The Lunar 100

Full Moon
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.

Lunar 100
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
L Feature Name Significance Lat. (°) Long. (°) Diam. (km) Rükl Chart
1 Moon Large satellite 3,476
2 Earthshine Twice reflected sunlight
3 Mare/highland dichotomy Two materials with distinct compositions
4 Apennines Imbrium basin rim 18.9N 3.7W 70 22
5 Copernicus Archetypal large complex crater 9.7N 20.1W 93 31
6 Tycho Large rayed crater with impact melts 43.4S 11.1W 85 64
7 Altai Scarp Nectaris basin rim 24.3S 22.6E 425 57
8 Theophilus, Cyrillus, Catharina Crater sequence illustrating stages of degradation 13.2S 24.0E 46, 57
9 Clavius Lacks basin features in spite of its size 58.8S 14.1W 225 72
10 Mare Crisium Mare contained in large circular basin 18.0N 59.0E 540 26, 27, 37, 38
11 Aristarchus Very bright crater with dark bands on its walls 23.7N 47.4W 40 18
12 Proclus Oblique-impact rays 16.1N 46.8E 28 26
13 Gassendi Floor-fractured crater 17.6S 40.1W 101 52
14 Sinus Iridum Very large crater with missing rim 45.0N 32.0W 260 10
15 Straight Wall Best example of a lunar fault 21.8S 7.8W 110 54
16 Petavius Crater with domed & fractured floor 25.1S 60.4E 177 59
17 Schröter's Valley Giant sinuous rille 26.2N 50.8W 168 18
18 Mare Serenitatis dark edges Distinct mare areas with different compositions 17.8N 23.0E N/A 24
19 Alpine Valley Lunar graben 49.0N 3.0E 165 4
20 Posidonius Floor-fractured crater 31.8N 29.9E 95 14

The Lunar 100 (continued)
L Feature Name Significance Lat. (°) Long. (°) Diam. (km) Rükl Chart
21 Fracastorius Crater with subsided & fractured floor 21.5S 33.2E 124 58
22 Aristarchus Plateau Mysterious uplifted region mantled with pyroclastics 26.0N 51.0W 150 18
23 Pico Isolated Imbrium basin-ring fragment 45.7N 8.9W 25 11
24 Hyginus Rille Rille containing rimless collapse pits 7.4N 7.8E 220 34
25 Messier & Messier A Oblique ricochet-impact pair 1.9S 47.6E 11 48
26 Mare Frigoris Arcuate mare of uncertain origin 56.0N 1.4E 1600 2–6
27 Archimedes Large crater lacking central peak 29.7N 4.0W 83 12, 22
28 Hipparchus First drawing of a single crater 5.5S 4.8E 150 44, 45
29 Ariadaeus Rille Long, linear graben 6.4N 14.0E 250 34
30 Schiller Possible oblique impact 51.9S 39.0W 180 71
31 Taruntius Young floor-fractured crater 5.6N 46.5E 56 37
32 Arago Alpha & Beta Volcanic domes 6.2N 21.4E 26 35
33 Serpentine Ridge Basin inner-ring segment 27.3N 25.3E 155 24
34 Lacus Mortis Strange crater with rille & ridge 45.0N 27.2E 152 14
35 Triesnecker Rilles Rille family 4.3N 4.6E 215 33
36 Grimaldi basin A small two-ring basin 5.5S 68.3W 440 39
37 Bailly Barely discernible basin 66.5S 69.1W 303 71
38 Sabine & Ritter Possible twin impacts 1.7N 19.7E 30 35
39 Schickard Crater floor with Orientale basin ejecta stripe 44.3S 55.3W 227 62
40 Janssen Rille Rare example of a highland rille 45.4S 39.3E 190 67, 68

The Lunar 100 (continued)
L Feature Name Significance Lat. (°) Long. (°) Diam. (km) Rükl Chart
41 Bessel ray Ray of uncertain origin near Bessel 21.8N 17.9E N/A 24
42 Marius Hills Complex of volcanic domes & hills 12.5N 54.0W 125 28, 29
43 Wargentin A crater filled to the rim with lava or ejecta 49.6S 60.2W 84 70
44 Mersenius Domed floor cut by secondary craters 21.5S 49.2W 84 51
45 Maurolycus Region of saturation cratering 42.0S 14.0E 114 66
46 Regiomontanus central peak Possible volcanic peak 28.0S 0.6W 124 55
47 Alphonsus dark spots Dark-halo eruptions on crater floor 13.7S 3.2W 119 44
48 Cauchy region Fault, rilles, & domes 10.5N 38.0E 130 36
49 Gruithuisen Delta & Gamma Volcanic domes formed with viscous lavas 36.3N 40.0W 20 9
50 Cayley Plains Light, smooth plains of uncertain origin 4.0N 15.1E 14 34
51 Davy crater chain Result of comet-fragment impacts 11.1S 6.6W 50 43
52 Crüger Possible volcanic caldera 16.7S 66.8W 45 50
53 Lamont Possible buried basin 4.4N 23.7E 106 35
54 Hippalus Rilles Rilles concentric to Humorum basin 24.5S 29.0W 240 52, 53
55 Baco Unusually smooth crater floor & surrounding plains 51.0S 19.1E 69 74
56 Australe basin A partially flooded ancient basin 49.8S 84.5E 880 76
57 Reiner Gamma Conspicuous swirl & magnetic anomaly 7.7N 59.2W 70 28
58 Rheita Valley Basin secondary-crater chain 42.5S 51.5E 445 68
59 Schiller-Zucchius basin Badly degraded overlooked basin 56.0S 45.0W 335 70, 71
60 Kies Pi Volcanic dome 26.9S 24.2W 45 53

