The Presence and Significance of Valleys on the Moon

The Moon is a treasure trove of fascinating geological features that can be observed with even small- to moderate-aperture telescopes. Among these features are valleys, rilles, and faults that add to the lunar landscape’s appeal. Four of these remarkable formations are the Alpine Valley (Vallis Alpes), Schröter’s Valley (Vallis Schröteri), Rima Ariadaeus and Rima Hyginus, and Rupes Recta, also known as the Straight Wall.

The Alpine Valley, with its 160-kilometer length, is one of the most impressive examples of a rift valley on the Moon. This crack-like formation cuts through the magnificent Vallis Alpes, giving it a unique appearance. Located between Mare Frigoris and Mare Imbrium, the Vallis Alpes region offers much more visual pleasure, including the stunning Plato crater that lies to the west of the valley. With widths reaching up to 20 kilometers and a floor lying 2,000 meters below its steep walls, the Alpine Valley is truly a gouge in the lunar terrain. The best time to observe this extraordinary formation is around the Moon’s first quarter phase or six days after full Moon, with optimal visibility on the early evening of February 17.

Schröter’s Valley, on the other hand, showcases a sinuous rille, a channel-like depression formed from volcanic activity. This particular sinuous rille spans 160 kilometers with an average width of six kilometers. Snaking across the Aristarchus Plateau, which rests on the Oceanus Procellarum to the northwest of Copernicus crater, Schröter’s Valley appears like a dried-up river bed. It originates from the ‘Cobra Head,’ a volcanic caldera located just north of Herodotus crater. To observe this remarkable lava flow, astronomers should wait for the Moon to be around four days after first quarter or three days after last quarter. On the evening of February 21, when the Moon is approximately 12 days old, the Aristarchus Plateau is beautifully bathed in morning sunlight. Telescopes with an aperture of 100mm are sufficient to resolve Schröter’s Valley, while larger telescopes around 250mm in diameter can unveil narrower sinuous rilles to the north, between Prinz and Krieger craters.

Another intriguing lunar sight is the interplay of Rima Ariadaeus and Rima Hyginus, which can be observed when observing the small lunar mare Sinus Medii and the western edge of Mare Tranquillitatis. Rima Ariadaeus extends west-northwest for over 200 kilometers, intersecting with the eastern end of Rima Hyginus—an amazing valley composed primarily of interlocked small craters that stretches over 110 kilometers on either side of crater Hyginus, which is 11 kilometers wide. These features are best viewed around six days after new Moon and five days after full Moon. On the evening of February 16/17, they emerge gloriously into the morning sunlight, with the morning terminator lying east of crater Hyginus. To fully appreciate Rima Ariadaeus and Rima Hyginus, a telescope with an aperture of 100-150mm will suffice.

Lastly, the Straight Wall, or Rupes Recta, is a fault that runs for an impressive 110 kilometers in southeastern Mare Nubium. This majestic linear formation, measuring one to three kilometers in width, is accompanied by Rima Birt—a narrow rille that runs parallel to Rupes Recta northwards from the small crater Birt. On the evening of February 17/18, as the Sun climbs across Mare Nubium, Rupes Recta is illuminated by morning sunlight, revealing a slightly curving narrow line that gradually blends into the lunar background as the Wall’s shadow shortens. While Rupes Recta can be observed with a small telescope, a larger telescope is necessary to view Rima Birt.

All of these lunar features offer awe-inspiring spectacles for amateur astronomers and lunar enthusiasts alike. With just a telescope and knowledge of the optimal viewing times, one can embark on a visual journey through the Moon’s valleys, rilles, and faults—exploring the wonders of our nearest celestial neighbor.