"Introducing the Lunar 100" article from Sky and Telescope

lunar notebook
Introducing the Lunar 100
Let this Moon observer's hit list guide your telescopic explorations of
Earth's nearest neighbor. \ By Charles A. Wood
J
UST ABOUT EVEKY TELESCOPE USER
is familiar with French comet
hunter Charles Messier's catalog
of fuzzy objects. Messier's 18thcentury listing of iO9 galaxies, clusters,
and nehuiae contains some ofthe 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
tew 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 al! and, more
important, to consider what each feature
tells us about lunar and Earth history.
Anatomy ofthe 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 Ll, 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. Highernumbered objects are smaller, less conspicuous, or positioned closer to the limb,
making them more challenging to locate
and view.
The Messier objects are scattered all
over the sky, but all are theoreticallv oh-
servable during marathon nights in
March and April every year. By contrast,
the Lunar 100 are concentrated in just
'/:° of sky, yet they can't all be seen in a
single night, or even in a single month.
Some lunar sights 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!
Many Lunar 100 selections are plainly visible in this view of the full Moon, while others require
a more detailed view, different illumination, or favorable libration. North is up in this and all
other images, unless otherwise noted.
Sky & Telescope | April 2004
113
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How big a telescope do you need to
view the Lunar 100? The smallest features listed arc 3 kilometers in diameter
and thus nominally visible in 3-inch (76millimeter) telescopes employing magnifications of about 150x to 200x. 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 under-
stand what they tell us abut the Moon.
Any selection of lunar features is
bound to lead to many difficult judgments, and Tm sure that at least a fev^' 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
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The spectacular ray crater Copernicus (L5) and its diminutive companion, Copernicus H (L74).
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Visible here are Mare Australe {L56), the Rheita Valley (L58}, and rilles in the crater Janssen (L40).
114
April 2004 I Sky&Telescope
governed hy a desire to include features
that tel! us something important or interesting about the Moon itself.
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Exploring and Understanding
Ohserving all 100 features (and understanding the geological significance of
each one) constitutes a short course in
lunar science. Here are some examples of
how our modern understanding of the
Moon is illustrated by particular objects:
Craters of different sizes result from
meteor and comet impacts of different
energies. Small craters such as Mosting
A (L61) have simple shapes, with steep,
smooth walls leading to small, flat floors.
Such craters look as if they were massproduced on a lathe. With higher-energy
impacts, the wall rocks collapse toward the
crater's center, creating irregular slumps
and jumhles of material on the wall and
floor. Compression of the rocks directly
under the impact energizes them into responding like a splash of water, with rocks
from below the surface rebounding to
make a small central peak.
As the energy of an impact event increases (due to a larger-mass or highervelocity projectile), the surrounding rocks
collapse more uniformly along a series
of circular faults, downdropping massive
terraces toward the crater's floor. The
central rebound is more intense and produces complex mountains composed of
rocks from deep below the surface. The
archetypal example is Copernicus (L5),
but most relatively fresh craters larger
than about 35 km have this morphology.
In the early 1960s, dozens of depressions bigger than normal impact craters
and featuring concentric and radial structures were first recognized. These are the
impact basins. The arcuate Apennine
(1-4) and Altai (L7) mountains are simply
the rims of the Imbrium and Nectaris
impact basins. And the line of tall peaks
near the south pole (the Leibnitz Mountains, L96) is the rim of a huge far-side
basin. The Alpine Valley (LI9) and Rheita
Valley (L58) are the most prominent of a
series of radial fractures and secondary
crater chains found around most basins.
Tbe great diameters and depths of impact basins make it clear tbat vast
amounts of excavated lunar material
must bave been strewn across tbe surface of tbe Moon. Ihis explains why
many craters close to basin rims are intensely battered and buried. Look near
the ruined craters Boscovich and Julius
Caesar (L63) close to the center of the
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The crater Triesnecker and its rilles (L35) and
the relatively conspicuous Hyginus rille (L24).
Located on the southeastern shore of Mare
Humorum are the ruined crater Hippalus and
its associated rilles ( LS4).
Moon's face and I. Herschel, Babbage,
and W. Bond (L76} near the north pole.
All were instantly degraded when ejecta
from the Inibrium impact surged over
them 3.84 billion years ago. The Apollo 14
astronauts landed in the Fra Mauro region
(L67) specifically to collect ejecta from
the Imbrium impact. Ejecta from smaller
craters made the glorious rays of Tycho
(L6), the bright nimbus surrounding
Linne (L82), and the pit-peppered surface
clearly visible east of Copernicus (L3).
