L74 Copernicus H
A tiny crater ringed by a dark halo of excavated mare basalt, once mistaken for a volcanic vent — orbital imagery instead confirmed it as an impact scar punched through Copernicus’s own bright ejecta.
L74 Copernicus H
Oceanus Procellarum · Dark-Halo Impact Crater📉 Vital Statistics
🔭 Field Notes
Copernicus H is a tiny crater with an outsized reputation — barely 5 km across, sitting on Copernicus’s southwestern flank, ringed by a distinctly dark halo that stands out against the bright Copernicus ejecta blanket surrounding it. That contrast is what earns it a place on this list: the impact that formed Copernicus H cut through the light-toned highland ejecta draped over the region and excavated buried mare basalt beneath the bright ejecta blanket, throwing that darker material back out as a halo around the new, much smaller crater.
- ▶ Solving a Volcanic Mystery: Dark-halo craters like this one were once suspected to be volcanic vents rather than impact scars. High-resolution Lunar Orbiter V images provided compelling evidence that Copernicus H was formed by impact rather than volcanism, a conclusion later reinforced by Apollo photography and subsequent orbital observations.
- ▶ A Window Beneath the Ejecta: Because its dark halo comes from excavated mare basalt rather than fresh highland material, Copernicus H acts like a natural drill core, showing geologists how thick the Copernicus ejecta blanket is at this spot and confirming that darker mare rock lies just beneath it.
- ▶ One of Many Small Windows: Dark-halo craters of this kind turn up wherever a thin, bright ejecta layer sits over darker mare material, and mapping them has become a useful, low-cost way to estimate the depth of buried basalt across many regions of the Moon.
📍 Nearby L100 Targets
- L5 Copernicus: The 93 km “Monarch of the Moon,” practically next door at roughly 100 km away, its own ejecta blanket the very material Copernicus H’s impact had to punch through to reach the darker basalt hidden beneath.
- L65 Hortensius Domes: A tight cluster of low, gently sloped volcanic shield domes roughly 290 km west, several capped with tiny summit calderas — genuine, if subtle, lunar volcanism, in contrast to Copernicus H’s superficially volcanic-looking but ultimately impact-formed halo.
- L69 Copernicus Secondary Craters: A chain of secondary craterlets strung along a Copernicus ray near Pytheas, roughly 390 km north — ejecta from the same parent impact, but debris that landed far out on the mare rather than punching a fresh window through it in place.
🚀 Mission Log
Target Acquisition
Find Copernicus, then work its southwestern flank
Start at Copernicus, the 93 km “Monarch of the Moon” and one of the most recognizable craters in any instrument, at almost any phase. Copernicus H sits on Copernicus’s southwestern flank, still well within the bright ejecta blanket surrounding the main crater rather than out on the open mare.
Wait for higher Sun — this target is about albedo, not shadow
Unlike most Lunar 100 targets, Copernicus H isn’t a relief feature you catch at a grazing terminator. Its dark halo is an albedo contrast — excavated mare basalt thrown out against the surrounding bright highland ejecta — and it’s usually less conspicuous under very low Sun, since long shadows from the surrounding terrain reduce the visibility of its subtle albedo contrast. Look for it under higher Sun, roughly a few days past when the terminator first crosses this longitude, through to near full Moon, when the darker halo reads clearly against the brighter blanket around it.
Push magnification — the crater itself is tiny
At only ~5 km across, Copernicus H is a genuinely small target. Center on the southwestern rim of Copernicus and work up to 250x–350x to resolve the crater itself, then look for the darker halo ringing it against the lighter ejecta further out. The halo is often easier to pick out than the crater’s own tiny rim.
From Copernicus’s ejecta to Hortensius and the ray secondaries
Roughly 100 km away, Copernicus itself (L5) is the source of the very ejecta blanket beneath which Copernicus H excavated darker mare basalt. About 290 km west, the Hortensius Domes (L65) are a cluster of genuine low volcanic shields, several capped with tiny summit calderas — real lunar volcanism, in contrast to Copernicus H’s superficially volcanic-looking but ultimately impact-formed halo. And roughly 390 km north near Pytheas, the Copernicus Secondary Craters (L69) are a chain of craterlets strung along one of Copernicus’s rays — ejecta from the same parent impact, but debris that landed far out on the mare rather than excavating buried basalt close to the parent crater.
