L12 Proclus Crater
A sharp, 28 km impact site perched on the western shore of the Sea of Crises — the proclus crater is defined by a brilliant “butterfly” ray system that marks where a low-angle projectile slammed into the lunar surface, scattering ejecta in every direction except one.
L12 Proclus
Palus Somni / Mare Crisium📉 Vital Statistics
🔭 Field Notes
Proclus is the textbook example of an oblique impact — the L12 hallmark. The projectile struck at a very low angle (~10–15°) from the east-southeast, creating a distinctive asymmetric ray system with a 120° forbidden zone to the southwest where no ejecta was emplaced.
- ▶ Asymmetric Rays: Prominent rays fan out to the NW, NNE, and NE — but the SW quadrant (the Palus Somni) is conspicuously dark and ray-free.
- ▶ Pentagonal Rim: The crater wall forms a distinctive pentagon shape — look for flat rim segments rather than a smooth circle.
📍 Nearby L100 Targets
- L25 Messier & Messier A: A grazing-impact ricochet pair in Mare Fecunditatis to the south — the projectile excavated Messier, then skipped downrange to form Messier A, leaving twin westward rays that make them a perfect companion study for Proclus’s own asymmetric ejecta.
- L31 Taruntius: A floor-fractured crater on the northwestern edge of Mare Fecunditatis — its concentric rilles, low rim, and central peak complex offer a striking contrast to Proclus’s crisp, youthful walls nearby.
- L48 Cauchy Region: Fault, rilles, and domes to the southwest across the Tranquillitatis border.
🚀 Mission Log
Target Acquisition
The Visual Anchor
Start with Mare Crisium — the Moon’s most distinctive isolated sea, easily visible to the naked eye as a dark oval near the eastern limb. Its isolation from all other maria makes it unmistakable on any map.
Find the Bright Spark
Look to the western border of Mare Crisium. Proclus sits just outside the mare itself, in the rough highland uplands of the Palus Somni that adjoin Crisium’s western shore — a strikingly brilliant pinpoint that stands out even at low magnification. It will likely catch your eye before you go looking for it.
Read the Rays
Increase to 100x – 150x and study the ejecta pattern. Bright rays fan asymmetrically to the NW, NNE, and NE — then stop abruptly. The dark tongue of Palus Somni to the west is conspicuously ray-free: this is the 120° forbidden zone, the signature of the oblique impact. Also look for the low central peak on the crater floor — it is a straightforward object even in a small telescope.
📝 Observation Log
0/4 CompleteIs Proclus visible tonight?
Proclus is best observed near Moon Day 5–6 for rim detail, or at Full Moon for the asymmetric ray system. Check the current phase below.
Check Moon Phase Today
🕐 When to Observe Proclus
Proclus sits in the highland uplands of the Palus Somni, just outside the western border of Mare Crisium. Unlike most craters, which have a single optimal window, Proclus genuinely rewards two separate sessions — each revealing a completely different feature.
Mare Crisium is your naked-eye anchor — the most isolated dark sea on the Moon. Proclus will catch your eye as a brilliant spark on Crisium’s western doorstep. It is the second brightest feature on the Moon after Aristarchus.
🔭 What to Look For
1. The Forbidden Zone
The headline feature of Proclus is its asymmetric ejecta pattern — the signature of an oblique impact. Bright rays fan out to the northwest, north-northeast, and northeast, but stop abruptly at the edge of the Palus Somni. The dark plain to the west is completely ray-free across a 120° arc.
2. The Pentagonal Rim
Most craters have roughly circular rims, shaped by the symmetrical shockwave of a vertical impact. Proclus is different. Its rim forms a distinct pentagon, with flat straight wall segments between each corner. This is thought to be influenced by pre-existing fractures or structural weaknesses in the lunar crust, though the exact mechanism is not fully established. Catch it near the terminator and look for the flat segments rather than a smooth curve.
3. The Central Peak
Unlike its heavily flooded neighbours such as Taruntius — whose interior lava buried any central structure — Proclus is young enough to have retained its central mountain. It is a low but distinct peak on the crater floor, described by observers as an easy object even in a small telescope. Spotting it is a useful reminder of why the peak exists: the rebound of rock after a massive impact, like the bounce at the centre of a water droplet splash.
⚗️ The Science: Reading an Oblique Impact
Proclus is the textbook example used to teach planetary scientists how to recognise and interpret an oblique impact — one where the projectile arrived at a very low angle rather than straight down.
In contrast to Archimedes — which is a lesson in time (lava flooding a basin) — Proclus is a lesson in geometry: what the shape of a crater and its ejecta can tell us about the direction and angle of the object that made it.