L25 Messier Craters
A pair of oblong impact craters in Mare Fecunditatis, widely interpreted as the result of an extraordinarily shallow, low-angle impact — Messier and Messier A sit nose to tail like a ricochet’s two footprints, while a pair of nearly straight bright rays streams 100+ km westward across the mare, giving the formation the appearance of a comet streaking across the lunar surface.
L25 Messier & Messier A
Western Mare Fecunditatis📉 Vital Statistics
🔭 Field Notes
Messier and Messier A are a closely paired set of impact craters sitting on the flat basalt floor of western Mare Fecunditatis, and together they represent one of the most studied examples of an oblique impact on the Moon. The two craters sit only about 20 km apart and are nearly identical in diameter — both just under 14 km across by IAU measurement — but their shapes differ markedly: Messier is elongated east-west, while Messier A has an irregular, roughly triangular form with its longer dimension oriented north-south. Extending westward from Messier A is the feature’s most distinctive signature: a pair of nearly parallel bright rays that stretch across the mare for roughly 100 km, narrowing as they go and maintaining a remarkably consistent separation — resembling, to early observers, a comet’s tail or a pair of searchlight beams. These rays are ejecta thrown out asymmetrically during a very low-angle impact. Various models have been proposed to explain the paired craters, including ricochet-like trajectories, fragmented impactors, or complex downrange ejecta processes — the low-angle origin is well accepted, but the exact mechanism remains a subject of discussion. Resolving the craters individually requires a modest telescope and steady seeing; the double ray is detectable in somewhat smaller instruments under high-Sun illumination.
- ▶ The Oblique Impact Signature: Several features distinguish Messier from a typical vertical impact. The primary crater is markedly elongated along the apparent direction of travel. The ejecta field is strongly asymmetric, concentrated downrange to the west rather than distributed in a symmetric blanket. The double ray extending from Messier A — rather than a radial ray system centered on either crater — is characteristic of very low-angle impacts where ejecta is concentrated in a narrow downrange plume. Taken together, these are generally interpreted as the product of an impactor arriving at an extremely low angle — probably under about 15° to the horizontal — though the exact geometry and whether one or two impactors were involved remains a subject of ongoing discussion.
- ▶ The Double Ray: The two parallel rays extending westward from Messier A are among the most geometrically striking ray features on the lunar near side. They remain bright and relatively narrow for roughly 100 km before fading into the mare. Their near-perfect parallelism and the absence of comparable rays in other directions — particularly the strong suppression of ejecta to the east, the direction from which the impactor is thought to have arrived — make the pair a compelling target at or near Full Moon when ray contrast is highest against the dark mare surface.
- ▶ Shape Asymmetry: Under low-Sun illumination near the day 5–6 or day 19–20 terminator, the morphological differences between the two craters become apparent in moderate apertures. Messier proper is visibly elliptical, elongated roughly east-west, while Messier A has an irregular, roughly triangular outline with curved sides — its longer axis oriented approximately north-south, at right angles to Messier. The interior floors of both are relatively flat compared with many fresh lunar craters, consistent with their formation within smooth mare basalt and the likely influence of impact-melt deposits pooling on the floor.
📍 Nearby L100 Targets
- L31 Taruntius: A 56 km flooded impact crater sitting on the northwestern floor of Mare Fecunditatis, roughly 200 km northwest of Messier. Taruntius is notable for its unusually low, subdued rim and its remarkably flat interior, which bears a prominent system of concentric rilles — curved fractures that roughly parallel the inner wall. These fractures are generally attributed to volcanic and tectonic modification of the flooded crater floor. The central peak is small and partially buried. Taruntius is among the better examples on the near side of a crater whose post-impact modification by mare volcanism is still legible in the landscape.
- L85 Langrenus Rays: The crater Langrenus — a 132 km walled plain on the eastern shore of Mare Fecunditatis, roughly 400 km northeast of Messier — produces a prominent ray system that extends across the mare floor and is visible near Full Moon. Langrenus itself is a geologically young, well-preserved impact structure with high terraced walls, a prominent central peak complex, and an ejecta blanket that includes one of the few ray systems easily visible on the eastern limb region under high-Sun conditions. Comparing the Langrenus rays with the narrow double-ray geometry of Messier A, visible in the same general field, offers a striking illustration of how dramatically impact angle shapes ejecta distribution.
