L61 Mösting A
A small, sharp bowl crater near the exact center of the Moon’s visible disk, once used to help anchor the entire lunar coordinate system — bright enough for its ray system, banded enough for its shadowed walls.
Source: IAU/Gazetteer & Sky & Telescope Lunar 100 (Wood, 2004)
L61 Mösting A
Mare Insularum · Simple Crater📉 Vital Statistics
🔭 Field Notes
Mösting A is a small, sharp-rimmed bowl crater sitting south-southeast of its larger, older namesake, near the southeastern margin of Mare Insularum in the highlands just adjacent to it, close to the middle of the lunar disk as seen from Earth. It isn’t large or dramatic — a simple, unremarkable-looking crater at a glance — but its central position and crisp, youthful preservation have made it one of the most closely watched small features on the Moon. Bright and sharply defined at almost any lunar phase, it holds up under both terminator lighting and high Sun, which is part of why it became such a convenient marker for early lunar cartographers.
- ▶ The Reference Point That Wasn’t Quite Fixed: For decades, astronomers defined the origin of the lunar coordinate system by tracking Mösting A’s apparent offset from the Moon’s visual center over many nights, then mathematically correcting for libration to estimate its true position. The best Earth-based telescopic estimate placed it at 3.18°S, 5.16°W — but when independently checked against optical observations from the Apollo 14 command module combined with precise spacecraft tracking, the result came out roughly 8 km away. The discrepancy is a useful reminder of how much uncertainty existed in pre-laser-ranging lunar cartography, and later control networks (the 1994 and 2005 Unified Lunar Control Networks) shifted the value again rather than fully adopting the Apollo result.
- ▶ A Bright, Banded Crater: Mösting A appears on both the ALPO’s list of bright ray craters and its list of banded craters, meaning that under good seeing its inner walls show alternating light and dark streaks in addition to a genuine ray system extending out across the surrounding highlands — features generally associated with a relatively young, well-preserved impact.
- ▶ A Thermal Anomaly: Mösting, Mösting A, and the nearby Mösting C were identified as thermal anomaly craters in an infrared eclipse-cooling survey — freshly exposed rock retains heat differently than surrounding, more mature regolith during a lunar eclipse, and such anomalies are generally associated with relatively fresh, rock-rich surfaces rather than strictly with age alone. It’s consistent with the crater’s crisp rim and bright ray pattern, though the exact age remains inferred from these traits rather than precisely dated.
📍 Nearby L100 Targets
- L51 Davy Crater Chain: A string of roughly two dozen small craters on Mare Nubium, roughly 240 km south-southwest, whose disputed origin — secondary impacts, volcanic collapse, or a tidally disrupted impactor — makes a good contrast with Mösting A’s straightforward, textbook simple-crater form.
- L75 Ptolemaeus B: A saucer-like ghost crater on the floor of the great walled plain Ptolemaeus, roughly 210 km southeast. Where Mösting A is a sharp, obviously young bowl crater, Ptolemaeus B is nearly the opposite — an old crater so thoroughly buried by lava that only a faint rise betrays its buried rim.
- L92 Gyldén Valley: One of the linear “grooves” of the Imbrium radial sculpture, roughly 170 km east-southeast near the crater Gyldén. It’s a useful reminder that the same Imbrium impact whose ejecta scarred the highlands around Ptolemaeus and Julius Caesar affected this stretch of Mare Insularum less dramatically — Mösting A itself postdates all of that older terrain by a wide margin.
🚀 Mission Log
Target Acquisition
Let first quarter do the work — the terminator runs right through it
Mösting A sits almost exactly at the center of the Moon’s visible disk, which means at first or last quarter the terminator passes directly through its neighborhood without any real hunting required. Look for its larger, older namesake Mösting, a 24 km crater just to the northwest, then drop south-southeast to the small, sharp, bright bowl crater beside it — that’s Mösting A, only about 13 km across but easy to confirm once you’re in the right patch of Mare Insularum’s highland margin.
Two lighting modes reward two different views
Unlike most Lunar 100 targets, which need one specific lighting angle, Mösting A is worth revisiting under both. Near the terminator, low sunlight reveals its banded interior walls — alternating light and dark streaks on the inner slope, visible under good seeing. Near full Moon, high sun instead washes out the relief but lights up its genuine ray system, radiating out across the surrounding highlands. Don’t treat a session under one lighting condition as a failed attempt to see the other — they’re simply different features of the same small crater.
Work up in power to confirm the banding
At 150x–200x near the terminator, look carefully at the crater’s inner wall for the faint alternating bands that place Mösting A on the ALPO’s lists of both bright ray craters and banded craters — a fairly small crater to be doing double duty on two separate observational catalogs. Its crisp, sharply defined bowl and rim, with no obvious central peak, is a good textbook example of a simple crater profile for comparison against larger, more complex craters elsewhere in the guide.
