A Month of Moonwatching
Some stargazers find the Moon an annoyance; its brightness makes it difficult to view faint nebulas, galaxies, and other "deep-sky" objects. But for me the Moon is forever an enchanted land, readily available even under less-than-ideal sky conditions. Whether presenting itself as a razor-thin slice or a circular orb lighting the night sky, the Moon always offers a visual feast. The interplay of shadows among the towering peaks and craters will appeal to your aesthetic interests and feed your curiosity. And its proximity allows even the most basic of astronomical instruments, the unaided eye, to explore some of the more prominent features.
Every 29 ½ days, as the Moon glides in its orbit around Earth, it steps through a complete cycle of its phases from delicate crescent to fat disk and back again. Our month-long tour begins shortly after new Moon, which astronomers count as "Day 0" of the lunar month, and continues through first quarter (Day 7), full Moon (Day 14), and last quarter (Day 22). I've picked times when some of my favorite lunar features are prominently positioned near the terminator, the border between lunar day and night, where a low-angle Sun creates dramatic shadows that help define the shapes of craters and mountains and gives them a three-dimensional appearance.
Days 3 and 4
Few astronomical sights are more beautiful than a crescent Moon lingering in the evening twilight. The Moon is now waxing, or growing more fully illuminated, night by night. You'll often see a faint glow on the unlit side of the disk, next to the crescent. Sometimes called "the old Moon in the new Moon's arms," this earthshine is sunlight that has been reflected onto the lunar landscape by Earth and has bounced yet again back toward us. Under the right conditions, you might even be able to identify prominent lunar features within the area lit up by earthshine.
One of the most obvious features on the waxing crescent is Mare Crisium (Latin for "Sea of Crises"), an oval dark spot north of the lunar equator that's about 350 miles across. (It's just above center in the image at right.) I like to check Mare Crisium with each new lunar cycle. If you keep a close eye on it from month to month or even over a few nights' time you'll find that Mare Crisium moves toward or away from the edge of the lunar disk. Sometimes it's very close and appears more elongated than at other times. You can monitor this phenomenon with the unaided eye. This slight nodding of the disk, called libration, results from the not-quite-constant velocity of the Moon in its orbit and other geometrical factors.