Fractures created by basin-forming im-
pacts provided conduits for magmas to
rise to the surface and fill deep basins.
The weight of the lavas caused the basin
floors to subside, with the greatest
amount of bending occurring near the
edges, forming concentric rilles such as
those near the crater Hippalus (L54) on
the rim of the Humorum basin. Some of
the lava flows folded as a result of compression, producing mare ridges like the
Serpentine Ridge (L33) in Mare Serenitatis.
Lavas erupted over hundreds of millions of years in some basins, and their
m
chemical compositions varied through
time. Multispectral imaging is usually required to identity lava flows of different
compositions and ages, but the dark collar around southeastern Serenitatis (L18)
is easily visible in a telescope. The faint
rilles hugging the southern shore of
Serenitatis occur only in this dark annulus
of lava. High-resolution spacecraft photos show that this older material tilted
toward the center of the basin before the
younger, lighter-hued flows erupted onto
the surface.
Feature name
Significance
Latitude p)
Longitude { ' )
Diameter (km)
1
Moon
Large satellite
—
3,476
2
Earthshine
Twice reflected sunlight
—
—
3
Mare/highland dichotomy
Two materials with distinct compositions
—
—
—
—
—
4
Apennines
Imbfium basin rim
18.9N
3.7W
400
5
Copernicus
Archetypal large complex crater
9.7N
20.1W
93
31
6
Tycho
Large rayed crater with impact melts
43.4S
11.1W
102
64
Altai Scatp
Nectaris basin rim
24.3S
22.6E
425
57
Theophilus, Cyrillus, Catharina
Crater sequence illustrating stages of degradation
13.2S
24.0E
no
46,57
9
Clavius
Lacks basin features in spite of its size
58.8S
14.1W
245
72
10
Mare Crisium
Mare contained in large circular basin
18.0N
59.0E
26,27,37,38
11
Ari^archus
Very bright crater with dark bands on its walls
23.7N
A7AW
12
Proclus
Oblique-impact rays
16.1 N
46.8E
540
40
28
52
18
13
Gassendi
Floor-fractured crater
17.6S
40.1 W
101
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
130
54
16
Petavius
Crater with domed and fractured floor
25.1S
60.4E
188
17
Schroter's Valley
Giant sinuous rille
26.2N
50.8W
168
18
Mare Serenitatis dark edges
Distinct mare areas with different compositions
17.8N
23.0E
N/A
24
Alpine Valley
Lunar graben
49.0N
3.0E
165
4
Posidonius
Floor-fraaured crater
31.8N
29.9E
95
14
Fracastorius
Crater with subsided and fractured floor
21,SS
33,2E
112
58
Aristarchus Plateau
Mysterious uplifted region mantled with pyroclastics
26.0N
5 LOW
18
23
Pico
Isolated Imbrium basin-ring fragment
45.7N
8.9W
24
25
Hyginus Rille
Rille containing rimless collapse pits
7.4N
7.8E
150
25
220
Messier and Messier A
Oblique ricochet-impact pair
1.9S
47.6E
W^
27
Mare Frigoris
Arcuate mare of uncertain origin
56.0N
1.4E
Archimedes
Large crater lacking central peak
29.7N
4.0W
83
12,22
Hipparchus
Subject of first drawing of a single crater
5.SS
4.8E
44,45
Aridaeus Rille
Long, linear graben
6.4N
14,0E
Schiller
Possible oblique impact
51.9S
39.0W
150
250
180
28
29
|30
31
11
1,600
*
34
71
Young floor-fractured crater
5.6N
46.5 E
56
37
Volcanic domes
6.2N
21.4E
26
35
Serpentine Ridge
Basin inner-ring segment
27.3N
25.3E
155
24
Lacus Mortis
Strange crater with rille and ridge
4S.0N
27.2E
35
Triesnecker Rilles
fiille family
4,3N
4.6E
33
36
Grimaldi basin
A small two-ring basin
5.5S
68.3W
152
215
410
37
Bailly
Barely discernible basin
66.SS
69.1W
303
71
38
Sabine and Ritter
Possible t w i n impacts
1.7N
19.7E
30
35
39
5chickafd
Crater floor with Orientale basin ejecta stripe
44.3 S
55.3W
206
62
40
Janssen Rille
Rare example of a highland rille
45.4S
39.3E
199
67,68
"34
^
11
Taruntius
33
^
34
48
2-6
Arago Alpha and Beta
«32
uo
o
26
14
|22
fD
22
7
21
«-•
RukI chart*
8
19
O
The Lunar 100
L
,18
C
3
at
14
39
41
Bessel ray
Ray of uncertain origin near Bessel
21,8N
17.9E
N/A
24
42
