L73 Smythii Basin
One of the Moon’s oldest recognized basins, squeezed by limb foreshortening into a thin, dark sliver most of the month — opening briefly into view only when favorable libration tips its eastern edge toward Earth.
L73 Smythii Basin
Eastern Limb · Near/Far Side Border📉 Vital Statistics
🔭 Field Notes
Mare Smythii sits almost exactly on the Moon’s eastern edge, roughly circular in reality but squeezed by limb foreshortening into a thin, elongated sliver most of the month. A significant portion of the basin lies out of sight on the far side entirely, and even the near-side fraction only reveals more of its outer basin wall — rising visibly above the local horizon — and interior lava flows under a genuinely favorable libration. Catch it at the wrong moment and there’s very little to see; catch it right, and one of the Moon’s oldest recognized basins briefly rises into view.
- ▶ A Basin You Catch Sideways: Because Smythii sits so close to the limb, its circular outline is almost always seen edge-on and compressed; only during peak eastern libration does the basin open up enough to distinguish its dark mare floor from the surrounding highland wall.
- ▶ Straddling Two Worlds: Smythii is one of the relatively few large lunar basins that spans both the near side and the far side, making it a rare naturally occurring bridge between the hemisphere we know well and the one we’ve only mapped from orbit.
- ▶ One of the Moon’s Oldest Recognized Basins: Photogeologic mapping has identified multiple concentric ring structures within the basin, evidence of an impact old enough to predate the Nectaris basin itself; subsequent bombardment and later, more modest lava flows have left Smythii’s outer walls heavily worn compared to younger basins like Crisium or Imbrium.
📍 Nearby L100 Targets
- L10 Mare Crisium: The 555 km, cleanly circular mare basin roughly 1,020 km northwest, sitting fully on the near side and easy to spot even naked-eye — a study in contrast with Smythii’s limb-hugging, half-hidden, foreshortened presence.
- L87 Humboldt: A roughly 207 km floor-fractured crater near the southeastern limb, about 780 km south-southwest, its cracked floor and central peaks lifted by intrusive volcanism after impact — a case of confirmed post-impact igneous activity, unlike the more ambiguous volcanic and impact history still being worked out at Smythii.
- L100 Mare Marginis Swirls: High-albedo swirl patterns on the mare floor roughly 465 km north, associated with a strong local crustal magnetic anomaly thought to deflect the solar wind and keep the underlying regolith brighter and less weathered — another of the eastern limb’s libration-dependent mysteries, in the same challenging neighborhood as Smythii itself.
🚀 Mission Log
Target Acquisition
Start at Mare Crisium, then push all the way to the limb
Locate Mare Crisium, the 555 km circular mare that’s easy to spot even naked-eye near the eastern limb. From there, continue east roughly 1,020 km, right out to the Moon’s edge. Mare Smythii sits almost exactly on that eastern limb itself, squeezed by foreshortening into a thin, dark sliver for most of the month rather than the roughly circular basin it actually is.
Check libration first — timing this target is about geometry, not phase
This is a libration target before it’s anything else. Smythii’s visibility has less to do with which day of the lunar month it is and more to do with whether favorable eastern libration is currently tipping the basin into better view. Check a libration table or app for a period of strong positive libration in longitude — that’s the window worth planning a session around, regardless of what phase it happens to fall on.
Make sure the region is also sunlit
Favorable libration alone isn’t enough — the basin also needs to be catching sunlight. Mare Smythii is generally illuminated from around waxing gibbous through waning gibbous, so plan your libration check within that broader window. Get both conditions to line up and the outer basin wall rises visibly above the local horizon, with the dark mare floor briefly distinguishable from the surrounding highland rim.
