While Tycho and Copernicus are famous for their jagged peaks and violent rays, the Archimedes crater is a masterpiece of silence. It is the largest crater in the Mare Imbrium basin, spanning 83 kilometers (52 miles), but its most striking feature is what itΒ lacks.
Archimedes has no central peak and a remarkably smooth, flat floor. Billions of years ago, lava welled up from beneath the lunar crust and filled this crater like a swimming pool, burying its central mountain and creating a perfect, dark mirror of stone.
L27 Archimedes
Imbrium Basinπ Vital Statistics
π Field Notes
Archimedes is the largest crater in Mare Imbrium. It is a “walled plain,” meaning its interior was completely flooded by lava after formation, burying its central peak.
- βΆ No Central Peak: The floor is famously smooth and flat, unlike its neighbor Aristillus.
- βΆ Terraced Walls: Look for the steep, terraced ramparts contrasting with the flat floor.
π Nearby L100 Targets
- L4 Apennine Mtns: The massive range directly Southeast.
- L66 Hadley Rille: Winding lava channel at the mountain base (Apollo 15).
- L63 Imbrium Sculpture: Radial grooves carved by the basin impact.
π Mission Log
Target Acquisition
The Visual Anchor
Start by locating the massive, curved spine of the Apennine Mountains (L4) on the eastern shore of Mare Imbrium. Follow this mountain range North until it ends in the dark lava plains.
Spot “The Trio”
Look just north of the mountains for a distinct triangle of three craters. You will see Aristillus and Autolycus (the smaller, deeper ones). The largest, flattest crater in this group is Archimedes.
The Optics Challenge
Increase your magnification to 100x – 150x. Your goal is not to find a central peak (it’s buried under lava), but to resolve the terraced steps on the inner walls. Look for the jagged, saw-tooth shadows cast onto the smooth floor.
π Observation Log
0/3 CompleteIs the Archimedes Crater visible tonight?
Check our real-time tool to see if the Moon is in the First Quarter or Waxing Gibbous phase.
Check Moon Phase Today
When to Observe the Archimedes Crater
Archimedes sits just on the eastern edge of the Mare Imbrium (“Sea of Rains”). Because it is relatively shallow, the shadows on the floor disappear quickly, so catching it near the terminator is critical for seeing the wall texture.
- Best Viewing: 7-8 days after New Moon (First Quarter / Waxing Gibbous) or 21-22 days after New Moon (Last Quarter / Waning Gibbous).
- The “Trio” View: Archimedes forms a famous trio with two neighbors, Aristillus and Autolycus. Comparing them is a great lesson in geology: Aristillus is deep and jagged (young), while Archimedes is flat and filled (old).
What to Look For
1. The Flat Floor
The floor of Archimedes is arguably the smoothest terrain on the entire moon. In a good telescope, it looks like a dark gray skating rink.
- Challenge: Can you spot any “craterlets” on the floor? It is almost featureless, but steady skies might reveal tiny pinprick impacts (micro-craters) on the lava plain.
2. The “Elephant Skin” Walls
While the floor is smooth, the rim is ancient and rugged. This texture is often called “elephant skin.” The rim rises about 2 kilometers above the surrounding mare, casting jagged saw-tooth shadows onto the nearby lava plains during sunrise.
3. The Spiteful Debris
Look closely at the bright terrain between Archimedes and its neighbor Aristillus. You will see a rough, “hummocky” texture. This is actually debris thrown out by the impact that formed the Imbrium Basin itselfβa chaotic reminder of the violent event that happened before the quiet lava flooding occurred.
The Science: The Law of Superposition
Archimedes is a “textbook” crater for geologists because it helps date the Moon’s history using the Law of Superposition (younger things sit on top of older things).
- Oldest: The Imbrium Basin formed (giant impact).
- Middle: The Archimedes impact happened, punching a hole in the basin.
- Youngest: Lava flooded the basin and flowed into Archimedes, smoothing it out.
Because the lava is inside Archimedes, we know the crater formed before the volcanic era ended. In contrast, its neighbor Aristillus has no lava inside, meaning it must have struck after the lava solidified.