L28 Hipparchus
One of the Moon’s oldest surviving walled plains, battered and filled by the debris of ancient impacts — its floor is a ghost-like landscape of flooded features, serving as a visual bridge between the heavily cratered highlands and the smooth, younger maria.
L28 Hipparchus Crater
Central Near Side Highlands📉 Vital Statistics
🔭 Field Notes
Hipparchus is one of the Moon’s oldest surviving large craters — so old that the Imbrium impact, roughly 3.8 billion years ago, battered and reshaped it from hundreds of kilometers away. Its western rim has been worn down to little more than low hills, while the eastern wall, though still heavily eroded, remains more continuous and easier to trace in a telescope.
- ▶ Twin Clefts: A pair of deep clefts cut through the western wall, running roughly parallel to similar scars that cross the south-central highlands. Their exact origin isn’t settled, though their alignment with the broader highland scarring is often noted.
- ▶ Partly Flooded Floor: The floor appears to have been resurfaced in part by basaltic lava, leaving a southwestern section slightly raised and noticeably rougher than the rest. Lunar Orbiter imaging also revealed a cluster of low, dome-like hillocks on the floor of uncertain origin.
- ▶ A Crowded Rim: Younger craters ring Hipparchus closely — Horrocks sits entirely within the eroded northeast rim, Halley is attached to the south rim, and Hind lies just southeast, making this a good spot to trace several stages of crater degradation in one field of view.
📍 Nearby L100 Targets
- L35 Triesnecker Rilles: A dense, net-like web of at least nine intersecting rilles spread across roughly 200 km east of the 26 km crater Triesnecker, running generally north–south between Rhaeticus and the area near Rima Hyginus. Tectonic graben formation is a leading interpretation of their origin, though the full picture is still debated.
- L92 Gyldén Valley: A pronounced groove cutting the western rim of the 48 km crater Gyldén, part of the wider “Imbrium sculpture” — the radial and non-radial grooves gouged into the highlands by debris from the Imbrium basin-forming impact. The valley itself runs roughly 10–15 km wide and well over 100 km long.
- L75 Ptolemaeus B: An 18–19 km saucer-shaped “ghost crater” on the northern floor of neighboring Ptolemaeus, just north of the small bowl crater Ammonius — the barely-there rim of an older crater almost entirely drowned by later lava, visible only as a shallow circular shadow under low sun.
🚀 Mission Log
Target Acquisition
Anchor on Sinus Medii, then drop southeast to the highland trio
Start with Sinus Medii, the small flooded bay sitting at the intersection of the Moon’s equator and prime meridian — the point directly facing Earth, and an easy naked-eye anchor near the center of the disc. Hipparchus lies immediately southeast of it. From there, three comparably large walled plains define the neighborhood: Ptolemaeus sits southwest of Hipparchus, and Albategnius lies south of it, forming a loose highland trio at low power. At 143.95 km across, Hipparchus itself is easy to hold in the field even at modest magnification once you’ve made the jump from Sinus Medii — and it’s worth remembering as you go that this is one of the Moon’s oldest surviving large Nectarian-age walled plains, among the most heavily modified highland structures on the near side.
Wait for the terminator — the worn rim and floor detail need raking light
Under high Sun, Hipparchus reads as a vague, shallow depression — its western rim has been eroded down to little more than low hills, and the floor’s subtle relief washes out entirely. The best detail appears when the morning terminator lies near Hipparchus’s own longitude. At that angle, the low sun catches the remaining rim hills in relief and throws just enough shadow to bring out the floor’s twin clefts and low hillocks — features that are essentially invisible under full illumination.
Work up in power to separate the rim craters from the floor detail
At 75x–100x, pick out the three craters crowding the rim: Horrocks sits entirely within the eroded northeast rim, Halley is attached to the south rim, and Hind lies just beyond it to the southeast — a good sequence for comparing degradation stages in one field. Push to 100x–150x under the same terminator light to look for the twin clefts cutting the western wall, running roughly parallel to similar scarring across the south-central highlands, and for the cluster of low, dome-like hillocks on the floor — seen clearly in Lunar Orbiter 3 and 5 imagery — whose exact formation history remains uncertain.
Step back and compare against the nearby L100 highlands targets
Once Hipparchus’s own features are confirmed, widen the field to take in the region it anchors. To the north, the Triesnecker Rilles (L35) form a dense net of intersecting graben east of Triesnecker crater. Gyldén Valley (L92) cuts the western rim of Gyldén. Both features record the extensive modification of the central highlands by major impact events, including material associated with the Imbrium basin. And on the floor of neighboring Ptolemaeus, Ptolemaeus B (L75) is a ghost crater drowned by lava, visible only as a shallow ring under the same kind of low-angle light you’re already using for Hipparchus.
📝 Observation Log — L28 Hipparchus Crater
0/4 CompleteIs Hipparchus visible tonight?
Hipparchus is a terminator target. Its degraded walls, floor ridges, and the crater Horrocks within its northeast rim are best seen around First Quarter (Day 7–8) or Last Quarter (Day 21–22). A distinctive clair-obscur effect — three sunlit rim spots resembling Orion’s Belt — appears when the morning terminator lies near 5–6°E longitude, making the timing worth getting right.
