The Lunar 100

Many Lunar 100 selections are plainly visible in this image of the full Moon, while others require a more detailed view, different illumination, or favorable libration. North is up.
S&T: Gary Seronik

Just about every telescope user is familiar with French comet hunter Charles Messier's catalog of fuzzy objects. Messier's 18th-century listing of 109 galaxies, clusters, and nebulae contains some of the largest, brightest, and most visually interesting deep-sky treasures visible from the Northern Hemisphere. Little wonder that observing all the M objects is regarded as a virtual rite of passage for amateur astronomers.

But the night sky offers an object that is larger, brighter, and more visually captivating than anything on Messier's list: the Moon. Yet many backyard astronomers never go beyond the astro-tourist stage to acquire the knowledge and understanding necessary to really appreciate what they're looking at, and how magnificent and amazing it truly is. Perhaps this is because after they identify a few of the Moon's most conspicuous features, many amateurs don't know where to look next.

The Lunar 100 list is an attempt to provide Moon lovers with something akin to what deep-sky observers enjoy with the Messier catalog: a selection of telescopic sights to ignite interest and enhance understanding. Presented here is a selection of the Moon's 100 most interesting regions, craters, basins, mountains, rilles, and domes. I challenge observers to find and observe them all and, more important, to consider what each feature tells us about lunar and Earth history.

Anatomy of the Lunar 100

Objects in the Lunar 100 are arranged from the easiest to view to the most difficult. This is more systematic than the haphazard approach that produced the Messier list. Indeed, just by knowing a feature's Lunar 100 number, you have some idea of how easy or challenging it will be to see. For example, the Moon itself is L1, while L2 is earthshine and L3 is the light/dark dichotomy between lunar highlands and maria ("seas"). I'd be surprised if anyone reading this couldn't tick those off the list right now. Higher-numbered objects are smaller, less conspicuous, or positioned closer to the limb, making them more challenging to locate and view.

Planetary scientist Charles Wood's Lunar 100 is a list of telescopic sights designed to ignite interest in the Moon and enhance understanding of its geology.
Source: Antonín Rükl

The Messier objects are scattered all over the sky, but all are theoretically observable during marathon nights in March and April every year. By contrast, the Lunar 100 are concentrated in just ½°of sky, yet they can't all be seen in a single night, or even in a single month. Some lunar objects can be observed only with grazing solar illumination, while others are albedo features that require full-Moon conditions to be seen. And others are positioned near (or sometimes even over) the limb of the Moon, requiring a very favorable libration to bring them into view. I don't know how quickly all 100 can be observed, but I'm sure that some competitive amateur will complete it faster than I dare guess!

How big a telescope do you need to view the Lunar 100? The smallest features listed are 3 kilometers in diameter and thus nominally visible in 3-inch (75-millimeter) telescopes employing magnifications of about 150× to 200×. And many can be found with smaller scopes at lower power. But a few Lunar 100 objects — such as narrow rilles — are best seen with 6- or 8-inch telescopes used at high power. The goal, however, is not just to find the objects, but to understand what they tell us abut the Moon.

Any selection of lunar features is bound to lead to many difficult judgments, and I'm sure that at least a few of my choices and rankings will generate considerable debate. Some of my choices were obvious, some were not. Some were influenced by my personal sense of what crater appears more dramatic than another, or which rille best demonstrates an aspect of the Moon's evolution. Aesthetics aside, my choices were principally governed by a desire to include features that tell us something important or interesting about the Moon itself.

I invite you to use the Lunar 100 to guide your explorations of the Moon.

