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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsMessier and Messier A are neighboring impact features in the Moon’s Mare Fecunditatis on the near side. They belong to one unusual impact system, but they are not identical: Messier is an east–west elongated crater, while Messier A is a nearby, roughly north–south, double-lobed satellite feature. NASA explains both shapes and their associated bright rays as the result of an impactor striking at an extremely shallow angle.
The most useful way to distinguish them is to combine position, orientation, catalog classification and measurement method. Messier is the eastern member of the pair; Messier A lies west of it. Their catalog diameters and NASA’s image dimensions do not use the same convention, so the figures should not be treated as contradictory measurements.
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Messier and Messier A at a glance
| Comparison point | Messier | Messier A |
|---|---|---|
| Gazetteer feature type | Crater | Satellite feature |
| Gazetteer center | 1.90° S, 47.65° E | 2.03° S, 46.94° E |
| Gazetteer nominal diameter | 13.80 km | 11.03 km |
| NASA descriptive dimensions | 15 × 8 km | 16 × 11 km |
| Overall form | Elongated, mainly east–west | Elongated, double-lobed form, mainly north–south |
| Position within the pair | Eastern member | Western neighbor |
| Origin | One extremely shallow-angle impact event, according to NASA | |
This table puts the basic distinction in one place, but each row answers a different question. Coordinates identify where the features are; the catalog type records how the International Astronomical Union naming system treats them; the diameter is a single catalog value; and the NASA dimensions describe the visible long and short spans of irregular shapes.
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Where the pair is located
Both features are in Mare Fecunditatis, the Sea of Fertility, on the Moon’s near side. The Gazetteer center for Messier is 1.90° south latitude and 47.65° east longitude. Messier A is centered at 2.03° south latitude and 46.94° east longitude. Because the longitude is lower for Messier A, it lies west of Messier; Messier is therefore the eastern member of the pair.
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The two coordinates are close enough to describe a neighboring system rather than unrelated craters scattered across the mare. At the same time, they are separate named features. A map that labels only “Messier” can therefore omit the western companion, while an image showing both may make the pair appear like one connected excavation unless the orientation and naming are checked.
Latitude and longitude are especially useful here because the shapes are not aligned in the same direction. Location alone tells you which feature is west or east, but it does not explain why their outlines look so different. For that, the long axes and the impact history matter.
Why the two shapes point in different directions
Messier is principally elongated from east to west. Messier A has an elongated, double-lobed appearance whose main direction is approximately north to south. The two orientations are close to perpendicular, which is the visual clue that most quickly separates the features in a photograph or lunar map.
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“Double-lobed” describes Messier A’s visible form; it does not mean the Gazetteer lists two independent craters under the name Messier A. The catalog treats Messier A as a satellite feature associated with Messier. In practical terms, you can see a distinct western feature with its own outline, but the official classification records its relationship to the principal crater rather than treating both names as equivalent entries.
Messier’s east–west axis and Messier A’s near north–south axis should be read as general orientations, not as perfectly measured compass lines. The outlines are irregular, and the dimensions depend on whether one measures a visible bounding span, a fitted diameter or another catalog convention. Their near-perpendicular appearance remains the important diagnostic.
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Why the size figures seem inconsistent
Two authoritative descriptions give different-looking numbers because they measure different properties. NASA’s image description gives Messier as 15 × 8 km and Messier A as 16 × 11 km. Those are two-dimensional dimensions: a long span and a short span intended to describe elongated, irregular outlines. The USGS/IAU Gazetteer instead lists one nominal diameter for each feature: 13.80 km for Messier and 11.03 km for Messier A.
A single catalog diameter is not required to equal either side of a rectangular long-by-short measurement. It is a standardized summary value for a feature whose shape does not form a perfect circle. NASA’s pair of dimensions preserves more information about elongation, while the Gazetteer’s single value makes the entries easier to compare in a catalog.
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|---|---|---|
| How long and wide does the visible outline appear? | NASA’s two-dimensional dimensions | Read the first number as the longer span and the second as the shorter span of the described shape. |
| What standardized size does the lunar feature catalog assign? | USGS/IAU nominal diameter | Use the single diameter for catalog comparisons, while recognizing that the outline is irregular. |
| Which feature is “bigger”? | Depends on the measurement convention | Messier has the larger Gazetteer diameter, but NASA’s long-span description gives Messier A a slightly greater maximum dimension. |
Therefore, the answer to “Which crater is bigger?” needs a qualifier. By the Gazetteer’s nominal diameter, Messier is larger. By NASA’s longest reported dimension, Messier A extends farther along its major axis. These statements are compatible because they summarize different geometric properties.
Are Messier and Messier A two separate craters?
They are two separately named lunar features with distinct outlines, but they are best understood as a related pair. Messier is cataloged as a crater. Messier A is cataloged as a satellite feature, a designation that identifies its association with a principal feature rather than presenting it as an unrelated crater elsewhere on the Moon.
This naming distinction prevents a common misunderstanding. “Messier A” is not simply a second spelling of Messier, and it is not evidence that the pair formed in two unrelated impacts. The names identify two parts of one unusual impact system: the main crater and its neighboring satellite feature.
