L37 Bailly
The largest walled plain on the near side, its degraded, discontinuous ring of peaks so heavily worn it barely reads as a basin despite its enormous scale — sometimes interpreted as a degraded peak-ring basin candidate, its floor was never resurfaced by mare lava, preserving ancient impact topography beneath.
L37 Bailly
Southwest Limb · Ancient Walled Plain📉 Vital Statistics
🔭 Field Notes
Bailly is the largest walled plain on the visible face of the Moon, lying near the southwest limb north of the crater Le Gentil and east of Hausen, with the unofficially named Montes Dörfel mountains further west. Its oblique position near the limb foreshortens the view badly, and the most favorable time for viewing it is near full Moon, when the terminator crosses the crater wall. Despite its enormous size, comparable in area to a small mare, Bailly’s degraded, discontinuous ring of peaks and worn walls mean it doesn’t read as a dramatic basin the way its scale would suggest — the challenge of picking it out is a large part of the appeal.
- ▶ Bailly A & B: A prominent pair of satellite craters on the floor. Bailly A is younger than Bailly itself, cutting into the walls of both Bailly and Bailly B, while Bailly B — only about 64 km across — is extraordinarily deep, its floor sunk more than 4,250 m below the crest of its own rim.
- ▶ An Unflooded Floor: Unlike many large lunar basins, Bailly’s floor was never resurfaced by mare basalt, leaving it covered in ridges and smaller craters that retain much of the detail from its original formation, even after billions of years of subsequent weathering.
- ▶ A Prominent Ridge: A north–south ridge crosses the floor, one of the more identifiable landmarks amid the crater’s otherwise chaotic jumble of smaller impacts.
📍 Nearby L100 Targets
- L9 Clavius: The second-largest crater on the near side at 231 km, lying to the northeast — an ancient walled plain that lacks the broad basin-like morphology interpreted for Bailly, making the two a natural study in contrasting degradation despite their comparable scale.
- L30 Schiller: A striking 179 × 71 km elongated “footprint” crater in the southwestern highlands, its unusual shape possibly linked to ejecta from the same basin-forming era as Bailly’s neighboring Schiller-Zucchius Basin.
- L59 Schiller-Zucchius Basin: A largely buried, 325 km Pre-Nectarian peak-ring basin southeast of Bailly, with Schiller crater sitting on its northeast margin — another badly degraded basin whose faint ridge arc is a fitting companion to Bailly’s own barely-discernible rim.
🚀 Mission Log
Target Acquisition
Head for the southwest limb, and be patient with the foreshortening
Bailly sits near the southwest limb, north of the crater Le Gentil and east of Hausen. At roughly 300 km across it’s the largest walled plain on the near side, but its oblique, near-limb position foreshortens the view badly — favorable libration matters more here than for almost any other target on this list, so don’t be surprised if a session or two goes by before the geometry cooperates.
Wait for near-full Moon, not the terminator you’d normally expect
Because Bailly lies at a far-western selenographic longitude, low-angle morning illumination there occurs when the Moon is approaching full phase — the opposite of most targets on this list, where full Moon is the condition to avoid. Exact timing still shifts slightly because of libration, so treat any specific date as approximate. When it lines up, the low-angle light finally gives the degraded, discontinuous ring of peaks enough shadow to separate from the surrounding highlands.
Work up in power to sort out the floor detail
At 75x–100x, trace as much of the worn, discontinuous wall as the foreshortening allows — don’t expect a clean circular outline, since the degradation here is part of what makes this a harder target than its size suggests. Push to 150x+ and look at the floor for the satellite pair Bailly A and Bailly B: A is the younger of the two, its rim cutting into the walls of both Bailly and Bailly B, while B — only about 64 km across — is remarkably deep, its floor sunk more than 4,250 m below its own rim crest. Look also for the north–south ridge crossing the floor, one of the few clean landmarks amid an otherwise chaotic scatter of smaller craters left undisturbed because this floor was never resurfaced by mare basalt.
Compare the degradation against nearby L100 targets
To the northeast, Clavius (L9) is the second-largest crater on the near side at 231 km — an ancient walled plain of comparable scale that never developed the broad, basin-like morphology proposed for Bailly, making the pair a useful study in how differently two large ancient structures can degrade. To the southeast, the striking elongated “footprint” crater Schiller (L30) sits on the margin of the largely buried Schiller-Zucchius Basin (L59), a 325 km Pre-Nectarian basin whose faint ridge arc is a fitting companion to Bailly’s own barely-discernible rim — both reminders of how thoroughly billions of years can erase even the largest impact structures.
📝 Observation Log — L37 Bailly
0/4 CompleteIs Bailly visible tonight?
Because Bailly lies at a far-western selenographic longitude (~69°W), low-angle light only reaches its degraded wall in one practical window per lunation — aim for Waxing Gibbous (Day 13–14), near Full Moon; the matching waning crossing happens too close to New Moon (~Day 28–29) to be observable. Exact timing shifts slightly with libration, and favorable libration matters more here than for almost any other target, since Bailly’s near-limb position foreshortens the view badly on an off night.
