A (6)

56 AUSTRALIAN SKY & TELESCOPE JULY 2016

A

ll 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 did most of the observing and mapping,

The ABCs of lunar craters

The alphabet has played a key role in naming and identifying features on the Moon.

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 the Reverend 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 designated Carpenter D, Lagrange F
and Alhazen F, respectively. You probably haven’t
knowingly seen these challenging features, but they’re
detectable with backyard telescopes when the libration
is favourable.
Orbiting spacecraft also revealed thousands of
CHRISTIAN VILADRICH / LPOD previously unknown craters on the farside. Of these,

Bliss (formerly A)

P

X

U

J

G

L

KA K

Mons

Pico

Plato

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.

CHARLES A.

WOOD

chairs the Lunar

Nomenclature

Task Group of

the International

Astronomical

Union’s

Working Group

for Planetary

System

Nomenclature.

Exploring the Moon