlands at 51.04°S, 19.10°E, south-southeast of Maurolycus. Best viewing is around Waxing Crescent (roughly Day 6), when the terminator crosses near 19°E and low sunlight reveals the worn rim and small craterlets that billions of years of minor impacts have left behind. The crater’s high southern latitude means it’s worth a second look under varying libration, but timing the terminator is still the key factor.
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When to Observe Baco
Baco doesn’t reward you with a dramatic profile — it rewards you with an absence. Its whole appeal is a floor that stays stubbornly calm in a stretch of highland where almost nothing else is.
- For the Rim and Floor: You need a low, raking terminator. Baco sits around 19°E in the southern highlands, so its best light comes when the terminator is crossing that longitude. Track the terminator’s position rather than counting on a fixed day, since exact timing shifts with libration and solar colongitude.
- For Orientation: Anchor on Maurolycus, a large, recognizable complex crater in the crowded southern highlands, then sweep roughly 300 km south-southeast to the 65 km crater whose floor calms down where the terrain around it doesn’t.
- Aperture and Seeing: Moderate power confirms the floor’s flatness; higher power near the terminator reveals just how worn the rim’s original terracing has become.
What to Look For
Scan Baco’s interior for small craters the way you would in any ordinary highland crater — and come up mostly empty. Aside from a few low ridges near the midpoint, the floor is comparatively free of the small impact scars that saturate the terrain immediately outside the rim.
Look at the rim and inner wall for signs of long-term erosion — the original terracing ground down to a smoother slope, with small craterlets now overlying the rim itself, evidence of just how much bombardment this stretch of highland has absorbed since Baco first formed.
Compare Baco directly against Maurolycus and its crowded neighbors nearby — a textbook example of a region at crater-saturation equilibrium, where new impacts have nowhere left to land except on top of older ones. Baco’s calm floor stands out precisely because of what surrounds it.
Look for Baco A, a 39 km crater to the south, and Baco B, a 41 km crater to the northwest — both confirmed and positioned in detail by Lunar Orbiter IV’s 1967 photographic survey of the region.
The Science: A Quiet Floor in a Loud Neighborhood
Baco’s significance isn’t in any single dramatic feature — it’s in what’s missing. A floor this calm, sitting in terrain this heavily reworked, is itself the interesting result.
An Old Crater, Selectively Smoothed
Baco is Pre-Nectarian in age, old enough that its rim has absorbed billions of years of subsequent minor impacts, grinding its original terracing down to a smoother slope. Yet its floor preserves a noticeably lower density of small craterlets than the surrounding highlands — an uneven distribution of wear that’s the whole reason Charles Wood selected it for the Lunar 100.
A Name Shortened by Convention
Baco is named for the medieval English natural philosopher Roger Bacon, but the German astronomer Johann von Mädler shortened the name to “Baco” in his 1837 map, following his preference for Latinized single-word crater names. The IAU formally adopted the name in 1935.
What Hasn’t Been Resolved
Exactly why Baco’s floor has stayed so much smoother than the highland surrounding it isn’t fully explained — whether it reflects local differences in surface properties, subtle resurfacing after the crater formed, or simply statistical variation in impact gardening over billions of years. Distinguishing genuine anomalies like Baco from statistical noise in a heavily cratered landscape remains a broader challenge in reading the Moon’s impact history.
Most Lunar 100 targets earn their place by showing you something extraordinary. Baco earns its place by showing you remarkably little. In one of the Moon’s most densely cratered landscapes, that quiet floor is the feature worth noticing.