L75 Ptolemaeus B
A crater erased almost to nothing, its rim reduced to a shallow saucer-shaped dip on the floor of Ptolemaeus — visible only in the narrow window when a grazing terminator throws a thin shadow across it.
L75 Ptolemaeus B
Floor of Ptolemaeus · Central Highlands📉 Vital Statistics
🔭 Field Notes
Ptolemaeus B sits just north of the small crater Ammonius, on the flat, dark-hued floor of the great walled plain Ptolemaeus. It isn’t a crater in any ordinary sense anymore — volcanic plains material flooded and buried it so completely that only a shallow, saucer-shaped dip in the floor and the faintest circular rise mark where its rim once stood. Ptolemaeus’s floor holds several of these barely-there “saucers,” first noticed by the 19th-century observer Kunowsky, and Ptolemaeus B is simply the best known and most frequently sought of the bunch.
- ▶ A Crater Erased, Not Destroyed: Ptolemaeus B wasn’t blasted away — it was buried. Later plains material flooded the floor of Ptolemaeus and drowned the crater almost entirely, leaving behind only the low, rounded ghost of its original rim rather than a sharp, bowl-shaped hollow.
- ▶ Best Caught at the Edges of Day: Because its remaining relief is only a few tens of metres or less, Ptolemaeus B becomes extremely difficult to detect under high sun. Catching it takes a low, grazing light near lunar sunrise or sunset, when the faint shadow inside the saucer briefly becomes visible before flattening out again as the sun climbs.
- ▶ One of Many “Saucers”: Ptolemaeus’s floor is dotted with several similar shallow depressions besides Ptolemaeus B, evidence that extensive plains flooding buried numerous earlier impact craters, leaving just a scatter of subtle rings for patient observers to pick out.
📍 Nearby L100 Targets
- L47 Alphonsus Dark Spots: The walled plain immediately south, roughly 190 km away, whose floor hosts a cluster of genuine explosive volcanic vents — real evidence of eruptive activity, in contrast to Ptolemaeus B’s quieter history as an ordinary impact crater that simply lies buried beneath later basaltic lava flows rather than erupting anything itself.
- L51 Davy Crater Chain: A roughly 50 km string of small craterlets on the floor of Davy Y, about 205 km south-southwest, its origin debated between secondary impacts, volcanic collapse, and a tidally disrupted impactor — a very different kind of puzzle than Ptolemaeus B’s comparatively straightforward burial by lava.
- L61 Mösting A: A crisp, sharp-rimmed 13 km bowl crater near the Moon’s visual center, roughly 210 km northwest, its bright rays and youthful profile making it Ptolemaeus B’s near-opposite: one is a textbook fresh impact, the other a crater so thoroughly erased by lava that only a faint rise betrays where its rim once stood.
🚀 Mission Log
Target Acquisition
Start at Sinus Medii, then drop south to the crater chain
Locate Sinus Medii, the small dark patch marking the Moon’s visual center — an easy naked-eye and binocular landmark near first quarter. From there, move south to a line of three large walled plains: Ptolemaeus, Alphonsus, and Arzachel. Ptolemaeus is the northernmost of the three and the largest, with the flattest, darkest floor of the group.
Chase a grazing terminator — this target has almost no relief left
Ptolemaeus B has been buried so thoroughly beneath later floor-filling material that only a few tens of metres of relief remain. It only shows itself when the terminator sits close enough to the floor of Ptolemaeus to throw a long, raking shadow across the shallow saucer. Wait even a day too long into higher Sun and the shallow depression blends into the surrounding floor, becoming indistinguishable from the terrain around it.
Work up in power once you’re on the floor
At 200x–300x, center on the north-central part of Ptolemaeus’s floor, just north of the tiny crater Ammonius. Under a grazing light angle, look for a faint circular rise with a shallow dip inside it — that low ghostly ring is Ptolemaeus B. Confirming the fainter companion “saucers” scattered nearby on the same floor is a good sign you’re catching the light at the right angle.
