Astronomy

48 ASTRONOMY • DECEMBER 2018

The effects of light and dark on the lunar

terminator offer some great observing fun.

by Phil Harrington

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T

he Moon’s terminator — the

line that divides the dark part

from the sunlit area — is a fas-

cinating sight through all tele-

scopes. Here, along the lunar

sunrise/sunset line, lighting can transform

familiar features into strange shapes that

disappear when the Sun climbs higher in

the Moon’s sky.

Some observers refer to such strange

lighting phenomena as clair-obscur

effects. The term is adopted from an oil

painting technique developed during the

Renaissance that uses varying shades of

colors and contrasts to create dramatic,

three-dimensional effects. The Italian

term chiaroscuro (translated as “light-

dark”) is also used often to describe the

same technique.

I have collected a baker’s dozen lunar

clair-obscur effects to whet your appetite.

To help zero in on when to look, each

description includes the prime time of vis-

ibility in terms of days after New Moon

(often called the Moon’s age). Be aware,

however, that some are more challenging

than others. A few remain visible for sev-

eral nights in a row, while others disappear

after a few hours because the Sun’s chang-

ing angle wipes them from view.

Another factor that makes viewing

some of these features trickier than others

is that they require knowing more than just

lunar age. You also need to know the

colongitude of the Sun. Colongitude is the

exact selenographic (lunar) longitude of the

“morning” terminator, the line of sunrise

on the Moon. Like longitude on Earth,

colongitude is measured in degrees.

By definition, the colongitude equals 0°

when the Sun stands exactly above lunar

longitude 90°. That alignment occurs

approximately at First Quarter, but can

actually take place several hours before or

after that moment due to libration — the

Moon’s slow back-and-forth wobble as it

orbits Earth.

Fortunately, you can use a freeware pro-

gram to find the Sun’s colongitude at any

given time: http://www.internetsv.info/

MoonCalc.html. The only data you’ll need

to enter is the date and the Universal Time.

LUNAR X

(Lunar Day 6.9) Let’s begin our hunt with

the one clair-obscur effect that has prob-

ably garnered more attention in online

discussion forums than any other. First

noticed in August 2004 by Canadian

amateur astronomer David Chapman, the

Lunar X — also known to some as the

Werner X or the Purbach Cross — is easy

to see if you look in the right place at the

right time. The shape is formed by the

confluence of four lunar craters: Purbach,

La Caille, Regiomontanus, and Blanchinus.

Purbach forms the eastern side of the X,

while Blanchinus creates the western side.

La Caille forms its northern boundary,

and Regiomontanus marks the southern

section. One of the X’s namesakes, the

43-mile-wide (70 kilometers) Werner

crater, does not contribute to the feature

directly. Instead, as Chapman suggests,

“Werner is the closest well-lit crater and

makes an obvious beacon for observers.”

If the timing is right, it’s fascinating to

watch the Sun rise over the X, slowly

unveiling its ragged form over the course

of an hour or so. The first rays catch the

southeastern wall of Purbach. As the Sun

climbs higher in the lunar sky, the X grows

as Purbach’s northeastern wall is lit, even-

tually merging with the southeastern rim

to form one side of the X. The southwest-

ern side of Blanchinus next sees the light,

followed finally by La Caille to complete

the X. If you are even a few hours late, the

shadowing effect is lost and the X-cellent

illusion disappears.

High magnification is not required to

see this effect. In fact, you can even spot it

through steadily held binoculars.

LUNAR V

(Lunar Day 6.9) While you’re enjoying the

X, be sure to catch the V, which lies nearby.

That’s right, we have another letter of the

alphabet visible at the same time just north

of the X. The V is sandwiched between

Mare Vaporum to its north and Sinus

Medii to its south. This puts it smack dab

in the center of the Moon’s disk.

The so-called Lunar V is formed by

low-angled sunlight spangled across several

small craters. The largest, 14-mile-wide

(23 km) Ukert, forms a portion of the V’s

western edge, while a pair of intersecting

ridges create the rest of this edge as well

as the eastern edge. Although the Lunar V

is every bit as obvious as the Lunar X, it

hasn’t quite attracted the same level of

attention among devout luna-tics.

ARIES’ HOOFPRINT

(Lunar Day 6.9) As the X and V draw your

attention, shift your concentration just