The Lunar 100
LFeature NameSignificanceLat. (°)Long. (°)Diam. (km)Rükl Chart
1MoonLarge satellite3,476
2EarthshineTwice reflected sunlight
3Mare/highland dichotomyTwo materials with distinct compositions
4ApenninesImbrium basin rim18.9N3.7W7022
5CopernicusArchetypal large complex crater9.7N20.1W9331
6TychoLarge rayed crater with impact melts43.4S11.1W8564
7Altai ScarpNectaris basin rim24.3S22.6E42557
8Theophilus, Cyrillus, CatharinaCrater sequence illustrating stages of degradation13.2S24.0E46, 57
9ClaviusLacks basin features in spite of its size58.8S14.1W22572
10Mare CrisiumMare contained in large circular basin18.0N59.0E54026, 27, 37, 38
11AristarchusVery bright crater with dark bands on its walls23.7N47.4W4018
12ProclusOblique-impact rays16.1N46.8E2826
13GassendiFloor-fractured crater17.6S40.1W10152
14Sinus IridumVery large crater with missing rim45.0N32.0W26010
15Straight WallBest example of a lunar fault21.8S7.8W11054
16PetaviusCrater with domed & fractured floor25.1S60.4E17759
17Schröter's ValleyGiant sinuous rille26.2N50.8W16818
18Mare Serenitatis dark edgesDistinct mare areas with different compositions17.8N23.0EN/A24
19Alpine ValleyLunar graben49.0N3.0E1654
20PosidoniusFloor-fractured crater31.8N29.9E9514

The Lunar 100 (continued)
LFeature NameSignificanceLat. (°)Long. (°)Diam. (km)Rükl Chart
21FracastoriusCrater with subsided & fractured floor21.5S33.2E12458
22Aristarchus PlateauMysterious uplifted region mantled with pyroclastics26.0N51.0W15018
23PicoIsolated Imbrium basin-ring fragment45.7N8.9W2511
24Hyginus RilleRille containing rimless collapse pits7.4N7.8E22034
25Messier & Messier AOblique ricochet-impact pair1.9S47.6E1148
26Mare FrigorisArcuate mare of uncertain origin56.0N1.4E16002–6
27ArchimedesLarge crater lacking central peak29.7N4.0W8312, 22
28HipparchusFirst drawing of a single crater5.5S4.8E15044, 45
29Ariadaeus RilleLong, linear graben6.4N14.0E25034
30SchillerPossible oblique impact51.9S39.0W18071
31TaruntiusYoung floor-fractured crater5.6N46.5E5637
32Arago Alpha & BetaVolcanic domes6.2N21.4E2635
33Serpentine RidgeBasin inner-ring segment27.3N25.3E15524
34Lacus MortisStrange crater with rille & ridge45.0N27.2E15214
35Triesnecker RillesRille family4.3N4.6E21533
36Grimaldi basinA small two-ring basin5.5S68.3W44039
37BaillyBarely discernible basin66.5S69.1W30371
38Sabine & RitterPossible twin impacts1.7N19.7E3035
39SchickardCrater floor with Orientale basin ejecta stripe44.3S55.3W22762
40Janssen RilleRare example of a highland rille45.4S39.3E19067, 68

The Lunar 100 (continued)
LFeature NameSignificanceLat. (°)Long. (°)Diam. (km)Rükl Chart
41Bessel rayRay of uncertain origin near Bessel21.8N17.9EN/A24
42Marius HillsComplex of volcanic domes & hills12.5N54.0W12528, 29
43WargentinA crater filled to the rim with lava or ejecta49.6S60.2W8470
44MerseniusDomed floor cut by secondary craters21.5S49.2W8451
45MaurolycusRegion of saturation cratering42.0S14.0E11466
46Regiomontanus central peakPossible volcanic peak28.0S0.6W12455
47Alphonsus dark spotsDark-halo eruptions on crater floor13.7S3.2W11944
48Cauchy regionFault, rilles, & domes10.5N38.0E13036
49Gruithuisen Delta & GammaVolcanic domes formed with viscous lavas36.3N40.0W209
50Cayley PlainsLight, smooth plains of uncertain origin4.0N15.1E1434
51Davy crater chainResult of comet-fragment impacts11.1S6.6W5043
52CrügerPossible volcanic caldera16.7S66.8W4550
53LamontPossible buried basin4.4N23.7E10635
54Hippalus RillesRilles concentric to Humorum basin24.5S29.0W24052, 53
55BacoUnusually smooth crater floor & surrounding plains51.0S19.1E6974
56Australe basinA partially flooded ancient basin49.8S84.5E88076
57Reiner GammaConspicuous swirl & magnetic anomaly7.7N59.2W7028
58Rheita ValleyBasin secondary-crater chain42.5S51.5E44568
59Schiller-Zucchius basinBadly degraded overlooked basin56.0S45.0W33570, 71
60Kies PiVolcanic dome26.9S24.2W4553

