Sky and Telescope - June 2016 - 53

Charles Wood chairs the Lunar Nomenclature Task Group of the IAU's
Working Group for Planetary System Nomenclature.

Charles A. Wood
X

B
C

North

MAPPA SELENOGRAPHICA

R
J

Q
P

S&T: LEAH TISCIONE;
SOURCE: EWEN WHITAKER

Left: Beer and Mädler's Mappa Selenographica (1834-36) used letters to identify small features near a large patronymic crater.
Right: Ewen Whitaker devised this lettering scheme for identifying small craters on the lunar farside.

mapper Ewen Whitaker (University of Arizona) devised
a clever system to assign letter designations for features
near them. Whitaker imagined each named crater to be
the center of a 24-hour clock, with 24 letters - omitting I and O - distributed around it. He assigned A to
craters near the 1 o'clock position, M to those halfway
around, and Z to 24 o'clock. Thus, unlike the somewhat
haphazard situation on the lunar nearside, all farside
letter designations follow a consistent scheme. Interestingly, the IAU earlier this year added two additional letter designations, Poincaré E and F, to farside craters now
being mapped with Lunar Reconnaissance Orbiter data.
One of the most famous letter carriers is Messier A,
the elliptical companion of Messier in Mare Fecunditatis. This dramatic duo probably formed when one
grazing projectile created Messier then ricocheted down
range to excavate Messier A. Another well-known lettered crater is Hesiodus A, the easiest concentric crater
to detect telescopically.
For those lunar observers wanting a greater challenge, I encourage you to seek out Byrgius A and Marian
T, and then to determine what's special about them.
(Hint: Two online image maps - http://is.gd/IAU_
lunar_names and http://is.gd/LRO_lunar_names - identify all named and lettered lunar craters. These show you
where they are, but they don't explain what they are.)
The IAU classiﬁes lettered craters as "satellite features," though typically they're not physically related to
their patronymics. Instead, lettered craters most often
are simply random impacts near a named crater. But
some lettered craters are true secondary craters, excavated by ejecta thrown out by the formation of their
named crater. For example, Copernicus B, E, F, G, and
N are all secondary pits to the south of Copernicus itself.
To its east are a dozen or so lettered craters that overlap
and occur in lines. They're obviously secondaries from

Copernicus as well - but because they are nearer to
Stadius, they bear the name of that ruined crater.
Despite their utility, lettered craters are not used anywhere else in the solar system. Perhaps this is because
in the early days of space exploration many members
of the International Astronomical Union's Planetary
Nomenclature Working Group had little experience
with lunar maps, and the idea at the time was to honor
fellow countrymen by placing their names on interplanetary bodies. Both Mars and Mercury bear thousands of
impact-excavated craters - and, without the use of letters, vast numbers of names ultimately will be necessary
to identify them. ✦

The Moon * June 2016
30

Phases

June
8

NEW MOON

June 5, 3:00 UT
FIRST QUARTER

June 12, 8:10 UT
FULL MOON

June 20, 11:02 UT
LAST QUARTER

June 27, 18:19 UT

Distances

21
18

NASA / LRO

For key dates, yellow dots indicate which part of
the Moon's limb is tipped the most toward Earth
by libration under favorable illumination.

Favorable Librations

Perigee
361,140 km

June 3, 11h UT
diam. 33′ 5″

Neper (crater)

June 8

Newton (crater)

June 18

Apogee
405,024 km

June 15, 12h UT
diam. 29′ 30″

Inghirami (crater)

June 21

Pythagoras (crater)

June 30

Sk yandTelescope.com June 2016

http://is.gd/IAU_lunar_names http://www.is.gd/LRO_lunar_names http://is.gd/IAU_lunar_names http://www.SkyandTelescope.com

Sky and Telescope - June 2016

Table of Contents for the Digital Edition of Sky and Telescope - June 2016