A (6)

http://www.skyandtelescope.com.au 57

670ofthemostprominenthavebeennamedbythe

International Astronomical Union, and in 1981 famed

lunar 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 centre 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 downrange to excavate Messier A. Another

well-known lettered crater is Hesiodus A, the easiest

concentric crater to detect telescopically.

If you’re a lunar observer 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 honour fellow countrymen by placing their

names on interplanetary bodies. Both Mars and

Mercury have thousands of impact-excavated craters

— and, without the use of letters, vast numbers of

names ultimately will be needed to identify them. ✦

MAPPA SELENOGRAPHICA.

S&T:

LEAH TISCIONE; SOURCE: EWEN WHITAKER

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Above: Beer and Mädler’s Mappa Selenographica (1834–36)

used letters to identify small features near a large patronymic

crater. Above right: Ewen Whitaker devised this lettering

scheme for identifying small craters on the lunar farside.

A B C D E F G H J K

M L

North

N

P

Q

R

S

T

U

V

W

X

Y Z