From Crisium’s clean circle to Humboldt and the Marginis swirls
Roughly 1,020 km northwest, Mare Crisium (L10) is a study in contrast: fully on the near side, cleanly circular, and easy to spot any night, versus Smythii’s limb-hugging, half-hidden presence. About 780 km south-southwest, Humboldt (L87) is a floor-fractured crater near the southeastern limb whose cracked floor and central peaks were lifted by confirmed post-impact igneous activity — a firmer volcanic story than Smythii’s still-debated history. And roughly 465 km north, the Mare Marginis Swirls (L100) are high-albedo patterns tied to a local crustal magnetic anomaly — another libration-dependent mystery in the same difficult neighborhood as Smythii.
📝 Observation Log — L73 Smythii Basin
0/2 CompleteIs Mare Smythii visible tonight?
Mare Smythii sits right on the eastern limb near 2.0°S, 87.0°E, squeezed by foreshortening into a thin, dark sliver most of the month. As with Humboldtianum, the day of the month matters less here than favorable eastern libration: check a libration table for a window when the basin tips into better view, ideally overlapping a period when the region is also sunlit, roughly between waxing gibbous and waning gibbous.
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When to Observe Smythii Basin
Mare Smythii lies very close to the Moon’s eastern limb, squeezed by limb foreshortening into a thin, dark sliver most of the month. Catching it as an actual basin, rather than a sliver, takes two conditions lining up together — libration and sunlight.
- Check Libration First: This is a libration target before it’s a phase target. Smythii’s visibility depends far more on whether favorable eastern libration is currently tipping the basin into better view than on which day of the lunar month it is. Check a libration table or app for a window of strong positive libration in longitude.
- Then Make Sure It’s Sunlit: Favorable libration alone isn’t enough — the region also needs suitable illumination, with the basin generally easiest once the morning terminator has moved well past the eastern limb.
- For Orientation: Start at Mare Crisium, the 555 km circular mare that’s easy to spot even naked-eye, then continue east toward the lunar limb.
What to Look For
Because Smythii sits so close to the limb, its circular outline is almost always seen edge-on and compressed. Only during peak eastern libration does the basin open up enough to distinguish its dark mare floor from the surrounding highland wall.
Smythii is one of the relatively few large lunar basins that spans both the near side and the far side, with a significant portion permanently out of view no matter how favorable the libration gets.
Photogeologic mapping has identified multiple concentric ring structures within the basin. Subsequent bombardment and later mare basalt flooding have left Smythii’s outer walls heavily worn compared to younger basins like Crisium or Imbrium.
The basin’s ring structure points to an impact generally classified as Pre-Nectarian, making the sliver you do catch one of the oldest impact basins visible from Earth.
The Science: An Ancient Basin at the Edge of Sight
Smythii’s interest lies as much in how hard it is to see as in what it actually is — a genuinely old basin whose true shape has to be pieced together from a foreshortened sliver and orbital data.
Vital Statistics
Located at 2.0°S, 87.0°E, on Rükl charts 38 and 49, the impact basin spans roughly 740 km, with Mare Smythii’s basalt fill covering about 375 km of that. It’s named for William Henry Smyth, a nineteenth-century Royal Navy admiral and astronomer. Its L100 distinction: a difficult-to-observe basin scarp and mare.
A Pre-Nectarian Survivor
Smythii is generally classified as Pre-Nectarian, making it older than Nectaris in current lunar stratigraphy. Multiple concentric rings, though heavily degraded, have been traced through photogeologic mapping, evidence of an early impact that has since been worn down by subsequent bombardment and later mare volcanism.
Mission Record
Lunar Orbiter IV provided the first systematic wide-angle photographic coverage of the limb-straddling region in 1967, essential for later photogeologic mapping of the basin’s ring structure. JAXA’s SELENE (Kaguya) mission used its onboard Lunar Radar Sounder between 2007 and 2009 to help identify subsurface interfaces beneath the mare basalts, improving understanding of the basin’s volcanic history and mare thickness. Since 2009, LRO’s high-resolution imaging and laser altimetry have helped refine the basin’s topography and geologic history.
Most Lunar 100 targets ask you to wait for the right light. Smythii asks you to wait for the right geometry — a rare window when libration and sunlight line up together to tip one of the Moon’s oldest basins briefly into view.