The Lunar CityMoon
MaidenQuincunxSinus IridumLunar VPtolemaeusMare
Nubium
Hesiodus
Sunrise RayEyes of Clavius Lunar LLunar XStraight Wall
and Huygens’ SwordEiN (^) Mountains
Aries’
Hoofprint
Nessie
WWW.ASTRONOMY.COM 51
their sky. The “E” is actually formed by the
shadows of three peaks cast onto the adja-
cent valley, while the “i” and “N” are cre-
ated by light striking the sides of adjacent
mountains. As its discoverer Joe Adlhoch
points out, the E-i-N effect is most obvious
in the inverted view of a ref lector.
MOON MAIDEN
(Lunar Day 11) One of the most striking
lunar features visible between the waxing
gibbous phases right through the wan-
ing crescent is Sinus Iridum, the Bay of
Rainbows. The bay is actually the remains
of a large impact crater that was partially
submerged after molten lava from Mare
Imbrium breached its southern wall about
3.8 billion years ago.
The southwest end of the bay, where the
crater rim sinks below the now-solidified
mare, is named Promontorium Heraclides.
Astronomers call the northeastern tip
Promontorium Laplace.
When he gazed toward Promontorium
Heraclides in the late 1670s, the renowned
Italian astronomer Giovanni Cassini imag-
ined a Moon Maiden: the profile of a wom-
an’s head with hair flowing behind her as
she looked across the bay toward Plato
Crater. Can you see her, too? There’s a
twist. Unlike some of the other clair-
obscur effects that take exceptional condi-
tions to be seen, the Moon Maiden is at her
best when viewed through a small tele-
scope and seeing (atmospheric steadiness)
is poor. That will soften the surface enough
to let you see her overall form. With optics
that are too sharp or seeing that is too
good, her form resolves into its individual
components, and her appearance is lost.
Come back to this spot during the wan-
ing crescent phase, and the Moon Maiden
will be gone. But in her place, the shadow
created by the jagged profile of the Juras
Mountains, which mark the perimeter of
Sinus Iridum, falls onto the smooth f loor
of the bay to create what some call the
Lunar Buzz Saw (Lunar Day 24.6).
STRAIGHT WALL AND
HUYGENS’ SWORD
(Lunar Day 21) One of my favorite lunar
targets is the Straight Wall, known more
properly by its official name Rupes Recta.
While fault lines on Earth are most often
associated with plate tectonics, Rupes
Recta was formed when a portion of Mare
Nubium succumbed to subterranean pres-
sures and buckled. The area to the west of
Rupes Recta sheared off and dropped more
than 1,000 feet (300 meters) along the fault
line that extends for 70 miles (113 km) from
tip to tip. On Lunar Day 8, the “wall” is
in shadow, turning it into a black line that
looks almost too straight to be natural. On
Lunar Day 21, however, sunlight fully illu-
minates the fault’s face, causing it to gleam.
Just south of Rupes Recta lies a small
clump of jumbled terrain and a half-buried
crater. The 17th-century astronomer
Christiaan Huygens, credited with discov-
ering Rupes Recta, likened the jumble and
half-crater to the handle of a sword, with
Rupes Recta forming the blade. Although a
better allusion is that of a fencer’s foil, we
know the combined appearance today as
Huygens’ Sword. The sword is most strik-
ing at lunar sunset.
Seeing these clair-obscur features will
take some persistence and favorable sky
conditions. For these challenges, though,
you won’t have to travel to a dark site.
Good luck!
Phil Harrington is a contributing editor
of Astronomy who loves to point different
telescopes at the Moon.
Use this guide to locate the general area of all the targets in this story. CO NSO LI DATED LU NAR ATL A S
Rupes Recta,
commonly called
the Straight Wall,
stretches for 70 miles
(113 km) just to the
east (right in this
image) of the double
crater Birt and Birt A.
West (left) of Birt is a
30-mile-long (50 km)
rille known as Rima
Birt. Just below the
Straight Wall is the
jumbled terrain and
half crater the author
described, which
form the handle and
guard (with Rupes
Recta being the blade)
of Huygens’ Sword.
BRIAN FORD
Birt