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When to Observe Bailly
Bailly sits right at the southwest limb, at roughly 66°S, 69°W, and that position is the whole story of observing it. At around 300 km across it’s the largest walled plain visible on the Moon’s near side, but its location means it’s badly foreshortened and often partly hidden by libration altogether. It’s telling that the Lunar 100 itself lists Bailly as a “barely discernible basin” — this is a target that rewards patience over a full lunation, not a single good night.
- Best Window: Near Full Moon, when the terminator is crossing the crater’s high-southern-latitude wall. Because Bailly sits so far south, low-angle sunlight reaches it even close to full phase, giving the terraced west wall real shadow relief that’s absent under flatter, more central illumination.
- Libration Requirement: A favorable libration toward the southwest is essential here — without it, Bailly rotates close to edge-on and becomes very hard to make out. The two conditions (right libration, right terminator position) don’t always line up in the same lunation, so treat this as a multi-month project.
- Also Worth a Look: Different librations expose different parts of the rim. A strong southerly libration favours the eastern rampart and its bright craters; a strong westerly one opens up the parallel curved valleys along the southwest border — worth returning to more than once to build a complete picture.
What to Look For
At 150x–200x, focus on Bailly’s west wall. A historical account from the astronomer T. G. Elger puts its height at nearly 15,000 feet (roughly 4.5 km) at its tallest point, and modern topographic surveys generally place rim relief around Bailly in the 3–5 km range, so treat any single figure as approximate. What’s more reliably reported is that this wall is unusually well terraced for such a degraded formation. Because the whole crater is viewed obliquely from Earth, the terracing reads more like a series of stacked shadows than the classic concentric rings you’d see on a crater nearer the disk’s center — that oblique, foreshortened look is itself part of what makes Bailly recognizable.
Look for two prominent craters on Bailly’s floor: Bailly A, a comparatively young, sharp-rimmed crater that cuts across the wall of Bailly itself and also breaks into its neighbour, and Bailly B beside it — a modest 64 km across but, per historical accounts, plunging over 4 km below its own rim crest, making it disproportionately deep for its size if that figure holds. The contrast between A’s crisp rim and the surrounding degraded terrain is a quick way to spot it first.
On a steady night, scan the floor for a north-south ridge running through the interior. The same historical Elger account that describes the west wall also reports a pair of fine dark lines crossing one another near the southern end, probably clefts rather than bold rilles — worth treating as a target to confirm rather than a guaranteed sighting. These are subtle, low-contrast features on a floor that, unlike many large formations, was never resurfaced by mare lava, so what you’re seeing is close to the crater’s original interior structure.
If timing allows, compare Bailly with another large, heavily degraded walled plain such as Clavius, favourably placed for comparison when Bailly’s libration and lighting cooperate. Both are enormous, ancient, and scarred by generations of smaller impacts — but Bailly’s extreme limb position and lack of lava flooding give it a rawer, less orderly floor than Clavius’s more famous arcing crater chain.
The Science: Why Bailly Never Filled With Lava
Bailly is old, big, and — unlike most of the Moon’s other giant impact scars — essentially untouched by mare volcanism. That combination makes it a useful window into both the Moon’s earliest bombardment history and the reasons lava did, or didn’t, reach any given basin.
Outside the Procellarum KREEP Terrane
Most of the Moon’s dark mare plains are concentrated on the near side within a region geochemically enriched in heat-producing elements, generally referred to as the Procellarum KREEP Terrane. That enrichment is thought to have kept the crust there warm enough, for long enough, to sustain the volcanism that filled basins like Imbrium and Serenitatis with basalt. Bailly sits well outside this region, in the southern highlands, and one of the leading explanations for its unflooded floor is that it never had the thermal conditions nearby to generate a comparable lava supply. That’s unlikely to be the whole story, though — crustal thickness, the basin’s own depth and structure, and whether any magma pathway existed to reach it at all would all have played a role alongside the regional heat budget.
Crater or Basin? A Size and Structure Debate
At close to 300 km across, Bailly sits right at the size where lunar features are sometimes reclassified from “crater” to “impact basin” — the Lunar 100 itself categorizes it as a basin, though a “barely discernible” one, a label that captures both its scale and how hard its ring structure is to actually resolve from Earth. Whether that structure reflects a true multi-ring basin origin or an unusually large complex crater is a point where descriptions still vary from source to source.
What Hasn’t Been Pinned Down
Because Bailly is so hard to image well from Earth and sits outside the areas most intensively sampled or dated by lunar missions, its precise age is not settled with the same confidence as better-placed basins. Its heavily broken rim confirms a long history of subsequent bombardment, but the timing and sequence of that bombardment relative to the basin’s original formation is still worked out mostly from crater-counting and morphology rather than direct sample dating.
What makes Bailly worth the effort is exactly what makes it frustrating to observe: because it was never buried under later lava flows, the floor you’re looking at is among the least modified large basin floors visible from Earth — you’re fighting libration and foreshortening for a genuinely rare view.