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Exploring the moon.

When I put my eye to the telescope, I'm not just pursuing a favorite hobby. I'm trying to make a virtual journey into space, to explore other worlds for myself. Sometimes I feel that I'm actually doing it. Especially when I observe the Moon.

Any telescopic view of the Moon is breathtaking. Its stark landscape offers a much richer field to explore than any other single celestial object. The Moon is more than 100 times closer than the closest planets (Mars and Venus) ever come. It shows more detail to the naked eye than any planet does in a telescope. And it's so bright that even the worst city light pollution never impairs the view!

Peering into the eyepiece, I'm looking down on a forbidding alien landscape as if from my own spaceship. The view in 10-power binoculars is what you'd see through the window of a spaceship just 24,000 miles from the Moon. A telescope will bring you even closer--a 150x view is like hovering just 1,600 miles up. But I find it best to start with a low-power eyepiece to see the Moon's entire face, then I switch to higher magnification to zero in on some of the features I've found.

Light and Shadow

Your eye quickly turns to the rich detail near the line that divides lunar day and night: the terminator. This is where the Sun is rising or setting on the lunar landscape, casting long, stark shadows that dramatize every hill and valley. On the airless Moon, daytime shadows are almost as black as night. A rising or setting Sun makes craters on the terminator look like bottomless pits--but looks deceive, as you can discover during a single evening of watching.

The Moon takes nearly a month to turn once on its axis (the same time it takes to go around Earth), so the Sun moves across the lunar sky much more slowly than in Earth's sky. Even so, in one hour the sunrise line advances about 9Vi miles at the Moon's equator. Fold in some hilly, cratered terrain, and the shifting shadows along the terminator can change dramatically in just a few hours.

To see for yourself, pick a feature on the terminator and check its appearance from time to time through the evening. And if you really want to witness a transformation, come back the next night and look again. By that time the terminator will have advanced more than 12[degrees] of longitude (225 miles near the lunar equator), and features that were just emerging from shadow the night before will appear more fully illuminated and very different--you might not even recognize them!

A Slow Unveiling

Before you take your scope out for a trip to the Moon, it's a good idea to think about your "flight plan." That's what the map on the page after next is for.

The lunar month--the Moon's cycle through all its phases as it orbits Earth--begins when the Moon is new and hidden in the glare of the Sun. In a day or two, the thin waxing crescent makes its first appearance low in the western sky in twilight. The waxing crescent is the right-hand edge of the Moon on the map and in the sky. Look for dim, gray earthshine illuminating the crescent Moon's night parts--the beautiful "old Moon in the new Moon's arm's" effect.

By the time the Moon is three days old (three days since new), the crescent is thicker and higher at dusk. The sunrise terminator is advancing leftward across the lunar landscape, revealing mountain-ringed Mare Crisium, then Mare Fecunditatis and Nectaris and the cratered highlands to their north and south. By now, the rugged-looking terrain near the terminator is an arresting sight.

Five or six days after new Moon, the moving sunrise line unveils Mare Tranquillitatis, then Serenitatis. On the latter, when the Sun is just right, you may notice the Serpentine Ridge: a string of low wrinkles snaking across the Sea of Serenity's lunar-eastern side (the right side on the map and pictures). Features like this reveal that the maria aren't as smooth as they appear when the Sun is high in their sky.

My favorite terminator views come when the Moon is half lit: at first-quarter phase 7 days after new (and again at last quarter, about 22 days after new). That's when the terminator is a straight line that divides the lunar disk in half. To the north, the curving Apennine Mountains shine in the morning sunlight, casting sawtooth shadows on the surrounding plains. In the south, the terminator struggles brokenly through the Southern Highlands, some of the Moon's most ancient and heavily cratered terrain. Here many giant craters overlap, and those astride the terminator are filled with dark.

