As the terminator sweeps across the smooth surface of Oceanus Procellarum during the waxing gibbous and waning crescent phases, it passes over an area of jumbled terrain that looks like a cluster of islands in the otherwise tranquil "ocean." The most obvious reference point in the vicinity is the extraordinarily bright crater Aristarchus. Its sunlit floor and interior walls shine with an unparalleled radiance once sunlight strikes them.

Aristarchus's located on the northwest part of the Moon's near side and is easily visible through a telescope. The crater is about 40 km (25 miles) in diameter and 4 km (2.5 miles) deep. It has a central peak and terraced walls, and is surrounded by a bright blanket of ejecta. Aristarchus is estimated to be about 450 million years old, which is relatively young for a lunar crater. It was formed by an impact from an asteroid or comet. The impact excavated material from the Moon's crust and mantle, which was then ejected outwards and deposited around the crater.

Lunar dawn and dusk over Aristarchus are wonderful times to view the area to the crater's immediate surroundings. Just to the west lies the crater Herodotus. Herodotus is just 4 miles smaller than Aristarchus, but it appears far more modest than its dazzling neighbor.  It's a relatively old crater, estimated to have formed around 3.9 billion years ago. Herodotus has a slightly irregular, narrow rim that appears somewhat oblong due to foreshortening. The inner floor has been flooded with lava, which causes a lower albedo than its brighter and more prominent neighbor, Aristarchus. There is a small craterlet overlapping the northwest rim, but otherwise the outer wall has not suffered significant wear. Nevertheless, the rim is unusually thin in relation to its size.

But Herodotus has something that Aristarchus doesn't; it has a tail! The tail curls away to Herodotus's north, twisting around toward the east as it flows through the mountainous terrain on its way toward the ocean's coastline. We know Herodotus's tail more properly as Schröter's Valley, named for the 18th-century German astronomer Johann Hieronymus Schröter.

Technically, Schröter's Valley, also known as Vallis Schröteri, is not a valley at all. It's a sinuous rille, the largest and most prominent sinuous rille on the Moon's near side. Although the "valley" looks to be connected to Herodotus, it actually begins at a 4-mile-wide (6 km) crater some 15 miles to the north called the "Cobra Head." It then meanders its way for over 100 miles (160 km) across the Aristarchus plateau, narrowing to less than 1/4 mile wide at its thinnest. It eventually empties into Oceanus Procellarum. The valley is believed to have been formed by ancient volcanic activity, specifically by a lava flow that carved out the channel. The estimated age of Schröter's Valley is around 3 to 4 billion years old, making it a relic of the Moon's early volcanic period.

As with many lunar features, it's best to look for Schröter's Valley when it is near the terminator. Try observing it 12 to 13 days after the New Moon, during the waxing phases. If you prefer early-morning viewing during the waning phases, the 24- to 26-day-old Moon is your best bet. The table at the top lists upcoming dates in 2025.

Magnification will probably need to be increased to at least 150x to spot the tiny "source crater." If you do make it out, notice how a pile of volcanic rocks surrounding the crater looks like a cobra's head? Believed to be around 3.5 billion years old, it formed during the Imbrian period. Its origin is volcanic, not impact-related, likely the result of a collapse above a lava tube. Research suggests that the Cobra Head marks the volcanic vent from which lava welled up to cut the rille and flood a portion of Oceanus Procellarum.

Good luck with this month's Cosmic Challenge! And be sure to post your results in this column's discussion forum.

Until next month, remember that half of the fun is the thrill of the chase. Game on!