L69 Copernicus Secondary Craters
A tight string of small craterlets gouged into the mare near Pytheas within minutes of the Copernicus impact — proof that lunar rays are debris fields with real, mappable landforms, not just streaks of bright dust.
L69 Copernicus Secondary Craters
Southern Mare Imbrium · Rays & Craterlets Near Pytheas📉 Vital Statistics
🔭 Field Notes
Just east of the small, sharp-rimmed crater Pytheas, in the open mare south of Lambert and Timocharis, a tight line of small craterlets sits strung out along one of Copernicus’s brightest rays. These aren’t independent impacts; they’re secondary craters, gouged out by boulders and blocks of ejecta flung roughly 300 km north from the Copernicus impact itself. Rather than the round, isolated bowl shape of a primary crater, this chain reads as an obviously linear, sometimes overlapping string, confined tightly within the boundaries of the ray that carried it here.
- ▶ A Ray Made of Craters: Secondary craters like these are one of the clearest demonstrations that lunar rays aren’t just streaks of bright dust — they’re debris fields with real, mappable landforms embedded in them, laid down within minutes of the Copernicus impact roughly 800 million years ago.
- ▶ On the Map Since 1680: The secondary crater field around Copernicus was distinctive enough to be captured in Giovanni Cassini’s pioneering 1680 lunar map, centuries before the impact origin of these features was understood.
- ▶ Diagnostic Shapes: Because secondary craters form from low-velocity, often oblique debris rather than a single high-speed primary impactor, they tend to cluster in chains and clumps, sometimes elongated or overlapping — a shape and grouping that lets observers trace them back to their parent crater even at a glance.
📍 Nearby L100 Targets
- L5 Copernicus: The 93 km “Monarch of the Moon,” roughly 300 km south, its terraced walls and triple central peak the source of the very ejecta that carved this craterlet chain — the parent crater and its secondary debris field, seen together.
- L74 Copernicus H: A small, roughly 5 km dark-halo crater on Copernicus’s own southwestern rim, roughly 390 km south. Once thought volcanic, it’s now understood as an impact crater excavating dark mare basalt from beneath the Copernicus ejecta — a different kind of hidden history than the straightforward secondary craterlets near Pytheas.
- L78 Lambert R: A roughly 55 km “ghost crater,” its rim reduced to a faint circular ridge by Imbrium’s lava flows, about 130 km north near the crater Lambert. Where the Pytheas secondaries are a young, sharp-edged scar written on top of the mare, Lambert R is the opposite — an ancient crater almost entirely erased by the mare itself.
🚀 Mission Log
Target Acquisition
Find Pytheas, then look east into the open mare
Locate Pytheas, a small, sharp-rimmed crater in the open mare south of Lambert and Timocharis. Just east of it, look for a tight line of small craterlets strung out along one of Copernicus‘s brightest rays — roughly 300 km north of Copernicus itself, this chain of secondary craters was flung out and gouged into the mare within minutes of the main impact.
Time it to a low terminator
These craterlets are a genuine relief feature, so a grazing light angle helps. Best viewing is usually around Day 9 waxing or roughly Day 24 waning, when low-angle sunlight throws shadows across the tiny rims and makes the chain stand out clearly from the surrounding mare. Under high or full Sun, individual craterlets become much harder to separate from the ray material around them.
Trace the chain’s shape, not just the craters themselves
At 150x–250x, look past the individual ~4 km craterlets to the pattern they form together — an obviously linear, sometimes overlapping string concentrated along the ray that carried it here. That elongated, clustered arrangement is what marks these as secondary craters rather than a scatter of unrelated small primary impacts.
From the Pytheas chain to Copernicus, Copernicus H, and Lambert R
Roughly 300 km south, Copernicus (L5) is the 93 km “Monarch of the Moon” and the source of the ejecta that created this chain of secondary craters — parent crater and secondary debris field, seen together. About 390 km south on Copernicus’s own southwestern rim, Copernicus H (L74) is a small dark-halo crater once thought volcanic but now understood as an impact that excavated dark mare basalt from beneath the ejecta — a different kind of hidden history than the straightforward secondaries here. And roughly 130 km north near the crater Lambert, Lambert R (L78) is a “ghost crater” almost entirely erased by Imbrium’s lava — the opposite case: where the Pytheas secondaries are a young, sharp-edged scar written on top of the mare, Lambert R is an ancient crater the mare has nearly swallowed.
