Glenn Chaple’s observing basics: Lunar spectacular

Have you ever gazed in spellbound silence at an aurora, admired the spangled beauty of the Pleiades, or pondered the mystical blue-green glow of a planetary nebula? By whatever means — unaided eyes, binoculars, or a telescope — a visual stroll through the night sky brings you face-to-face with some strange and amazing sights. It’s easy to feel like Alice adrift in a cosmic Wonderland, particularly when you encounter the Cheshire-cat grin of a crescent Moon.

March offers favorable conditions for viewing a waxing (visually growing) crescent Moon. The angle between the ecliptic (the pathway traversed by the Sun across the starry background) and the western horizon is particularly high at sunset now. Thus, Sun-hugging objects, like a “young” crescent Moon and the inner planets Mercury and Venus during their evening-sky forays, rest high above the setting Sun.

Now is the time when Moon-watchers try for a glimpse of a young Moon — one that is roughly 1 day past new phase. Your best dates to see a day-old crescent are February 28 and March 30. You’ll need an observing location that affords a low, open western horizon. Skies must be clear — low-lying haze will conceal the Moon.

Begin your search 10 to 15 minutes after sunset. The Moon will be no more than 10° above the horizon (remember, the Full Moon appears ½° wide), so concentrate on the area above and slightly left of the point on the horizon where the Sun set. If your unaided eyes fail to capture the Moon, try using binoculars. A young Moon will appear as a glimmering hairline curve, barely visible in the evening twilight.

The waxing crescent Moon most casual observers notice appears between 2 and 4 evenings after New Moon. We’ll have two opportunities this month to enjoy an early evening audience with this “Cheshire cat” Moon — the 1st through the 3rd, and again from March 31 through April 2.

As the sky darkens, you’ll notice there’s more to the crescent Moon than just a luminous grin. The “dark” part of the lunar disk appears awash in a ghostly glow. You’re looking at earthshine, also known as “the old Moon in the New Moon’s arms.” Your unaided eyes can spot earthshine, especially when a branch or rooftop masks the bright crescent. Binoculars or a small telescope really shows it off.

What causes earthshine? Leonardo da Vinci explained it 5 centuries ago. A master at understanding the interplay of light and shadow, he reasoned that sunlight reflected from Earth illuminates the lunar surface. He understood that, viewed from a crescent Moon, Earth would appear as a dazzling, nearly full orb. The nighttime lunar landscape would be bathed in brilliant earthlight — the light we see as earthshine.

Because earthshine is a reflection of Earth’s atmospheric conditions, its intensity varies. Clouds reflect more sunlight than land or sea, so a cloudy Earth produces a more prominent earthshine than a fair-weather Earth. Earthshine diminishes as the Moon’s crescent widens and brightens.

March is Messier marathon month. I’ll be “duking it out” with Phil Harrington again. For the past 2 years, clouds have scuttled the event here in the Northeast. Last year, I tried a make-up marathon in April. Final tally: 98 “kills.” I’ll have to do much better this year if I want to beat the Binocular Man — especially now that I’ve learned the secret to his Messier marathon success.

Around sunset on a Messier marathon evening, Phil drives out to his dark-sky site on Long Island. On the side of a barn, he tacks up a poster depicting all of the Messier objects. Once evening darkness sets in, he illuminates the poster with a lantern, then trudges to a spot 100 yards away. He scans the chart with his binoculars, dutifully checks off everything on his Messier list, then packs up and heads home for a good night’s sleep. When Phil says he was “clouded out,” he really means the batteries to his lantern failed!

Next month: Will the Cheshire cat devour the Pleiades? Plus, it’s party time — astronomically speaking.