Sky and Telescope - June 2016 - 52

OBSERVING

Exploring the Moon

The ABCs of Lunar Craters
The alphabet plays a key role in identifying features on the Moon.
All lunar observers know Copernicus, Plato, Tycho, and
probably a handful more of the 895 named craters on
the nearside. But did you know that an additional 5,433
nearside craters have designations? These craters bear
letters, not names. For example, a small dark halo crater
southeast of Copernicus is called Copernicus H. And
Petavius B, just north of its namesake, is a wonderful
oblique impact crater with asymmetric rays.
Imagine the nightmare it would be for observers who
wanted to learn their way around the Moon's surface if
each of those 6,328 nearside craters had a distinct name
honoring some historical ﬁgure. That's why letter designations exist for smaller or less prominent craters. When
I wrote "Copernicus H" above, you probably knew immediately in what part of the Moon this feature is located -
which would be a big help in ﬁnding it.
Many of the letter designations we use today derive
from the famous Mappa Selenographica, published by
German astronomers Wilhelm Beer and Johann Heinrich von Mädler from 1834 to 1836. Apparently Mädler

Bliss (formerly A)
P

X
Plato
G
L
U

CHRISTIAN VILADRICH / LPOD

J
Mons
Pico

MARE IMBRIUM

A dramatic image of 101-km-wide Plato also reveals several smaller lettered
craters surrounding it - including enigmatic Plato K and Plato KA.

June 2016 sky & telescope

did most of the observing and mapping, and Beer was
a wealthy benefactor who supplied the telescope and
observatory. Mädler systematically used capital letters as
designations for craters of secondary interest. The aforementioned Petavius B appears on his map, for example.
In general, Mädler gave the letter A to the most
conspicuous crater positioned near the named primary
feature. He reserved letters near the end of the alphabet
for battered features, with the letter R often used for
ghost rings, such as the beautiful but elusive Lambert R,
and similar ruined craters.
On average, you'll ﬁnd about six lettered craters for
each named one on the Moon's nearside, but the counts
vary greatly. Abulfeda is the crater with the most lettered
features, 24, with only the letters I and V missing. Plato
used to have 23 lettered craters, but in 2000 the surprisingly deep crater Plato A was renamed Bliss, after Rev.
Nathaniel Bliss, who brieﬂy served as Astronomer Royal
in 18th-century England. (The story goes that Bliss was
the only Astronomer Royal not honored with a crater
name, so Patrick Moore lobbied for it.)
Two named craters have a double-lettered companion
nearby: Abulfeda BA and Plato KA. Both cases involve
two nearly same-size craters that touch. The Plato pair
is intriguing because they formed simultaneously, and
a curved ridge between them shows where ejecta from
the two impacts collided. As with human twins, it seems
reasonable that each of these lunar twins should have its
own designation, thus Plato K and Plato KA.
Many formerly lettered craters near the lunar limb
acquired names in the 1960s and '70s, as Space Age
images replaced the foreshortened telescopic perspectives available from Earth. The new overhead views
provided by spacecraft showed so many diﬀerences
that much improved mapping, and nomenclature, was
needed. Newly named craters included Pascal, Krasnov,
and Cannon, which were formerly designatedCarpenter D, Lagrange F, and Alhazen F, respectively. Few of
you have probably knowingly seen these challenging
features, but they're detectable with backyard telescopes
when the libration is favorable.
Orbiting spacecraft also revealed thousands of previously unknown craters on the farside. Of these, 670 of
the most prominent have been named by the International Astronomical Union, and in 1981 famed lunar

Sky and Telescope - June 2016

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