L20 Posidonius
Mare Serenitatisπ Vital Statistics
π Field Notes
Posidonius is a “floor-fractured crater” (FFC) on the eastern rim of the Sea of Serenity. Magma pressure from beneath uplifted the floor, creating a complex system of rilles and a “crater within a crater” appearance.
- βΆ Rimae Posidonius: A distinctive zig-zagging rille system running across the crater floor.
- βΆ Posidonius A: An 11km crater located near the center, often used as a focus test.
π Nearby L100 Targets
- L33 Dorsa Smirnov: The “Serpentine Ridge,” a massive wrinkle ridge directly West.
- L18 Mare Serenitatis: The dark lava edges of the basin bordering the crater.
- L34 Lacus Mortis: “Lake of Death,” a flooded crater complex due North.
π Mission Log
Target Acquisition
The Visual Anchor
Locate Mare Serenitatis (The Sea of Serenity) in the Moon’s northeast quadrant. Scan the eastern “shoreline” where the dark grey lava meets the bright, rugged highlands of the Taurus Mountains.
Locate the “Great Ring”
Posidonius is the largest, most distinct ring on this eastern edge. Look for a massive, shallow depression that appears almost like a bay. It is situated just South of the prominent crater pair Atlas and Hercules.
The Optics Challenge
Increase your magnification to 100x – 150x. The goal is to resolve the inner details: the smaller crater Posidonius A (slightly off-center) and the fine, zig-zagging rille (canyon) cutting across the floor.
π Observation Log
0/3 CompleteIs the Posidonius Crater visible tonight?
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When to Observe the Posidonius Crater
Posidonius sits prominently on the northeastern edge of Mare Serenitatis (“Sea of Serenity”). Because the crater floor is uplifted and shallow, it does not hold shadows for long. Catching it when the sun is low (near the terminator) is essential to seeing the complex network of cracks on its floor.
Best Viewing: 5β6 days after New Moon (Waxing Crescent) or 21 days after New Moon (Waning Gibbous).
The “Guardian” View: Posidonius acts as a massive, ruined gateway between the Sea of Serenity and the Lacus Somniorum (“Lake of Dreams”). It provides a stunning contrast to the nearby Dorsa Smirnov (Serpentine Ridge), allowing observers to compare a fractured crater against a tectonic wrinkle ridge in the same field of view.
What to Look For
1. The Fractured Floor
Unlike the smooth skating rink of Archimedes, the floor of Posidonius is chaotic. It is a “Floor-Fractured Crater” (FFC). In a good telescope, the floor looks tilted and rough, not flat.
Challenge: Can you spot Rimae Posidonius? This is a distinct, zig-zagging rille (lava channel) that cuts through the crater floor. It looks like a winding river or a lightning bolt etched into the grey rock.
2. The “Double Wall” Effect
Posidonius appears to have a crater within a crater. This isn’t actually a second impact; it is a massive section of the inner rim that slumped and separated from the main wall. This creates a terraced “double rim” look on the eastern side, while the western rim is almost non-existent where the mare lava breached the walls.
3. The “Bullseye” Crater (Posidonius A)
Look for a distinct, perfectly round crater located just off-center inside the main walls. This is Posidonius A (11 km wide). It is often used by astronomers to test the “seeing” conditionsβif you can see the sharp shadow inside this smaller crater clearly, the atmosphere is stable enough to hunt for the floor rilles.
The Science: Magma Uplift
Posidonius is a geological marvel because it tells a story of subterranean pressure, rather than just surface impact.
The Intrusion: After the original impact crater formed, magma rose from beneath the Moon’s crust but didn’t fully flood the crater.
The Uplift: Instead of breaking through, the magma pooled underneath the crater floor, pushing the ground upward like a blister.
The Fracture: This pressure caused the crater floor to crack, creating the famous rilles and giving Posidonius its shallow, plate-like depth. While Archimedes was filled from above, Posidonius was pushed up from below.