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Messier and Messier A are neighboring impact features in the Moon’s Mare Fecunditatis, but they are not identical craters. Messier is the eastern, mainly east–west elongated crater; Messier A is the western, larger-looking double-lobed feature whose long axis runs approximately north–south. NASA interprets the pair and their bright rays as the products of one extremely shallow-angle, or grazing, impact.
The two features at a glance
The names describe a closely related pair rather than two unrelated objects scattered across the Moon. Both lie on the lunar near side in Mare Fecunditatis, the Sea of Fertility. Messier is east of Messier A; the Gazetteer coordinates place Messier A about 0.71 degrees farther west in longitude.
| Comparison | Messier | Messier A |
|---|---|---|
| USGS/IAU 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 image dimensions | 15 × 8 km | 16 × 11 km |
| Overall appearance | Elongated, mainly east–west | Elongated, double-lobed or double-crater form, mainly north–south |
| Position in the pair | Eastern member | Western neighbor |
| Associated rays | Two bright rays extending away from the pair, attributed to the same impact event | |
The table contains two kinds of size information. The USGS/IAU Gazetteer assigns one nominal diameter to each named feature, while NASA’s image description gives two dimensions that capture the long and short axes of these irregular shapes. Those measurements answer different questions and should not be treated as a contradiction.
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Mare Fecunditatis is a broad basaltic plain on the Moon’s near side. In the USGS/IAU Gazetteer, Messier is centered at 1.90° south latitude and 47.65° east longitude. Messier A is centered at 2.03° south and 46.94° east. Because lunar longitude increases toward the east, the smaller longitude places Messier A west of Messier.
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The latitude values are close, so the pair occupies nearly the same north–south zone. Their separation is therefore most apparent as an east–west relationship: locate Messier on the eastern side and look west for Messier A. Coordinates identify the catalog centers; they do not imply that either feature is perfectly circular or that the listed point marks the visual center of every lobe.
Both names are official International Astronomical Union names recorded by the USGS Gazetteer. The catalog’s classification is significant: Messier is entered as a crater, whereas Messier A is entered as a lunar satellite feature. “Satellite feature” is a catalog relationship to a primary feature and does not mean that Messier A is a moon orbiting Messier.
Why the published sizes appear inconsistent
NASA’s two-dimensional description
NASA describes Messier as 15 × 8 km and Messier A as 16 × 11 km. A pair of numbers is useful for an elongated object because it records the approximate length and width visible in an image. It preserves the fact that the features are not well represented by circles.
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The Gazetteer’s single nominal diameter
The USGS/IAU Gazetteer lists a nominal diameter of 13.80 km for Messier and 11.03 km for Messier A. A single catalog diameter is a standardized field for a named lunar feature, not a promise that every point on an irregular rim lies exactly that distance from the center. Cataloging conventions can therefore produce a value that differs from either the long or short dimension measured from an image.
How to read the comparison
It is not sound to ask which source has the “correct” size without specifying the measurement convention. NASA’s values describe the apparent two-axis footprint in the cited image description. The Gazetteer values are nominal catalog diameters. For a nearly circular crater, those numbers would be expected to be similar; for an elongated or lobed pair, a difference is normal and informative.
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| Question | Most useful value | Reason |
|---|---|---|
| How long and wide does the feature look in the image? | NASA’s dimensions | They preserve the long and short axes. |
| What standardized diameter is attached to the official catalog entry? | USGS/IAU nominal diameter | It supplies one comparable catalog field. |
| Does one feature have a more complicated outline? | The combination of both sources and the image | A single diameter cannot express a double-lobed or strongly elongated outline. |
Orientation is the clearest visual difference
Messier: the east–west member
Messier’s long dimension runs generally east–west. In a standard lunar map with north at the top, its outline is stretched sideways. The feature is therefore recognized not simply by its size, but by the direction of its elongation.
Messier A: a north–south, double-lobed form
Messier A is elongated in an approximately north–south direction and has a double-lobed or double-crater appearance. Its long axis is close to perpendicular to Messier’s. This near-crosswise arrangement is why the pair can be identified even when the image scale makes their individual rims difficult to distinguish.
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The cited descriptions establish the outline, orientation and lobed character. They do not provide a quantified comparison of rim heights, floor depths, central peaks, wall terraces or ejecta thickness. An image may suggest differences in interior shading, but those visual impressions depend on illumination and viewing geometry. The defensible morphological distinction is the plan-view shape: Messier is a single east–west elongated member, while Messier A has the north–south double-lobed form.
Why the pair is elongated
NASA attributes both unusual shapes to one impactor moving along an extremely shallow trajectory across the lunar surface. Its published explanation 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 impact rather than a vertical or steeply inclined strike.
This model explains why two neighboring depressions have different orientations yet appear connected as one system. A low-angle projectile can interact with the surface over an extended path instead of producing one nearly circular bowl. The resulting excavation is distributed along the direction of travel, helping account for Messier’s sideways elongation and Messier A’s separate, lobed geometry.
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The source does not give a measured numerical impact angle. “Extremely shallow-angle” is therefore the appropriate level of precision; assigning a degree value would go beyond the cited evidence. Likewise, the available authoritative descriptions do not establish an absolute formation age or a specific geological period for either feature.