📝 Observation Log — L69 Copernicus Secondary Craters
0/3 CompleteAre the Copernicus Secondary Craters visible tonight?
This craterlet chain sits in southern Mare Imbrium near 19.6°N, 19.1°W, just east of the crater Pytheas. Best viewing is around Waxing Gibbous (roughly Day 9) or Day 24 waning, since these tiny, ~4 km secondary craters need a low sun angle to separate them from the surrounding ray material. Libration is not a meaningful factor at this longitude.
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When to Observe the Copernicus Secondary Craters
Unlike Copernicus H nearby, this chain is a genuine relief feature, so the usual advice applies here — you want a low, grazing terminator, not high Sun.
- Time It to a Low Terminator: Best viewing is generally around first quarter to waxing gibbous, or the equivalent waning phase, when the morning terminator crosses southern Mare Imbrium and low-angle sunlight throws shadows across the tiny rims. Under high or full Sun, individual craterlets become much harder to separate from the ray material around them.
- Trace the Chain, Not Just the Craters: At 150x–250x, look past the individual roughly 4 km craterlets to the pattern they form together — an obviously linear, sometimes overlapping string concentrated within one of Copernicus’s prominent rays.
- For Orientation: Locate Pytheas, a small, sharp-rimmed crater in the open mare south of Lambert and Timocharis, then look just east of it for the craterlet chain.
What to Look For
Look for a tight line of small craterlets strung out along one of Copernicus’s brightest rays, roughly 300 km north of Copernicus itself, reading as an obviously linear, sometimes overlapping chain rather than the round, isolated bowl shape of a primary crater.
Secondary craters like these are one of the clearest demonstrations that lunar rays aren’t just streaks of bright dust — they’re debris fields that include both bright ejecta and numerous secondary impact craters, laid down within minutes of the Copernicus impact.
Secondary craters tend to form from lower-velocity, often oblique debris rather than a single high-speed primary impactor, so they tend to cluster in chains and clumps, sometimes elongated or overlapping — a shape and grouping often allowing them to be traced back to their parent crater.
Early telescopic maps by Giovanni Cassini recorded the prominent Copernicus ray system centuries before the origin of secondary craters was understood.
The Science: Reading a Debris Field Written in Craters
The Pytheas chain’s interest lies in what it demonstrates about impact ejecta generally — that a bright ray isn’t just fine dust, but a scatter of real landforms carrying a legible signature of the impact that made them.
Vital Statistics
Located at 19.6°N, 19.1°W, on Rükl chart 20, near the crater Pytheas, itself named for the ancient Greek astronomer and explorer, the individual craterlets run about 4 km across. The chain formed during the Copernicus impact about 800 million years ago and therefore belongs to the Copernican System. Its L100 distinction: rays and craterlets near Pytheas.
Formed in Minutes, Not Millennia
These craterlets aren’t independent impacts — they were gouged out by boulders and blocks of ejecta flung roughly 300 km north from the Copernicus impact, arriving within minutes of the primary event and concentrated within one of Copernicus’s prominent rays.
Mission Record
Giovanni Cassini’s 1680 lunar map is among the earliest known telescopic maps to record the Copernicus ray system, produced centuries before impact ejecta processes were understood. Lunar Orbiter IV’s 1967 wide-area photographic survey captured the chains of secondary craters radiating from Copernicus across southern Mare Imbrium, including the craterlets near Pytheas. Apollo 12 landed on a Copernicus ray roughly 350 km to the southwest in November 1969, and its returned samples helped establish the age of the Copernicus impact at about 800 million years, anchoring the age of its entire ray and secondary-crater system.
Most Lunar 100 targets show you a single feature. The Pytheas chain shows you a process instead — a scatter of small, linear scars that trace an impact’s debris field back to the moment it was thrown across the mare.