Narrow cracks or rilles may come into view here and there. When the Moon is near first or last quarter, find the small crater Agrippa near the middle of the map. Look a little to its celestial northeast (upper left on the map) for the long, narrow Hyginus Cleft, shown in the photo on the previous page. Although this valley is no more than 4 miles wide, the low Sun angle makes it visible even in small telescopes.

For something more challenging look a little south of it (below it), using the photo, for the crater Triesnecker, and try to spot the delicate Triesnecker Rilles nearby. You'll usually need at least an 8-inch scope to see this complex of narrow valleys.

Be sure to take in a trio of huge, flat-floored craters farther south as they emerge from night: Ptolemaeus, Alphonsus, and Arzachel. The latter two have central peaks. All three start out looking like black lakes but undergo an amazing transformation as their floors greet the new lunar morning.

After first quarter the terminator crawls across vast Mare Imbrium, where you can spot not only wrinkle ridges and small craters but lava-filled Plato (photo on previous page) and isolated peaks such as Pico and Piton. Their sharply pointed shadows at very low illumination might suggest that the Moon's mountains are very steep and jagged, but they're actually lower and more rounded.

In the following days as the Moon waxes to become more gibbous, the terminator crosses the crater Copernicus and enormous Oceanus Procellarum. Copernicus, in splendid isolation and surrounded by its bright rays (rubble splashes), is one of the Moon's grandest sights.

When the Moon turns full, about 15 days from new, the terminator lies all around its rim and hence is invisible (or nearly so). Because of the lack of shadows, the full Moon is often called the least interesting phase in a telescope. But now you get to examine the Moon's face purely in terms of its brighter and darker surface material, rather than in terms of shadow-casting topography. The steepest slopes do often display the brightest material. And look for subtle variations in the gray of the maria.

Then for the next two weeks, the Moon wanes back to new. The sunset terminator first appears along the Mare Crisium limb, then creeps leftward night by night. With the direction of illumination reversed, you'll quite literally see familiar features in a different light. The waning Moon is best seen (and finally only seen) in the morning hours. So plan to get up for some late-night exploration of the waning phases.

No matter how many times I look at the Moon, I'm always impressed by the changes wrought by the moving sunlight. The Apollo astronauts, the only people to see features along the terminator up close, had the same impression. One of them, Apollo 12's Alan Bean, recalls that no matter where the terminator was located, whenever he orbited over it he thought to himself, "There's the Moon's really rough part."

Which Way Is Which?

To use this Moon map at your telescope, you need to know which directions are which in your view.

North is up on the map, but north may be any direction in your telescope's eyepiece. Start with low power to see the whole Moon at once, and turn the map around until it matches. That's all you need to do if your telescope gives a correct-reading image (not mirror reversed). That's what you see in a Newtonian reflector, for instance.

But if you use a refractor or compound scope with a right-angle attachment at the eyepiece, you'll probably see a mirror image instead. In that case, you'll need to mentally flip the map left for right after you turn it around. Or you can scan the map, flip it left for right in software, and print out the mirrored version for use at the telescope. Or, go to ShopatSky.com and search for "mirror image Moon map."

A VERY ANCIENT LAND

Why is the Moon covered with giant asteroid impact craters when the Earth isn't? The reason is that the Moon, unlike Earth, went geologically dead vast ages ago. Its biggest craters date from the Late Heavy Bombardment ending 3.8 billion years ago, and the last of the lava floods that created the flat, gray maria ended 3.1 billion years ago. Not much has happened since, so we see truly ancient terrain. Earth, by contrast, is geologically active and keeps resurfacing itself. Earth's landforms also erode in wind and rain.

Andrew Chaikin, author of A Man on the Moon: The Voyages of the Apollo Astronauts, explores lunar nooks and crannies from his home in southern Vermont.
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Title Annotation:Moonwatching: Visiting Another World
Author:Chaikin, Andrew
Publication:SkyWatch
Date:Jan 1, 2015
Words:1578
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