The Lunar 100 (continued)
L Feature Name Significance Lat. (°) Long. (°) Diam. (km) Rükl Chart
61 Mösting A Simple crater close to center of lunar near side 3.2S 5.2W 13 43
62 Rümker Large volcanic dome 40.8N 58.1W 70 8
63 Imbrium sculpture Basin ejecta near & overlying Boscovich & Julius Caesar 11.0N 12.0E 34
64 Descartes Apollo 16 landing site; putative region of highland volcanism 11.7S 15.7E 48 45
65 Hortensius domes Dome field north of Hortensius 7.6N 27.9W 10 30
66 Hadley Rille Lava channel near Apollo 15 landing site 25.0N 3.0E 22
67 Fra Mauro formation Apollo 14 landing site on Imbrium ejecta 3.6S 17.5W 42
68 Flamsteed P Proposed young volcanic crater & Surveyor 1 landing site 3.0S 44.0W 112 40
69 Copernicus secondary craters Rays & craterlets near Pytheas 19.6N 19.1W 4 20
70 Humboldtianum basin Multi-ring impact basin 57.0N 80.0E 650 7
71 Sulpicius Gallus dark mantle Ash eruptions northwest of crater 19.6N 11.6E 12 23
72 Atlas dark-halo craters Explosive volcanic pits on the floor of Atlas 46.7N 44.4E 87 15
73 Smythii basin Difficult-to-observe basin scarp & mare 2.0S 87.0E 740 38, 49
74 Copernicus H Dark-halo impact crater 6.9N 18.3W 5 31
75 Ptolemaeus B Saucerlike depression on the floor of Ptolemaeus 8.0S 0.8W 16 44
76 W. Bond Large crater degraded by Imbrium ejecta 65.3N 3.7E 158 4
77 Sirsalis Rille Procellarum basin radial rilles 15.7S 61.7W 425 39, 50
78 Lambert R A buried "ghost" crater 23.8N 20.6W 54 20
79 Sinus Aestuum Eastern dark-mantle volcanic deposit 12.0N 3.5W 90 33
80 Orientale basin Youngest large impact basin 19.0S 95.0W 930 50

The Lunar 100 (continued)
L Feature Name Significance Lat. (°) Long. (°) Diam. (km) Rükl Chart
81 Hesiodus A Concentric crater 30.1S 17.0W 15 54
82 Linné Small crater once thought to have disappeared 27.7N 11.8E 2.4 23
83 Plato craterlets Crater pits at limits of detection 51.6N 9.4W 101 3, 4
84 Pitatus Crater with concentric rilles 29.8S 13.5W 97 54
85 Langrenus rays Aged ray system 8.9S 60.9E 132 49
86 Prinz Rilles Rille system near the crater Prinz 27.0N 43.0W 46 19
87 Humboldt Crater with central peaks & dark spots 27.0S 80.9E 207 60
88 Peary Difficult-to-observe polar crater 88.6N 33.0E 74 4, II
89 Valentine Dome Volcanic dome 30.5N 10.1E 30 13
90 Armstrong, Aldrin & Collins Small craters near the Apollo 11 landing site 1.3N 23.7E 3 35
91 De Gasparis Rilles Area with many rilles 25.9S 50.7W 30 51
92 Gylden Valley Part of the Imbrium radial sculpture 5.1S 0.7E 47 44
93 Dionysius rays Unusual & rare dark rays 2.8N 17.3E 18 35
94 Drygalski Large south-pole region crater 79.3S 84.9W 162 72, VI
95 Procellarum basin The Moon's biggest basin? 23.0N 15.0W 3200
96 Leibnitz Mountains Rim of South Pole-Aitken basin 85.0S 30.0E 73, V
97 Inghirami Valley Orientale basin ejecta 44.0S 73.0W 140 61
98 Imbrium lava flows Mare lava-flow boundaries 32.8N 22.0W 10
99 Ina D-shaped young volcanic caldera 18.6N 5.3E 3 22
100 Mare Marginis swirls Possible magnetic field deposits 18.5N 88.0E 27, 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.

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