L54 Hippalus Rilles
A fan of arcuate clefts curling along the eastern shore of Mare Humorum, cracked into the crust as the whole basin sagged under the weight of its own lava fill — one of the clearest concentric rille systems anywhere on the Moon.
Source: IAU/Gazetteer & Sky & Telescope Lunar 100 (Wood, 2004)
L54 Hippalus Rilles
Eastern Mare Humorum · Concentric Rilles📉 Vital Statistics
🔭 Field Notes
The Hippalus Rilles trace a graceful arc along the eastern shore of Mare Humorum, running roughly parallel to the mare’s edge before curving southward through the mountainous Rupes Kelvin. They’re best caught with the terminator near 32° west longitude, when low sunlight throws each narrow rille into sharp relief against the surrounding flooded plain. The rilles take their name from Hippalus, an ancient Greek navigator and geographer of the 1st century AD — the crater bearing his name has a southwest rim that has vanished entirely into the mare, and fittingly, one of the main rilles cuts directly through what remains of it, splitting the old crater floor cleanly in two.
- ▶ Basin Flexure in Action: Unlike a simple linear graben carved by crustal tension, the Hippalus Rilles formed as the lunar lithosphere surrounding Mare Humorum flexed beneath the sheer weight of the basalt filling the basin’s center, cracking in a set of concentric rings around the rim as the middle subsided. It’s a different mechanism entirely from the straight-line grabens seen elsewhere in the highlands, and one of the clearest visual demonstrations of basin-loading flexure on the Moon.
- ▶ A Timeline Written in Rock: Where one of the main rilles cuts straight through the crater Hippalus, it leaves behind a clean geologic timeline in a single eyepiece view — features it slices through predate the rille, while anything sitting undisturbed on top of it formed afterward. It’s a rare case where cause and sequence are both directly visible rather than merely inferred.
- ▶ Not Officially Named, Still Numbered: The individual rilles were never given proper IAU names, only Roman numerals — I through III recognized in the original System of Lunar Craters, with additional rilles identified in later mapping. In 1973 the IAU floated replacing this numeral system with real names for lunar rimae, but no such renaming was ever carried out, so the numbered convention has simply stuck.
📍 Nearby L100 Targets
- L13 Gassendi Crater: A 110 km floor-fractured crater roughly 355 km northwest, on the opposite, northern shore of the same Mare Humorum. Gassendi’s rille network was carved by impact fracturing later exploited by rising magma, while the Hippalus Rilles instead trace crustal bending from the basin’s own subsidence — two very different rille-forming processes bookending the same mare.
- L60 Kies π: A solitary volcanic dome roughly 215 km southeast, on the floor of Mare Nubium just east of Mare Humorum. The pairing makes a good contrast in the same general region: one target defined by tectonic cracking around a basin’s edge, the other by quiet effusive volcanism out on the open mare.
- L39 Schickard: A huge, ancient walled plain roughly 860 km southwest, itself Pre-Nectarian in age. Schickard’s rim survives largely intact despite its age, offering a useful contrast to Hippalus, whose southwest wall was buried by mare lavas, while the nearby Hippalus Rilles formed as those same lava deposits flexed the surrounding crust.
🚀 Mission Log
Target Acquisition
Find Gassendi, then trace the shore south
Start at Gassendi, a large, unmistakable floor-fractured crater sitting right on the northern shore of Mare Humorum. From there, follow the mare’s eastern edge south roughly 355 km, past the open basalt, until you reach a graceful arc of narrow clefts running parallel to the shoreline — those are the Hippalus Rilles, named for the crater whose remains they cut directly through.
Chase the terminator near western Mare Humorum
Like most lunar rilles, the Hippalus system is a low, narrow feature that needs raking light to cast a visible shadow. Catch the terminator crossing near 32°W — timing shifts a little with libration and solar colongitude — and each rille resolves into a thin, dark line against the mare. Under high or full Sun, the whole family fades back into the surrounding basalt, an expected lighting effect rather than a sign of a bad session.
Work up in power to follow one rille straight through Hippalus itself
At 100x–150x, trace the individual rilles as they curve south and then southwest through the mountainous Rupes Kelvin. Look closely at the crater Hippalus, whose southwest rim has vanished into the mare — one of the main rilles cuts directly across what remains of its floor, giving you a clean cross-cutting relationship in a single view: whatever the rille slices through predates it, and anything sitting undisturbed on top of it came later.
