L19 Alpine Valley
A 166 km straight-walled rift slicing through the Montes Alpes on the northeastern rim of Mare Imbrium — easy to find in any small telescope, but home to a slender central rille so faint that most reported sightings of it are probably mistaken.
L19 Alpine Valley
Montes Alpes📉 Vital Statistics
🔭 Field Notes
The Alpine Valley is a straight-walled rift slicing through the Montes Alpes on the northeastern rim of Mare Imbrium. Easy to find in any small telescope, it is one of the most visually striking non-crater features on the near side.
- ▶ Asymmetric Walls: The southern wall runs fairly straight along nearly the full length; the northern wall is more irregular and slightly bowed, giving the valley a lopsided look in profile.
- ▶ The Guardians: At the southwestern end, two mountains pinch the valley nearly shut before it reaches Mare Imbrium — nicknamed “the Guardians” by observers.
- ▶ The Central Rille: A slender sinuous rille runs down the lava-flooded floor. It’s notoriously hard to spot — low sun angle doesn’t help here as it does elsewhere, and many reported sightings in small scopes are likely mistaken.
📍 Nearby L100 Targets
- L83 Plato Craterlets: The dark, flat-floored 101 km crater Plato sits just southwest of the valley’s western mouth — its tiny floor pits are a classic resolution test.
- L14 Sinus Iridum: The flooded “Bay of Rainbows,” roughly 500 km to the southwest, shares the Alpine Valley’s connection to the Imbrium impact’s basin-forming aftermath.
- L4 Apennines: The much larger Imbrium basin rim segment on the opposite (southeastern) side of Mare Imbrium — the same ring system the Alpine Valley cuts through on this side.
🚀 Mission Log
Target Acquisition
Anchor on Plato
Start at Plato — the dark, lava-flooded 101 km crater that stands out instantly against the bright Montes Alpes and Mare Frigoris highlands to its north. Its flat, near-black floor makes it one of the easiest features on the Moon to spot at a glance, even at low power.
Look immediately east of Plato
The Alpine Valley sits right next to Plato, cutting through the Montes Alpes just to its east. You’re looking for a long, straight-walled gash slicing clean through the mountain range — unmistakable once the terminator is nearby, since nothing else in the area has that ruler-straight profile.
Center of the valley is at approximately 49.2°N, 3.6°E — close enough to Plato (~51.6°N, 9.4°W) that both fit in the same low-power field of view.Trace the valley to its Guardians
Follow the valley’s 166 km length from its wide northeastern mouth near Mare Frigoris down to its narrow southwestern end, where two mountains — nicknamed “the Guardians” — pinch it almost shut just before it opens onto Mare Imbrium. The southern wall runs nearly straight along the full length; the northern wall is more irregular and slightly bowed, giving the valley a lopsided look in profile.
Hunt the central rille at higher power
Switch to higher magnification and look down the lava-flooded floor for a slender, sinuous rille running nearly the valley’s full length. This is a genuinely tough target — low Sun angle doesn’t help here the way it does elsewhere, and many reported sightings in small scopes are likely mistaken. Larger apertures and a terminator positioned right on the valley give the best odds.
📝 Observation Log — L19 Alpine Valley
0/4 CompleteIs the Alpine Valley visible tonight?
Vallis Alpes shows best as the morning terminator crosses the Montes Alpes — check if the Moon is near First Quarter (or ~Day 20–21, six days after Full).
Check Moon Phase Today
When to Observe the Alpine Valley
The Alpine Valley sits at colongitude ~0° near the Moon’s center, just east of Plato — well clear of any limb foreshortening, and at high enough latitude (~49°N) that it benefits noticeably from favorable libration in latitude, which can cause the valley to visually “open up” on a given night. Its most reliable viewing window comes around First Quarter, or roughly six days after Full Moon, as the morning terminator crosses the Montes Alpes and lights the valley floor.
- Best Viewing: Around First Quarter (or ~Day 6 after Full Moon), when the morning terminator is crossing the Montes Alpes. The valley and its surrounding peaks are easy at almost any phase once lit, but the floor detail and especially the central rille are only well placed for a narrow window either side of the terminator.
- No Limb Distortion, But Libration Matters: At roughly 0° longitude, the valley sits near the Moon’s optical center and isn’t stretched by perspective the way limb features are. But its northern latitude (~49°N) means latitudinal libration has an outsized effect — a favorable northward libration opens the valley up for observation, while an unfavorable southward tilt can hide sections of the rille behind the valley’s own walls. Sources differ slightly on overall length (166–190 km) and width (10–20 km) depending on where along the valley the measurement is taken; the valley narrows sharply at both ends and widens in the middle.
What to Look For
At low power the valley reads as an unmistakable straight-edged cut through the Montes Alpes, running southwest to northeast between Mare Imbrium and Mare Frigoris — nothing else nearby has its ruler-straight profile. The southern wall runs fairly straight along nearly the full length; the northern wall is more irregular and slightly bowed, giving the valley a distinctly lopsided look in cross-section.
The valley is flanked by some of the tallest peaks in the Montes Alpes, with Mont Blanc — the range’s highest point at roughly 3.6 km — rising prominently to its southeast. At sunrise the sun first catches the higher peaks and ridge crests, throwing long jagged shadows across the floor while the rest of the terrain remains dark.
At the valley’s narrow southwestern end, two mountains — nicknamed “the Guardians” — pinch the valley to its narrowest point, nearly cutting it off from Mare Imbrium. Just north of the Guardians, the walls open out briefly into a broad, oval widening, sometimes called the amphitheatre by observers, before the valley narrows again and continues along its straight main run.
A slender, broken rille runs down the lava-flooded floor for nearly the valley’s full length, and continues beyond the obvious valley walls in places — measured end to end it’s actually a bit longer than the valley itself, while only about half a kilometer wide. It is one of the most difficult rilles on the Moon to observe visually: apertures of 8 inches or more are typically needed under excellent seeing, and even then, success depends heavily on having the terminator positioned almost exactly over the rille and a favorable northward libration to keep it from hiding behind the valley’s own walls.
The Science: Still an Open Question
Unlike some lunar features with a settled, well-documented origin story, the Alpine Valley’s formation is genuinely still debated. Most lunar geologists interpret it as a graben — a section of crust that dropped between two roughly parallel faults — but exactly what caused those faults to form is not fully resolved.
Leading Theory — Imbrium Impact Stress
The most widely favored explanation ties the valley’s formation to the same colossal impact that created the Imbrium basin roughly 3.8 billion years ago. The valley is not dissimilar to other linear features that radiate outward from Imbrium, and the stresses generated by that basin-forming impact may have fractured the crust along the lines that became the Alpine Valley graben.
Alternative Theory — Global Expansion or Cooling Contraction
A separate hypothesis ties the valley to broader, basin-independent tectonic stress: gravity data show the Moon’s radius actually grew slightly in its early history through internal heating and magma upwelling, then began contracting as the mantle cooled — and stress from either phase could have helped open the Alpine Valley’s faults. Both lava flooding and thermal or tensional stress are recognized mechanisms for graben formation in general, and it isn’t yet settled which combination applied here.
After the graben formed — by whichever mechanism — magma from the Imbrium and Frigoris mare flows later poured in, flooding the valley floor and leaving behind the sinuous rille that traces a former lava channel down its center. The crust, the fracture, and the flood are agreed on; only the trigger for the fracture itself remains an open question.