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The pair is accompanied by two bright rays that extend along the surface away from the craters. NASA explains that the same shallow impact produced these rays, describing material extending to the right beyond the photographed scene. In an image, the rays are a clue that the two depressions belong to one unusual impact system rather than being a chance juxtaposition of unrelated craters.
“Right” refers to the orientation of NASA’s presentation, not to a universal compass direction on every lunar map. Rotate the image, change the map projection or reverse the viewing direction and the rays will no longer appear on the page’s right-hand side. The stable geological point is that they extend outward from the pair along the impact-related surface track.
The rays should not be used to calculate a crater diameter. They are deposits spread across the surrounding mare, whereas the diameter fields describe the named depressions. Their brightness is an image-visible property of the ray material and lighting, not a separate crater measurement.
Are Messier and Messier A two separate craters?
They are two separately named lunar features with a shared origin. Messier has its own official crater entry. Messier A is cataloged separately as a satellite feature associated with Messier. Calling them a “pair” refers to their close placement, complementary shapes, common impact explanation and shared ray system; it does not mean that the catalog treats them as one single crater.
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The distinction matters when reading lunar maps or databases. A search for Messier may return the primary crater entry, while Messier A appears as the related satellite feature. The letter A is part of the official designation and is not an indication that the feature is merely a temporary label or an informal nickname.
How to identify the pair in an image
Use the following sequence when a lunar photograph or map shows this region:
- Confirm the setting. Check that the scene is in Mare Fecunditatis on the near side, rather than in another mare containing a similarly named feature.
- Find the east–west elongated crater. The member stretched mainly sideways is Messier.
- Look immediately west. The neighboring feature at the lower east longitude is Messier A.
- Check the axis contrast. Messier A should appear elongated roughly north–south, close to perpendicular to Messier’s long axis.
- Look for the ray pattern. Two bright surface rays extending away from the pair support the shared-impact interpretation.
- Account for image rotation. Use the feature shapes and map coordinates rather than assuming that “left,” “right,” “up” or “down” always correspond to a compass direction.
This procedure is more reliable than comparing apparent area alone. Shadows, foreshortening and illumination can make one lobe or rim look larger in a particular photograph.
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NASA’s stereo presentation of the pair uses high-resolution scans of Apollo 11 photographs AS11-42-6304 and AS11-42-6305. Those frame identifiers are useful when tracing the provenance of the view or comparing the stereo pair with a map. The photographs show the morphology and ray arrangement that underpin NASA’s shallow-impact explanation; they do not turn the visual dimensions into a new catalog standard.
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Stereo imagery can make relief easier to perceive, but the apparent depth and prominence of a rim still depend on viewing conditions. The most secure cross-source comparisons remain the catalog coordinates, the stated nominal diameters, NASA’s two-axis dimensions and the documented orientations.
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Which crater is bigger?
There is no single answer unless “bigger” is defined first. By NASA’s long-by-short image dimensions, Messier A is listed as 16 × 11 km, compared with Messier at 15 × 8 km; that description gives Messier A the larger two-dimensional footprint in both listed directions. By the USGS/IAU nominal-diameter field, Messier is 13.80 km and Messier A is 11.03 km; that catalog measure makes Messier’s nominal diameter larger.
The two results are not a mathematical error. They compare different representations of irregular outlines. If the question concerns the visible elongated footprint, quote the NASA dimensions. If it concerns the official single diameter attached to each Gazetteer entry, quote the Gazetteer values and identify them as nominal diameters.
What is established, and what remains unspecified
- Established: both features are in Mare Fecunditatis on the lunar near side.
- Established: Messier is east of Messier A, based on the listed longitudes.
- Established: Messier is mainly east–west elongated; Messier A is approximately north–south and double-lobed.
- Established: NASA links both shapes and the two bright rays to an extremely shallow-angle impact.
- Established: the official catalog treats Messier as a crater and Messier A as a satellite feature.
- Not established by these sources: a numeric impact angle.
- Not established by these sources: an absolute formation age or named geological period.
- Not established by these descriptions: a complete quantitative comparison of rim height, floor depth, central peaks or ejecta thickness.
Keeping those boundaries clear prevents two common errors: treating a descriptive image measurement as interchangeable with a catalog diameter, and presenting a qualitative impact explanation as though it supplied a precise angle or date.
Terminology for reading lunar maps
Nominal diameter
A standardized single diameter recorded for a named feature in a catalog. It is useful for consistent indexing and comparison, but it compresses an irregular outline into one number.
Long-by-short dimensions
Two approximate dimensions that describe the major and minor extents visible in an image. They are especially informative for elongated, oval or lobed forms.
Satellite feature
An officially cataloged feature designated in relation to a primary named feature. In this case, Messier A is the satellite feature associated with Messier; the term does not describe an orbiting body.
Ray
A bright streak or deposit extending across the lunar surface from an impact site. The two rays associated with Messier and Messier A are part of the evidence for a common shallow-angle event.
Bottom line for the comparison
Messier and Messier A are neighboring, officially separate features created by the same unusual grazing impact. Messier is the eastern crater stretched mainly east–west; Messier A is west of it, has a roughly north–south double-lobed outline and is cataloged as a satellite feature. NASA’s 15 × 8 km and 16 × 11 km image dimensions describe their elongated footprints, while the Gazetteer’s 13.80 km and 11.03 km values are nominal single diameters. The contrasting axes and shared bright rays are the defining clues that the two features form one impact system.
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