Pair it with the dome and the walled plain nearby
Back at your starting point, roughly 355 km northwest, Gassendi (L13) shows a very different kind of rille — one carved by impact fracturing later exploited by rising magma, rather than the basin-flexure origin on display at Hippalus. Roughly 215 km southeast, Kies π (L60), a solitary volcanic dome on Mare Nubium, offers a good contrast in mechanism: tectonic cracking around a basin’s edge versus quiet effusive volcanism out on the open mare. And roughly 860 km southwest, Schickard (L39), a huge Pre-Nectarian walled plain with its rim still largely intact, is a useful reminder of how much more of Hippalus crater’s own wall has been lost to the same lava that later flexed the crust into these rilles.
📝 Observation Log — L54 Hippalus Rilles
0/4 CompleteIs Hippalus Rilles visible tonight?
Hippalus lies on the eastern shore of Mare Humorum at 24.8°S, 30.2°W, a moderate southwestern longitude with no serious limb effects. Best viewing is around Waxing Gibbous (roughly Day 10), when the terminator crosses near 32°W and low, raking light throws each narrow rille into sharp relief. Under high or full Sun the whole family fades back into the mare, so timing the terminator matters far more than libration here.
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When to Observe the Hippalus Rilles
This is a family of narrow clefts curling along the eastern shore of Mare Humorum, and like almost every rille on the Moon, it needs light skimming nearly flat across the ground before any of it shows up at all.
- For the Full Arc: You need raking light. The rilles sit around 30°W along the eastern edge of Mare Humorum, so their 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 a little with libration and solar colongitude.
- For Orientation: Anchor on Gassendi, the large floor-fractured crater on the mare’s northern shore, then follow the eastern edge of the mare south roughly 355 km to the arc of rilles curving through the mountainous Rupes Kelvin.
- Aperture and Seeing: Moderate power traces the main arc; higher power near the terminator resolves individual rilles and the section where one crosses the flooded floor of Hippalus crater.
What to Look For
Follow the rilles as they run roughly parallel to the mare’s edge before curving south and then southwest through Rupes Kelvin. The three principal rilles are traditionally designated Rimae Hippalus I, II, and III, with additional smaller rilles recognized in later mapping.
Look closely at the crater Hippalus, whose southwest wall has been buried by mare lava, and find the rille slicing directly across what remains of its floor — a clean cross-cutting relationship visible in a single view.
Unlike a simple linear graben, these rilles curve in a broad arc concentric to Mare Humorum’s center — a shape that’s the direct visual signature of how they formed.
The individual rilles were never given proper IAU names, only Roman numerals. The IAU floated replacing this numbering with real names for lunar rimae back in 1973, but no such renaming was ever carried out, so the numbered convention has simply stuck.
The Science: A Basin Bending Under Its Own Weight
The Hippalus Rilles aren’t cracks from crustal stretching in the usual sense — they’re the visible signature of an entire basin sagging under the weight of the lava that filled it.
Basin Flexure, Not Simple Extension
The Hippalus Rilles formed as the lunar lithosphere surrounding Mare Humorum flexed beneath the sheer weight of the basalt filling the basin’s center, fracturing in concentric patterns around the basin as the interior subsided. It’s a different mechanism entirely from the straight-line grabens seen elsewhere in the highlands, and one of the clearest visual demonstrations of basin-loading flexure on the Moon.
Two Related but Distinct Processes at Hippalus Crater
Hippalus crater’s missing southwest wall was buried by Mare Humorum’s lava flooding, while the surrounding crust later flexed under the weight of the broader mare fill. The two effects share a common cause — the accumulated weight of Mare Humorum’s basalt — but they aren’t the same process: one is burial, the other is bending.
What Hasn’t Been Resolved
The exact relationship between Mare Humorum’s interior structure, lava thickness, and the timing of rille formation remains difficult to reconstruct. Orbital gravity data have improved models of buried basins, but the detailed sequence of flooding and flexure that produced the Hippalus system is still inferred rather than directly observed.
Most Lunar 100 rilles trace back to a single, obvious fracture. The Hippalus Rilles tell a slower story — an entire basin gradually bending under the weight of its own lava fill, leaving behind a curved pattern of fractures that still records the stresses of an ancient lunar sea.