📝 Observation Log — L74 Copernicus H
0/3 CompleteIs Copernicus H visible tonight?
Copernicus H sits on Copernicus’s southwestern flank near 6.9°N, 18.3°W, still within the crater’s bright ejecta blanket. Unusually for this list, a grazing terminator actually hurts here: this dark-halo crater is an albedo feature, so it’s best seen under higher Sun, roughly Day 12–16 waxing through near full Moon, when its dark halo reads clearly against the brighter ejecta around it. Libration is not a meaningful factor at this longitude.
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When to Observe Copernicus H
Copernicus H flips the usual rule for this list — it’s an albedo feature, not a relief feature, so a grazing terminator actually works against you here. You want higher Sun, not a low one.
- For the Dark Halo: Wait for higher Sun, roughly Day 12–16 waxing through near full Moon. Copernicus H sits at 6.9°N, 18.3°W, on Copernicus’s southwestern flank. Under low Sun, long shadows from the surrounding terrain tend to mask its subtle contrast.
- Push Magnification: At only about 5 km across, the crater itself is a genuinely small target. Work up to 250x–350x once you’re centered on Copernicus’s southwestern rim.
- For Orientation: Start at Copernicus itself, the 93 km “Monarch of the Moon,” instantly recognizable at almost any phase, then look along its southwestern flank while still well within the bright ejecta blanket.
What to Look For
Look for a small crater ringed by a distinctly dark halo, standing out against the light-toned Copernicus ejecta blanket surrounding it. The halo is often easier to pick out than the crater’s own tiny rim.
The dark halo comes from mare basalt excavated from beneath the bright ejecta rather than from fresh highland material — a small natural drill core, hinting at how thick the Copernicus ejecta blanket is at this spot and showing darker mare rock lies close beneath it.
Dark-halo craters like this one were once suspected to be volcanic vents rather than impact scars. High-resolution Lunar Orbiter V imagery, later reinforced by Apollo photography and subsequent orbital observations, provided strong evidence for an impact origin instead.
Dark-halo craters of this kind turn up wherever a thin, bright ejecta layer sits over darker mare material, and mapping them has become a useful method for estimating the depth of buried basalt across many regions of the Moon.
The Science: An Impact Scar Mistaken for a Vent
Copernicus H’s interest lies in how a tiny, unremarkable-looking crater helped resolve a real debate about the origin of dark-halo features on the Moon.
Vital Statistics
Located at 6.9°N, 18.3°W, on Rükl chart 31, Copernicus H spans about 5 km and is named as a satellite designation of the crater Copernicus. Its L100 distinction: a dark-halo impact crater, younger than the roughly 800 million year old Copernicus impact itself.
From Volcanic Suspect to Impact Scar
Dark-halo craters were once suspected to be volcanic vents. High-resolution Lunar Orbiter V images provided compelling evidence that Copernicus H formed by impact rather than volcanism, punching through the light-toned highland ejecta draped over the region and throwing out buried mare basalt as a halo around the new, much smaller crater.
Mission Record
Lunar Orbiter II returned high-quality regional photographs in 1966 that helped place Copernicus H within the broader ejecta blanket. Lunar Orbiter V’s 1967 high-resolution imagery revealed blocky ejecta around the crater, helping resolve the long-running debate over its origin. Apollo 12 landed near Copernicus in November 1969 and sampled ejecta rays from the crater at its own landing site; those samples helped constrain the age of the Copernicus impact, providing an upper age limit for younger features like Copernicus H.
Most Lunar 100 targets ask you to chase shadows. Copernicus H asks the opposite — wait for the Sun to climb, then look for a small dark ring telling a story about what lies just beneath a much larger crater’s bright blanket of debris.