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Appreciating the Moon

Our natural satellite has many treasures to observe.
RELATED TOPICS: THE MOON | LUNAR OBSERVING
Chaple
The lunar disk is shown in crisp detail in this mosaic of an 11-day-old-gibbous Moon imaged on March 17, 2019.
Alan Dyer

Mario Motta really hates the Moon. In fact, he would nuke it into oblivion if he had the means. You see, Mario is an accomplished astroimager whose work appears in the second edition of Stephen O’Meara’s Deep-Sky Companions: The Messier Catalog. And as anyone who images clusters, nebulae, and galaxies knows, a bright Moon obscures these cosmic delights.

But even the most vocal Moon-hater has to acknowledge the jaw-dropping visual impact of the Moon when viewed through a telescope. Nothing in the evening sky packs a “Wow!” punch like the sight of its rugged, cratered surface. No cluster. No nebula. No galaxy. Saturn, with its fabled rings, is a distant second.

The Moon is resplendent with lofty mountain ranges, sinuous ridges and rilles, and hundreds of craters. And, to sample a variety of lunar features, you can do no better than to explore the expanse that runs along the northern border of Mare Imbrium (Sea of Rains) from the Montes Alpes (Lunar Alps) westward to the Montes Jura (Jura Mountains).

The Lunar Alps come into view around First Quarter. Located at the northeast edge of the Sea of Rains, this range is about 155 miles (250 kilometers) long. Most of its peaks reach elevations between 6,000 and 8,000 feet (1,800 to 2,400 meters). They arose from the impact that created the Sea of Rains some 4 billion years ago. Bisecting the Lunar Alps is the Vallis Alpes (Alpine Valley), a gash a little over 100 miles (160 km) long and 5 to 6 miles (8 to 9.6 km) wide. Once thought to be the result of an oblique strike by an object thrown out by the Sea of Rains impact, it’s now thought to be a graben — a segment of the lunar crust thrust downward by the event.

Near the southern end of the Montes Alps and jutting out of the grayish Sea of Rains is the bright peak Mons Piton. When near the terminator, Piton casts a long, pointed shadow, giving the impression that this mountain is a tall, needle-shaped structure. In reality, Piton rises just 1.4 miles (2.2 km) above its 16-mile-wide (25 km) base. The lofty spire is actually a gently rolling hill!

By the evening following First Quarter, the lunar terminator (the boundary between day and night on the Moon) has drifted westward to reveal a dark oval patch encircled by a bright rim. This is the 63-mile-wide (101 km) crater Plato, which appears dark because its floor is covered by low-reflectivity lava rocks. A popular observing challenge for lunar observers is to search for the tiny craterlets that dot the crater’s floor. If seeing conditions are favorable, a 6-inch scope and a magnification of 175x or more should reveal a few. How many can you spot?

Directly south of Plato and rising out of the Sea of Rains is the 8,000-foot-high (2,400 m) Mons Pico. Like Piton, Pico casts a sharp and lengthy shadow when near the terminator, belying its relatively flattened shape.

Another evening will reveal more mountains that emerge from the Sea of Rains. Northwest of Pico is a 70-mile-long (113 km) jumble of peaks known as the Montes Teneriffe (Teneriffe Mountains), whose tallest members reach a height of 8,000 feet (2,400 m). Further west is a mountain chain that’s shorter (56 miles [90 km]) and less lofty (average height 6,000 feet [1,800 m]) but far more eye-catching. Appearing more like a massive, artificially created wall than a natural lunar feature, this line of mountains is appropriately named the Montes Recti (Straight Range).

Three nights after First Quarter, the terminator has moved westward enough to reveal the entire north shore of the Sea of Rains and our pièce de résistance, the magnificent Sinus Iridum (Bay of Rainbows). The “bay” is actually the remnant of a crater 160 miles (258 km) across. Its northern rim forms the craggy Jura Mountains. The remaining rim was erased by the lava flow that created the Sea of Rains. The Bay of Rainbows is flanked by the two mountainous capes of Promontorium Laplace and Promontorium Heraclides.

So, my deep-sky-imaging friends, stop cursing the Moon. Set up a small telescope in the backyard and spend some time exploring this amazing world. Questions, comments, or suggestions? Email me at gchaple@hotmail.com. Next time: Here comes the Sun! Clear skies!

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A chronicle of the first steps on the Moon, and what it took to get there.