Moon Maiden
(Lunar Day 11) One of the most striking lunar features visible between the waxing gibbous phases right through the waning 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 imagined a Moon Maiden: the profile of a woman’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 conditions to be seen, the Moon Maiden is at her best when viewed through a small telescope 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 waning 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 floor 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 pressures 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 illuminates 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 discovering 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 striking 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!