- L48 Cauchy Region: The Cauchy region, lying on the floor of Mare Tranquillitatis roughly 300 km northwest of Messier, encompasses a rare combination of features in close proximity: the 12 km crater Cauchy, the straight fault scarp Rupes Cauchy running roughly 120 km east-west, and the sinuous graben Rima Cauchy running parallel to it on the opposite side of the crater. Having a fresh impact crater, a compressional fault scarp, and a tensional rille within the same small field of view makes this one of the more instructive regions on the near side for demonstrating the range of tectonic processes that operated as the mare lavas cooled and the crust adjusted after flooding.
🚀 Mission Log
Target Acquisition
Locate Mare Fecunditatis, then anchor on Langrenus
Mare Fecunditatis is the large dark oval on the Moon’s southeastern quadrant — visible naked-eye as a distinct patch below and right of Mare Tranquillitatis. With binoculars, pick up Langrenus, the prominent 132 km walled plain on the eastern shore of the mare with a bright ray system visible at high sun. That gives you a hard anchor in the right part of the field. Messier sits on the flat mare floor in the western half of the basin, just south of centre.
Step west from Langrenus to the Messier pair
From Langrenus, sweep westward across the mare floor. At 50x – 100x near the day 5–6 terminator, two small craters will come into view close together — Messier to the east and Messier A roughly 20 km to its west. At this illumination, shape differences start to emerge: Messier reads as distinctly elongated east-west, while Messier A has a rounder but irregular rim. Neither is large — both sit just under 14 km — so steady seeing matters. If the seeing is poor, wait; these are not targets for a turbulent night.
Switch to high sun to catch the double ray
The craters themselves are the terminator target. The double ray is the opposite: it shows best under a high-Sun or near-Full Moon view. With shadows gone and albedo contrast at its peak, scan westward from Messier A. Two nearly parallel bright streaks extend roughly 100 km across the dark mare floor, maintaining a consistent separation as they go. They are narrower and more geometrically precise than the broad fan rays of Langrenus to the east — that contrast in ray geometry, visible in the same field, is part of what makes this area instructive.
Compare ejecta suppression to the east
Once you have the double ray, note what is missing. The region east of Messier — the direction the impactor is thought to have come from — shows strong suppression of ejecta compared with the bright plume extending west. No comparable ray system extends eastward from either crater. This asymmetry is the observational signature of the oblique impact and is visible even at modest magnification under high sun, though it requires a deliberate look rather than a casual sweep.
📝 Observation Log — L25 Messier & Messier A
0/4 CompleteAre Messier & Messier A visible tonight?
Messier rewards two separate sessions. The crater shapes and elongation are best seen near the terminator around Waxing Crescent (Day 5–6) or Waning Gibbous (Day 19–20). The double ray extending from Messier A shows best under high-Sun illumination near Full Moon — the two targets need opposite lighting conditions.
Check Moon Phase TodayReturn to the List
Ready to find the next target? Go back to the full map to see what else is visible tonight.
← Back to Lunar 100 Map
When to Observe Messier & Messier A
Messier and Messier A are one of the few Lunar 100 targets where the two defining features require opposite lighting conditions. The crater shapes and morphological differences between the pair are terminator targets — low raking light throws the elongated form of Messier and the irregular outline of Messier A into relief. The double ray extending westward from Messier A is a high-Sun feature — it washes out under low illumination and shows best when the mare floor is in flat, shadowless light near Full Moon.
- Crater Shapes and Morphology: Best around Day 5–6 (Waxing Crescent) or Day 19–20 (Waning Gibbous), when the terminator crosses western Mare Fecunditatis. Under low sun the elongation of Messier and the irregular outline of Messier A are most legible, and the morphological differences between the two craters become apparent at 100x or more.