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When reading a map, use the official feature type as a clue. A label for Messier marks the crater itself. A label for Messier A marks the western companion. If a source uses “the Messier craters” informally, check whether it means the pair as a system or only the principal crater. The formal Gazetteer categories resolve that ambiguity.
How a grazing impact produced the pair
NASA attributes the unusual geometry to an impactor traveling along an extremely shallow trajectory across the lunar surface. Its description states: “Their elongated shapes are explained by the extremely shallow-angle trajectory followed by an impactor, moving left to right, that gouged out the craters.” The wording describes a grazing or shallow-angle impact, not a vertical strike and not a precisely measured angle.
In a conventional mental picture of an impact, the projectile approaches almost straight down and excavates a roughly circular cavity. A very shallow approach changes that geometry. The moving body interacts with the surface over a longer horizontal path, so the excavation can be stretched rather than circular. The resulting pair has the two directional signatures seen here: Messier is elongated mainly east–west, and Messier A is elongated mainly north–south with a double-lobed outline.
The two features should not be treated as proof that two separate impactors arrived from unrelated directions. NASA’s explanation links their shapes to one grazing event. The pair’s alignment and shared ray pattern are the visible evidence for that interpretation.
What the bright rays mean
The same shallow impact also produced two bright rays of material extending along the surface to the right, beyond the area shown in NASA’s image description. These rays are ejecta: bright material thrown or spread outward during the excavation. Their paired, directional appearance is a visual clue that the neighboring features belong to a single unusual impact system.
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“Right” in NASA’s description refers to the orientation of the presented image, not a universal lunar compass direction. The safe conclusion is that the rays extend away from the pair along the impact path shown in that view. It would be misleading to convert that wording into a fixed eastward or westward direction without preserving the image’s orientation.
The rays also explain why a photograph can make the area look more extensive than the crater outlines alone suggest. The bright streaks continue beyond the immediate rims, while the named features remain the excavated forms at the center of the system. Do not count the rays as part of either crater’s diameter.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Rim and interior morphology: what can be identified reliably
The supplied NASA and Gazetteer descriptions establish the elongation, the double-lobed appearance of Messier A, the pair’s positions and the shared ray-producing impact. They do not provide a detailed inventory of rim terraces, central peaks, floor textures or wall heights for each feature. Those details should not be inferred from the names or from the diameter table.
What can be stated confidently is the contrast in plan view. Messier presents an elongated east–west outline. Messier A presents a longer north–south form with two visible lobes. The contrast is morphological, but it is primarily about the outline and orientation rather than a documented claim that one crater has a particular interior structure absent from the other.
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How to identify each feature in an image
If you are looking at a lunar map or a photograph that includes the pair, use the following sequence:
- Find the pair in Mare Fecunditatis. Confirm that the scene is on the Moon’s near side and in the Sea of Fertility.
- Locate the east–west elongated feature. This is Messier, the eastern member of the pair.
- Look immediately west for the companion. The western feature is Messier A, centered at the lower east longitude listed by the Gazetteer.
- Check the axes. Messier’s long direction runs generally east–west; Messier A’s elongated, double-lobed form runs approximately north–south.
- Trace the bright material. Two bright rays extend away from the system along the impact path shown in NASA’s presentation. They are ejecta rays, not additional craters.
- Read the label carefully. A catalog may identify Messier as a crater and Messier A as a satellite feature, even when both are visible together.
This method works better than judging apparent area alone. Illumination, image scale and the irregular outlines can make the larger-looking feature change from one view to another, while the east–west versus north–south orientation remains comparatively stable.
The Apollo photographs behind NASA’s stereo presentation
NASA’s stereo presentation uses high-resolution scans of Apollo 11 photographs AS11-42-6304 and AS11-42-6305. Those frame identifiers establish the imaging provenance for the presentation and explain why the pair can be examined with a stereoscopic effect rather than as a generic modern telescope image.
A stereo view can help the eye separate the excavated forms from surrounding mare, but it does not change the naming or measurement conventions. The catalog diameter remains a single standardized value, and NASA’s descriptive dimensions remain long-by-short spans of the visible shapes. The photographs provide the visual context for the orientation, lobes and rays; they do not supply a numeric impact angle or an absolute formation age.
What is not established by these sources
- No defensible absolute age is given. The NASA and USGS/IAU material identified here does not establish when either feature formed in years or assign it to a dated geological period.
- No numeric impact angle is given. The impact is described qualitatively as extremely shallow-angle or grazing. A precise degree value would require a source that actually reports one.
- No single size number answers every comparison. The Gazetteer diameter and NASA’s two-dimensional dimensions serve different purposes.
- No claim about unreported interior details should be added. The documented comparison concerns location, outline, orientation, classification, rays and origin.
Keeping these limits visible prevents a familiar lunar feature from acquiring invented precision. The evidence supports a strong explanation of how the pair differs without assigning an unsupported age, angle or detailed interior geology.
Bottom line
Messier and Messier A are neighboring, separately named parts of one grazing-impact system in Mare Fecunditatis. Messier is the eastern crater with an east–west elongation; Messier A is the western satellite feature with a roughly north–south, double-lobed form. Messier is larger by the USGS/IAU nominal-diameter convention, while Messier A has the slightly longer maximum span in NASA’s image description. Their unusual shapes and two bright rays are explained by one extremely shallow-angle impact, not by two unrelated ordinary craters.
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