From the December 2015 issue

The Moon: still the best

The best observing wonder might be closest to home.
By | Published: December 28, 2015
The year’s best lunar viewing now begins. And, for some reason unclear even to me, in its quarter century in Discover and then Astronomy, this column never once focused on observing the Moon. How weird is that?

Time to atone. I assume you own a telescope. Doesn’t matter the size or quality. The Moon is the sky’s most forgiving target. Even many binoculars make it look good, especially 15x to 20x tripod-mounted or image-stabilized models. All of them are far better than Galileo’s pathetic instruments, which nonetheless revealed things that blew the cranky man’s mind. Nothing else in the universe shows as much detail. It wows every crowd.

But you have to know when to look. If you like observing between 6 and 10 p.m., then your best Moon season is January through March. Circle the calendar for the First Quarter Moon, plus the next three nights. That’s your optimum window. January 16–19, February 14–17, and March 15–18.

That’s when the Moon floats high up at nightfall. It’s not hidden behind hills even if you live at the bottom of the Grand Canyon. But the best thing is the lighting. Intricately detailed craters and mountains best materialize when struck by a low-angled Sun, and this is when that happens.

The First Quarter Moon is heaven. It’s great even in a low-power eyepiece, which keeps turbulent seeing conditions to a minimum. Bring this page out with you and start with the curved mountain range almost in the middle of the Moon. These are the lunar Apennines, fabulous from First Quarter until two nights later. Some 400 miles (600 kilometers) long, they stick 18,000 feet (5,000 meters) straight up toward you like shark’s teeth. They’re the Moon’s best mountains, and an easy guide to other features. Observe two days after First Quarter and follow this curved mountain chain toward the Moon’s unlit part: They end abruptly at the crater Eratosthenes. It’s worth saying his name because he was the guy who first figured out the true size of Earth without ever setting foot outside of Egypt. That was 2,200 years ago.


Two and three days after First Quarter, those curved Apennines and Eratosthenes together point you to the most beloved lunar crater, Copernicus. At Full Moon it’s just a white spot surrounded by emanating white rays. It doesn’t look like a crater at all then. But now? It’s worth buying a telescope for Copernicus alone. Some 58 miles (93km) wide, its terraced walls, looking like Asian rice paddies, step their way to a nosebleed height 12,000 feet (3,800m) above the crater floor. At the crater’s uneven bottom stand two majestic mountains and a third smaller one, all casting shadows whose profiles reveal their intricate vertical shapes.

As the day/night line, or terminator, creeps over the lunar surface at 10 mph (16 km/h) — jogging speed, if you’re quick — lunar detail at its equator changes in just a 4-hour observing session. Aiding the process is the Moon’s small size and thus steeply curved surface. The lunar horizon falls away so abruptly that Apollo 15, 16, and 17 astronauts driving in the rover disconcertingly watched the lander almost immediately vanish over the skyline. If an astronaut stood at the center of Copernicus and looked out, she wouldn’t be able to see its miles-high rim. The crater’s lofty encircling walls would all be invisible — over the horizon!

The Apennines’ curve forms the southeastern boundary of the Moon’s most dramatic “sea” — Mare Imbrium. This large, dark, smooth mare lies at the Moon’s upper left as seen with the naked eye or binoculars. Notice how all the seas are pockmarked by few craters. By contrast, the highlands at the bottom (south end) of the Moon are chaotic with impact holes. This quick inspection reveals the Moon’s timeline. Those highlands are the oldest features. Then came a period of volcanism that swallowed mountains and craters and left behind dark flat areas. Finally, here and there, a crater like Archimedes (the biggest crater just to the upper left of the Apennines) marks a recent impact. These are the precious new features.

The scarcity of craters embedded in the smooth maria is absolute proof that the wild west years of the solar system, when crazy-frequent meteors kept pounding Earth and the Moon, ended a long time ago. Our current state of calm stares at us from the Moon’s surface. Otherwise those seas would have lots of craters.

This dramatic visual storybook hovers in the late winter sky. And the scene even changes nightly, like no other celestial object. Is it possible the best telescope target happens to be the closest? I don’t know about you, but after all these years I’m still not over the thrill of the Moon.

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