Marius Hills
Complex of volcanic domes and hills
12.5N
S4.0W
125
28,29
43
Wargentin
A crater filled to the rim with lava or ejecta
49,6S
60,2Vi/
84
70
" Chart numbers refer to Anionin Riikl's Atlas of the Moon
Sky & Telescope , April 2004
117
O
The Lunar 100 (continued)
1
o
L
Feature name
Significance
Latitude {°)
Longitude {°)
(V
44
Mersenius
Domed floor cut by secondary craters
21.5S
49.2W
84
o
c
45
46
47
48
49
50
51
52
53
54
55
Maurolycus
Region of saturation cratering
42.0S
14.0E
0.6W
114
108
119
130
20
c
Regiomontanus central peak
Possible volcanic peak
28.0S
Alphonsus dark spots
Dark-halo eruptions on crater floor
13.7S
3.2W
Cauchy region
Fault, rilies, and domes
10.5N
38.0E
Gruithuisen Delta and Gamma
Volcanic domes formed with viscous iavas
36.3N
40.0W
Cayley Plains
Light, smooth plains of uncertain origin
4.0N
15.1E
14
Davy crater chain
Result of comet-fragment impacts
ll.lS
6.6W
Cruger
Possible volcanic caldera
16.7S
66.8W
Lamont
Possible buried basin
4.4N
23.7E
Hippalus Rilles
Rilles concentric to Humorum basin
24.5S
29.0W
Baco
Unusually smooth crater floor and surrounding plains
51.OS
19.1E
49,8S
84.5E
34
45
106
240
69
132
70
445
335
45
13
70
56
Mare Australe
A partially flooded ancient basin
57
Reiner Gamma
Conspicuous swirl and magnetic anomaly
7.7N
59.2W
58
Rheita Valley
Basin secondary-crater chain
42.5S
51 5E
59
Schiller-Zucchius basin
Badly degraded overlooked basin
56.0S
45.0W
60
Kies Pi
Volcanic dome
26.9S
24.2W
61
Mbsting A
Simple crater close to center of lunar near side
3.2S
5.2W
62
Rumker Hills
Large voicanic dome
40.8N
58.1W
63
64
65
66
67
68
69
70
71
72
Imbrium sculpture
Basin ejecta near and overiying Boscovich and Julius Caesar
11.ON
12.0E
Descartes
Apollo 16 landing site; putative region of highland volcanism
11.7S
15.7E
Hortensius domes
Dome field north of Hortensius
7.6N
27.9W
Hadley Rille
Lava channel near Apollo 15 landing site
25.0N
3.0E
Fra Mauro formation
Apollo 14 landing site on Imbrium ejecta
3.6S
17.5W
73
74
75
76
77
78
79
80
81
Flamsteed P
Proposed young volcanic crater; Surveyor 1 landing site
3.0S
44.0W
Copernicus secondary craters
Rays and craterlets near Pytheas
19.6N
19.1W
Humboldtianum basin
Multi-ring impact basin
57.ON
80.0E
Sulpicius Gallus dark mantle
Ash eruptions northwest of crater
19.6N
11.6E
Atlas dark-halo craters
Explosive volcanic pits on the floor of Atlas
46,7N
44.4E
Smythii basin
Difficult-to-observe basin scarp and mare
2.0S
87.0E
Copernicus H
Dark-halo impact crater
6.9N
18.3W
Ptolemaeus B
Sauceriike depression on the fioor of Ptoiemaeus
8.0S
0.8W
W, Bond
Large crater degraded by Imbrium ejecta
65.3N
3.7E
Sirsaiis Riile
Procellarum basin radial rilles
15.7S
61.7W
Lambert R
A buried "ghost" crater
23.8N
Sinus Aestuum
Eastern dark-mantle volcanic deposit
12,0N
Orientale basin
Youngest large impact basin
19.0S
95,OVI/
Hesiodus A
Concentric crater
30.1 S
17.0W
82
Linne
Smaii crater once thought to have disappeared
27,7N
n.8E
83
Plato craterlets
Crater pits at limits of detection
51.6N
9.4W
84
Pitatus
Crater with concentric rilles
29.8S
13.5W
85
Langrenus rays
Aged ray system
8.9S
60.9E
86
Prinz Rilles
Rilie system near the crater Prinz
27.0N
43.0W
87
Humboidt
Crater with central peaks and dark spots
27.OS
Difficult-to-observe polar crater
88.6N
Valentine Dome
Volcanic dome
30.5N
1.3N
237E
25.9S
50.7W
88
Peary
—
—
10
—
—
—
4
650
12
87
74
76
28
68
70,71
53
43
8
34
45
30
22
42
40
20
7
23
15
54
3.5W
20
33
50
54
3,4
80.9E
90
930
15
2.4
109
97
132
46
189
95.3E
104
4,11
lO.lE
13
35
51
44
35
90
Armstrong, Aldrin, and Coiiins
DeGasparis Rilles
Area with many rilles
92
Gylden Valley
Part of the Imbrium radial sculpture
5.1S
0.7E
93
94
Dionysius rays
Unusual and rare dark rays
2.8N
17,3E
Drygalski
Large south-pole region crater
79.3S
84,9W
95
96
97
Procellarum basin
The Moon's biggest basin?
