…continuedA Month of Moonwatching
Days 5 and 6
Three prominent craters straddle the terminator on Day 5 and they can be seen easily with binoculars. The first, Theophilus, is a 60-mile-wide ring that borders the shore of Mare Nectaris (Sea of Nectar). Through a telescope magnifying at about 80x, you should see a long, narrow shadow cast by the 4,600-foot-high cluster of mountain peaks at Theophilus's center. I like to use high power to try to distinguish individual peaks and explore the terraced inner walls of the crater.
Equal in size is the adjoining crater, Cyrillus. It's also ring-shaped, though part of its circumference has been breached by Theophilus. Tonight's shadows outline its rim, which to me appears slightly elongated in a north-south direction. The nearby crater Catharina is like-sized, but it lacks a central peak and its walls have partly disintegrated. Look for the mountain range Rupes Altai as it passes near Catharina's rim.
Some moongazers consider the date of first-quarter Moon (Day 7) to offer the most impressive lunar vistas. But waiting just one day later brings sunlight onto a few more dramatic features.
For example, Rupes Recta, the "Straight Wall," is 800 to 1,000 feet high. It's an easy find for a small telescope look south-southeast of the prominent crater Ptolemaeus, which lies to the right of the terminator near the center of the Moon. Although the Straight Wall might look like a steep cliff, it has a slope of only about 15°. Tonight this 80-mile-long feature appears as a thin, dark line but 15 days from now it will look very different.
Now sweep to the northern edge of Mare Imbrium (Sea of Rains), where you'll find Plato. This 60-mile-wide crater, sometimes likened to a "walled plain," has no central peak and a lava-filled floor that appears darker than its surroundings. It's truly circular, but it looks like an oval because of foreshortening at that far-northern latitude. In my 8-inch-aperture telescope at low power (40x), I see one wall cast in shadow and the opposite wall brightly lit. Try increasing the power to see if you can locate tiny craterlets on Plato's floor.
Just to the south lies Pico, a solitary mountain roughly 10 by 15 miles in breadth that abruptly rises 7,900 feet above the surrounding terrain. The slopes aren't really all that steep, but at this time of the lunar cycle I see a long slender shadow cast by Pico, which gives me the impression of a towering mountain. Some observers think it looks like a cathedral spire. What does it look like to you?