The Lunar 100 (continued)
LFeature NameSignificanceLat. (°)Long. (°)Diam. (km)Rükl Chart
61Mösting ASimple crater close to center of lunar near side3.2S5.2W1343
62RümkerLarge volcanic dome40.8N58.1W708
63Imbrium sculptureBasin ejecta near & overlying Boscovich & Julius Caesar11.0N12.0E34
64DescartesApollo 16 landing site; putative region of highland volcanism11.7S15.7E4845
65Hortensius domesDome field north of Hortensius7.6N27.9W1030
66Hadley RilleLava channel near Apollo 15 landing site25.0N3.0E22
67Fra Mauro formationApollo 14 landing site on Imbrium ejecta3.6S17.5W42
68Flamsteed PProposed young volcanic crater & Surveyor 1 landing site3.0S44.0W11240
69Copernicus secondary cratersRays & craterlets near Pytheas19.6N19.1W420
70Humboldtianum basinMulti-ring impact basin57.0N80.0E6507
71Sulpicius Gallus dark mantleAsh eruptions northwest of crater19.6N11.6E1223
72Atlas dark-halo cratersExplosive volcanic pits on the floor of Atlas46.7N44.4E8715
73Smythii basinDifficult-to-observe basin scarp & mare2.0S87.0E74038, 49
74Copernicus HDark-halo impact crater6.9N18.3W531
75Ptolemaeus BSaucerlike depression on the floor of Ptolemaeus8.0S0.8W1644
76W. BondLarge crater degraded by Imbrium ejecta65.3N3.7E1584
77Sirsalis RilleProcellarum basin radial rilles15.7S61.7W42539, 50
78Lambert RA buried "ghost" crater23.8N20.6W5420
79Sinus AestuumEastern dark-mantle volcanic deposit12.0N3.5W9033
80Orientale basinYoungest large impact basin19.0S95.0W93050

The Lunar 100 (continued)
LFeature NameSignificanceLat. (°)Long. (°)Diam. (km)Rükl Chart
81Hesiodus AConcentric crater30.1S17.0W1554
82LinnéSmall crater once thought to have disappeared27.7N11.8E2.423
83Plato craterletsCrater pits at limits of detection51.6N9.4W1013, 4
84PitatusCrater with concentric rilles29.8S13.5W9754
85Langrenus raysAged ray system8.9S60.9E13249
86Prinz RillesRille system near the crater Prinz27.0N43.0W4619
87HumboldtCrater with central peaks & dark spots27.0S80.9E20760
88PearyDifficult-to-observe polar crater88.6N33.0E744, II
89Valentine DomeVolcanic dome30.5N10.1E3013
90Armstrong, Aldrin & CollinsSmall craters near the Apollo 11 landing site1.3N23.7E335
91De Gasparis RillesArea with many rilles25.9S50.7W3051
92Gylden ValleyPart of the Imbrium radial sculpture5.1S0.7E4744
93Dionysius raysUnusual & rare dark rays2.8N17.3E1835
94DrygalskiLarge south-pole region crater79.3S84.9W16272, VI
95Procellarum basinThe Moon's biggest basin?23.0N15.0W3200
96Leibnitz MountainsRim of South Pole-Aitken basin85.0S30.0E73, V
97Inghirami ValleyOrientale basin ejecta44.0S73.0W14061
98Imbrium lava flowsMare lava-flow boundaries32.8N22.0W10
99InaD-shaped young volcanic caldera18.6N5.3E322
100Mare Marginis swirlsPossible magnetic field deposits18.5N88.0E27, III
Chart numbers refer to Antonín Rükl's Atlas of the Moon.

For the convenience of observers, the Lunar 100 is also available on Sky & Telescope's 9-by-12-inch laminated Lunar 100 Card ($6.95), featuring a high-quality Moon map by Antonin Rukl on the front. The reverse side shows the locations and sizes of 100 features, together with brief descriptions of each.