Pair it with the chain, the ghost crater, and the sculpture nearby
Roughly 240 km south-southwest, the Davy Crater Chain (L51) strings a line of small craters across Mare Nubium, its disputed origin a good contrast to Mösting A’s straightforward, textbook simple-crater form. About 210 km southeast, Ptolemaeus B (L75) is nearly the opposite kind of crater — a saucer-like ghost so thoroughly buried by lava that only a faint rise marks its rim, next to Mösting A’s crisp, obviously young bowl. And roughly 170 km east-southeast, Gyldén Valley (L92), part of the Imbrium radial sculpture, is a reminder that this whole quiet stretch of Mare Insularum postdates the far older, heavily scarred terrain surrounding it.
📝 Observation Log — L61 Mösting A
0/4 CompleteIs Mösting A visible tonight?
Mösting A sits almost exactly at the Moon’s visible center, at 3.2°S, 5.2°W, south-southeast of the larger crater Mösting. Best viewing is around First Quarter (roughly Day 8) for its banded interior walls under low light, though it’s also worth a return visit near Full Moon when high Sun instead reveals its bright ray system. Its near-central position keeps it well clear of any libration effects.
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When to Observe Mösting A
Mösting A is small, but it’s worth two separate visits rather than one — its banded interior walls and its bright ray system each need opposite lighting to appear, and neither view is a substitute for the other.
- For the Banded Walls: You need raking light. Mösting A sits almost exactly at the Moon’s visible center, at 3.2°S, 5.2°W, so the terminator crosses it naturally around first or last quarter. Track the terminator’s position rather than counting on a fixed day, since exact timing shifts a little with libration and solar colongitude.
- For the Ray System: Come back near full Moon. High Sun washes out the crater’s relief entirely, but lights up the bright rays radiating out across the surrounding highlands — a genuinely different view of the same small crater.
- For Orientation: Anchor on Mösting, the larger 24 km crater just to the northwest, then drop south-southeast to the small, sharp bowl crater beside it — Mösting A, only about 13 km across.
What to Look For
Look carefully at the crater’s inner slope near the terminator for faint alternating light and dark streaks. This banding, along with its genuine ray system, is what places Mösting A on both the ALPO’s list of bright ray craters and its list of banded craters — a fairly small crater to be doing double duty on two separate observational catalogs.
Near full Moon, look for bright rays extending outward from the crater across the surrounding terrain — a sign generally associated with a relatively young, well-preserved impact.
Mösting A is a crisp, sharply defined bowl crater with no central peak and simple, unterraced walls — a good baseline example of an uncomplicated impact crater, useful for comparison against larger, more structurally complex craters elsewhere in the Lunar 100.
Nothing in its appearance suggests its historical importance, but for decades, this small crater served as the classical reference point for the lunar coordinate system. Every other named feature’s telescopically measured longitude was once anchored, indirectly, to this crater’s position.
The Science: The Crater That Anchored a Coordinate System
Mösting A’s real significance has little to do with its own geology and everything to do with how astronomers once used it to make sense of everywhere else on the Moon.
A Reference Point Built From Patient Tracking
Astronomers defined the classical origin of the lunar coordinate system by tracking Mösting A’s apparent offset from the Moon’s visual center across many nights, then mathematically correcting for libration to estimate its true position. Wollenhaupt et al.’s 1972 telescopic study placed it at 3.18°S, 5.16°W — the reference value used to anchor lunar longitude for decades.
An 8 km Discrepancy From Apollo 14
When Apollo 14’s optical observations, combined with precise spacecraft tracking of the command module, independently checked that classical position, the result came out roughly 8 km away from the accepted telescopic value — a small but telling reminder of how much uncertainty existed in pre-laser-ranging lunar cartography. Later control networks, the Unified Lunar Control Networks of 1994 and 2005, incorporated spacecraft tracking and laser-ranging measurements, superseding both the original telescopic solution and the Apollo-era determination.
A Thermal Anomaly, Not Fully Explained
Mösting, Mösting A, and the nearby Mösting C were identified as thermal anomaly craters in an infrared eclipse-cooling survey — freshly exposed, rock-rich surfaces retain heat differently than surrounding, more mature regolith during a lunar eclipse. Such anomalies are generally associated with relatively fresh, rock-rich surfaces rather than strictly with age alone, so Mösting A’s exact absolute age has not been directly measured.
Most Lunar 100 targets are chosen for what they show you. Mösting A is chosen for what it once did for everyone else — a small, unremarkable-looking crater that quietly served as the fixed point the rest of the lunar map was measured against, discrepancies and all. Because it lies only a few degrees from the Moon’s mean center, foreshortening here is minimal, making it one of the easiest small craters whose true circular shape can be fully appreciated.