Pair it with nearby L100 targets
Roughly 190 km south, Alphonsus Dark Spots (L47) sit on the floor of the next walled plain in the chain, marking real explosive volcanic vents — a sharp contrast to Ptolemaeus B’s quieter history as an ordinary impact crater later buried beneath smooth floor deposits. About 205 km south-southwest, the Davy Crater Chain (L51) has a debated origin, though a tidally disrupted impactor is now generally considered the leading explanation. And roughly 210 km northwest, Mösting A (L61) is Ptolemaeus B’s near-opposite: a crisp, bright-rayed fresh impact rather than a crater erased almost to nothing.
📝 Observation Log — L75 Ptolemaeus B
0/4 CompleteIs Ptolemaeus B visible tonight?
Ptolemaeus B sits on the floor of Ptolemaeus near 8.0°S, 0.8°W, just north of the tiny crater Ammonius. Best viewing is around Waxing Gibbous (roughly Day 7) or Day 22 waning, and getting a very low sun angle matters more than the exact day: this ghost crater’s rim has been buried down to a few tens of metres of relief and needs a near-grazing terminator to cast any shadow at all. Libration is not a meaningful factor at this longitude.
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When to Observe Ptolemaeus B
Ptolemaeus B has almost no relief left to see — this ghost crater was almost completely buried during ancient volcanic resurfacing, and it only reveals itself in the narrow window when a grazing terminator throws a shadow across its shallow saucer.
- For the Faint Rim: You need a very low, grazing Sun angle. Ptolemaeus B sits at 8.0°S, 0.8°W, on the floor of Ptolemaeus, with only very slight relief remaining. Best viewing falls around Day 7 waxing or Day 22 waning. The exact lunar day varies slightly with libration, so what matters most is having the terminator crossing the crater under a very low Sun angle.
- Avoid High Sun Entirely: Wait even a day too long and the shallow depression blends completely into the surrounding floor, becoming indistinguishable from the terrain around it.
- For Orientation: Start at Sinus Medii, drop south to the Ptolemaeus–Alphonsus–Arzachel chain, then center on Ptolemaeus, the northernmost and largest of the three, with the flattest, darkest floor.
What to Look For
At 200x–300x, center on the north-central part of Ptolemaeus’s floor, just north of the tiny crater Ammonius. Under grazing light, look for a low, rounded ghost of a rim enclosing a shallow saucer-shaped depression — that’s Ptolemaeus B.
Ptolemaeus B wasn’t blasted away — later smooth plains deposits flooded the floor of Ptolemaeus and drowned the crater almost entirely, leaving only a low rounded ghost of its original rim.
Ptolemaeus’s floor holds several similarly shallow depressions besides Ptolemaeus B, among the subtle floor craters described by Georg Kunowsky in the early nineteenth century — evidence that extensive plains flooding buried numerous earlier impact craters here, leaving just a scatter of subtle rings for patient observers.
Roughly 190 km south, the Alphonsus Dark Spots are interpreted as deposits from explosive volcanic eruptions. Roughly 210 km northwest, Mösting A is a crisp, bright-rayed fresh impact. Ptolemaeus B is the quiet counterpoint to both — an ordinary impact crater with no eruptive history of its own, simply buried beneath later plains material.
The Science: Reading a Crater That’s Almost Gone
Ptolemaeus B’s interest lies less in its own form, largely buried, and more in what its burial says about the floor it sits on.
Vital Statistics
Located at 8.0°S, 0.8°W, on Rükl chart 44, Ptolemaeus B spans about 16 km and is named as a satellite designation of Ptolemaeus, honoring the astronomer Claudius Ptolemy. Its L100 distinction: the saucer-like depression on the floor of Ptolemaeus.
Buried, Not Fractured
Plains material flooded and buried the crater so completely that only a shallow saucer-shaped dip and the faintest circular rise mark where its rim once stood. The crater clearly predates the smooth plains material that buried it.
Mission Record
Ranger 8 captured wide-field approach imagery of the region in February 1965 en route to impact in Mare Tranquillitatis. Lunar Orbiter IV’s 1967 survey documented the floor’s subtle ghost craters in far greater detail. Since 2009, LROC images together with LOLA topography clearly reveal the crater’s extremely subdued rim against the surrounding lava-flooded floor.
Most Lunar 100 targets reward you with a clear, dramatic view. Ptolemaeus B rewards patience instead — a crater largely buried, caught only in the brief window when grazing light picks a faint ring out of an otherwise smooth-looking floor.