23.0N
15.0W
3,200
Leibnitz Mountains
Rim of South Pole-Aitken basin
85.0S
30,0F
Inghirami Valley
Orientale basin ejecta
44.0S
73.0Ur
98
Imbrium lava flows
Mare lava-flow boundaries
32,8N
22.0W
99
Ina caldera
D-shaped young volcanic caldera
18.6N
5.3E
Possible magnetic-field deposits
18.5N
88.0E
—
140
—
3
—
April 2004 | Sky&Telescope
52,53
20.6W
91
' Chart numbers refer to Antonin ROkl's Atlas ofthe Moon.
51
66
55
44
36
9
34
43
50
35
38,49
Small craters near the Apollo 11 ianding site
100 Mare Marginis swirls
RukI chart*
740
5
164
158
425
30
3
30
47
18
149
89
118
Diameter (km)
31
44
4
39,50
23
54
49
19
60
72, VI
—
73, V
61
10
22
27,111
:3
Mare lava seeped under and into craters
along mare shores. The rising magniii lifted and tilted crater floors, creating concentric cracks and rilles and often leaking
onto the surface. Gassendi (L13), Posidonius (L20), and Taruntius (L31) are all
variations of this floor-fracturing process.
Other craters, mostly on the floors of
basins, were deeply filled by mare lavas
that rose up through crater fractures
produced by the crater-forming impact.
Thus, deep pools of solidified lavas conceal the cenlnil peak of Archimedes (L27).
Lunar lavas were much less viscous
than those on Earth and consequently
flowed much greater distances. Most
lunar Java flows were not very deep, and
their edges became feathered by subsequent small-scale impact cratering. However, with low-angle solar illumination,
keen-eyed observers may spot the edges
ot the hundred-kilonieter-iong lava flows
(L981 passing into Imbrium from vents
near the I.a Hire mountains.
Svi/iftly moving lavas flowing downhill
from small volcanic craters also produced
snakelike channels. The Apollo 13 landing
site was selected partly to study the sinu-
The floor-fractured crater Gassendi {113).
North is to the upper left.
Sky & Telescope | April 2004
119
r
Arizona
Deep Skies &
Deserts
Above: Plato and its craterlets (L83).
Right:lhe Valentine Dome (L89).
Below: Sinus Iridum {L14), the "Bay of Rainbows."
destinations in the world,
including Kitt Peak National
Observatory, the Steward Observatory Mirror Laboratory,
and the US Geological Survey's
Astrogeology Branch, as well as
magnificent Barringer Meteor
Crater and Whipple and Lowell
observatories. We'll aiso enjoy
specially arranged stargaa
sessions under the dark, t
Space is limited for
these astrotiomical
adveritures —
make your
reservation today!
Call toll free 800-830-1998 or visit
www.tq-international.com
www.tq-ifitemational.com
120
April 2004 \ Sky&Telescope
ous Hadley Rille (L66). Numerous barely
visible sinuous rilles (L86) also cascade
downslope north of the crater Prinz..
Lavas that erupted slowly onto the lunar
surface cooled sufficiently to solidify before
they could flow very far. These slow tlows
formed circular mounds or domes. For
reasons we do not understand, domes did
not form in all lunar maria but are concentrated in certain areas. Subtle crater-topped
domes are visible at low-angle illumination near the craters Hortensius (L65) and
Arago (L32). And a field of hundreds of
pronounced domes and small hills is con-
centrated west of the crater Marius (L42).
I invite you to use the Lunar 100 to
guide your explorations of the Moon.
The complete list appears on pages 115
and 116. Forthcoming columns will provide detailed descriptions of each feature
listed in the Lunar 100.
The Moon awaits!
CHARLES A. W O O D (S a dedicated
planetary
scientist and lunar expiorer. He has a new
Web site (www.observingthesky.org) and a new
book — The Modern Moon: A Personal View,
now available from Sky Publishing.