- Double Ray from Messier A: Best under high-Sun illumination near Full Moon (Day 14–15). Two narrow parallel bright rays extend westward across the mare floor, maintaining a consistent separation. The strong suppression of ejecta to the east — the direction of impactor arrival — is also most obvious at this phase.
What to Look For
At 100x or more near the terminator, Messier often appears distinctly elongated in the east-west direction — roughly 15 km along its long axis versus about 8 km across. This is not foreshortening; the crater sits close enough to the centre of the near side that its true shape is readable. The elongation is one of the most direct visual signatures of an oblique impact available anywhere on the Moon, and confirming it at the eyepiece is the primary observational goal for the terminator session.
Under high-Sun illumination, scan westward from Messier A. Two narrow, nearly parallel bright rays extend across the dark mare floor, maintaining a consistent separation across their length. They are geometrically tighter and more parallel than the broad radial fans produced by most fresh impact craters — Langrenus, visible in the same general area of Mare Fecunditatis, provides an immediate comparison. The parallelism of the Messier A rays is the feature that puzzled observers for two centuries before oblique impact experiments explained it.
Under high Sun, the area immediately surrounding both craters rewards close study at moderate magnification. The bright ejecta blanket is noticeably concentrated to the west and northwest of the pair — downrange from the inferred impactor trajectory — while the terrain to the east is comparatively unmarked. This asymmetry in the local ejecta field, combined with the elongation of Messier and the double ray from Messier A, forms a strong observational case for the oblique impact interpretation that is now widely accepted.
Under any illumination, both Messier and Messier A have noticeably higher albedo interiors than the surrounding mare basalt. The bright walls and floors make the pair stand out even under moderate magnification. This brightness reflects the youth of the craters relative to the ancient Fecunditatis lava plain — the impact excavated fresh, unweathered material, which has had less time to darken through space weathering than the surrounding surface. The craters are considered geologically young, though their precise age has not been directly determined.
The Science: How One Impact Made Two Craters
The Messier crater pair is one of the most studied examples of oblique impact on the Moon. The unusual shapes, asymmetric ray system, and paired structure have generated competing explanations for over two centuries. Modern impact physics has narrowed the field considerably, but the exact formation sequence remains a subject of discussion.
The Oblique Impact Consensus
The mainstream interpretation, supported by laboratory impact experiments, is that a single impactor arrived from the east at an extremely low angle — probably under about 15° to the horizontal. At such shallow angles, experimental impacts in sand and rock consistently produce elongated primary craters with strongly asymmetric ejecta concentrated downrange, matching Messier’s east-west elongation and the westward double ray. The oblique trajectory also explains the near-total suppression of ejecta to the east: at very low angles, the impactor’s momentum carries almost all excavated material forward into the downrange hemisphere.
How Messier A Formed: Three Proposals
The origin of Messier A — the second, westward crater — is where the precise formation sequence remains debated. Three mechanisms have been proposed: (1) a ricochet or skip, in which the impactor bounced off the surface after forming Messier and struck again to the west; (2) a fragmented impactor that broke apart before or during impact, with one fragment forming each crater; (3) complex downrange ejecta processes in which the primary impact threw a coherent mass of material westward that formed a secondary impact crater. Each mechanism is physically plausible, and the observational evidence from Earth or orbit has not been sufficient to rule any of them out definitively.
What the Laboratory Experiments Showed
The foundational experimental work by Don Gault and John Wedekind at NASA’s Ames Research Center in 1978 demonstrated that the shapes of both Messier and Messier A can be reproduced by firing projectiles into sand targets at angles of 4° or less from the horizontal. Both elongated primary craters and paired secondary structures appeared in their experiments at the most oblique angles, establishing the physical plausibility of the oblique impact explanation and ending the earlier era of speculative or non-impact interpretations. The Gault-Wedekind experiments remain the observational anchor for understanding the Messier system.
What makes Messier genuinely instructive as a telescopic target is that the impact physics is directly readable in what you can see. The elongation, the double ray, the ejecta suppression to the east — each is a different consequence of the same shallow trajectory, and each is accessible to a modest amateur telescope under the right illumination. Few features on the near side let you watch physics happen quite so legibly.