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When to Observe Hipparchus
Hipparchus is a case of a crater that vanishes in plain sight. Its walls were never very tall to begin with, and roughly three to four billion years of subsequent bombardment have worn them lower still, so under anything but a low sun the whole 144 km outline all but disappears into the surrounding highlands. Positioned close to the Moon’s central meridian, it’s a feature you catch briefly rather than one that’s always available.
- Best Window: Roughly Day 6–8 after New Moon, as the morning terminator sweeps across the central meridian near First Quarter. This is when the low, eroded rim throws just enough shadow to trace the crater’s rhombus-like outline against the highlands.
- Second Window: Around Day 20–22, as the evening terminator returns from the west. Shadows fall the opposite direction here, which is useful for confirming which rim segments are real relief and which were tricks of the earlier lighting.
- Also Worth a Look: Near Full Moon, when the whole rim washes out almost completely. This isn’t a bonus view so much as the point — spend a few minutes at Full Moon and you’ll see exactly why Hipparchus was picked for this list: an “obvious” crater on a map that becomes nearly invisible at the eyepiece.
What to Look For
At 100x–150x near the terminator, trace the walled enclosure as far as you can around its circumference. The east and northeast sections, near the craters Hipparchus G and Horrocks, hold together reasonably well. The southwest section is another matter — it has been so heavily eroded by later impacts that only scattered low hills and rises mark where the wall used to be. Some observers describe the overall shape as a rough hexagon or a rhombus with curved sides rather than a clean circle.
Part of the crater floor has been resurfaced by basaltic lava, which probably reached the floor through fractures or low sections of the northwest rim connecting through to the mare terrain nearby. Look for a subtle tonal and textural difference between the smoother, darker-toned flooded areas and the rougher ground elsewhere on the floor. The southwest portion of the floor is distinctly different again — slightly raised and noticeably more rugged than the rest, and generally interpreted as remnants of the crater’s original uplifted interior that the lava never fully buried.
Hipparchus hosts a small population of younger, sharper craters that make good waypoints once the ghost outline is hard to hold. Horrocks sits entirely within the northeast rim and is the most prominent single feature on the floor. Halley is attached to the south rim, and Hind lies just to the southeast, forming a loose curved line with a couple of the smaller lettered craters. Pickering, bowl-shaped and easy to pick out, sits off the north-northeast rim, and the flooded remnant Saunder lies further out to the northeast.
In October 1664, Robert Hooke turned a 36-foot telescope on Hipparchus and produced what’s generally regarded as the first detailed telescopic illustration of a single lunar feature, later published as a plate in his 1665 book Micrographia. Hooke was already arguing that the Moon’s surface was shaped by ongoing physical processes rather than being fixed and unchanging, and he floated both impact and volcanic origins for lunar craters — three centuries before either idea was settled. He also speculated, less usefully, that the crater floor might host something like grass and shrubs.
The Science: A Crater Worn Down by Deep Time
Hipparchus is classified as a pre-Nectarian feature, meaning it predates the Nectaris basin-forming impact — one of the oldest generally recognized impact structures on the Moon’s near side. Its present appearance owes less to its original formation than to everything that has happened to it since.
Billions of Years of Impact Gardening
The most direct explanation for Hipparchus’s ruined state is simple accumulated damage. Over roughly 3.9 billion years, an ancient crater this size is repeatedly struck by smaller impactors, has chunks of its rim excavated or buried by ejecta from neighbours like Albategnius, and gets steadily rounded off and lowered by the general churn of micrometeorite gardening. This process alone accounts for a great deal of the wall’s low relief and its gaps and transverse valleys, without requiring any additional mechanism.
Slow Structural Relaxation
Alongside outright erosion, very old, large highland craters may also have undergone limited long-term structural relaxation — a slow settling of the crust beneath the structure under its own weight. The Moon’s cold, rigid lithosphere means this process is not generally considered a dominant mechanism for highland craters, and impact-driven degradation almost certainly did most of the work on Hipparchus’s walls. Even so, some modest contribution from relaxation over billions of years is plausible and difficult to rule out from surface observations alone.
A Later, Separate Chapter of Volcanism
The basaltic infill on part of the floor is generally understood to be a distinct, much later event — mare-style lava reaching the crater floor long after Hipparchus itself formed, likely during the same broad era of volcanic activity that filled the nearby maria. What isn’t well pinned down is a precise date for this local resurfacing episode, or exactly why the lava covers some parts of the floor and not the raised, rugged southwest section. Whether that untouched area survived because it stood too high for the lava to reach, or for some other reason tied to the crater’s original uplifted interior, remains an open question rather than a settled one.
What makes Hipparchus worth the effort is that it rewards exactly the kind of looking that most other L100 targets don’t ask for: not a dramatic feature caught at the right moment, but a faint, damaged outline that only makes sense once you understand how much time and how many subsequent impacts stand between what Hipparchus looked like on day one and what’s left of it now.