June 13, 2003
 |
SPACE.com's "Skywatcher's Guide to the Moon" answers these questions and more. It's your road map to the most easily observed object in the cosmos. With our printable Moon map, some tips and facts, along with a pair of eyes, you're ready to explore. No telescope required.
Avid Moonwatchers say the best time to view it is not just during a full Moon, but also around the time of first quarter or last quarter [See our Moon Phase chart & Sky Calendar]. For about a week around each of these times, the Sun strikes the Moon's surface at a steep angle, rendering craters in sharp relief.
Though it's tempting to use a telescope, your first explorations should be with the naked eye. Become familiar with the Moon's major features. Then use binoculars to study the features more closely. Learn the names of a few craters and plains (called maria, after the Latin word "mare," which means sea).
If you use a telescope, zero in on the line where light and shadow meet. Here you'll see the greatest contrast and the most detail.
The Moon has a story to tell. And much of its story is like an open book, born of a violent past, worn on the lunar surface as ancient round pockmarks and streaks of fresh, bright material. It's a story that scientists started piecing together in 1609 when Galileo Galilei first turned a telescope on the Moon.
Galileo and You
Galileo Galilei did not invent the telescope, but he was the first to realize its importance for astronomy, and he used his great engineering skills to craft improved lenses for the crude devices, which were at first called spyglasses or eyeglasses (a colleague of Galileo's later dubbed them telescopes).
When Galileo turned his own handmade device on the Moon in 1609, he turned conventional thinking on its head. Amazed at what he saw, Galileo wrote that the Moon "is like the face of Earth itself," marked with mountains and valleys.
The discovery surprised the heck out of his contemporaries, too, most of whom still believed Aristotle's idea that the heavens were made of unearthly matter -- ether or quintessence -- that was immutable and incorruptible, meaning it couldn't have jagged edges.
With a small telescope, or even a pair of binoculars, you can share Galileo's thrill as the Moon's mountains and valleys jump out in stark relief.
The book evolved slowly. In the 1800s, people claimed to see settlements on the Moon. And it wasn't until the middle 20th Century that scientists agreed on what caused the Moon's craters.
Then beginning in the 1960s, Apollo astronauts added many chapters to the book by bringing back rocks and photographs from the surface.
You can easily read the Moon's story with a touch of guidance, so we asked Andrew Chaikin, Apollo historian, to share some insights and simple tips for anyone who might want glance up at the sky, with or without a telescope:
Even if you don't have a telescope or a pair of binoculars, you can go out and look at the Moon and make a discovery. There are two kinds of rock that you can see from Earth, even with the naked eye.
When you look up at the Moon you see a bright disk and dark splotches. The bright areas are the ancient crust leftover from the Moon's formation, and the dark areas are relatively newer rock that formed from lava that erupted from the interior. Those are the so-called seas.
They are made out of lava rock like we find in Hawaii. Except that in Hawaii, if you pick up a rock, it's likely to be maybe 100 years, or maybe it's a year old, or maybe it formed last week. But on the Moon, it's likely to be 3.5 billion years old.
With binoculars, you'll start to see some of the surface features. In particular, you can see some of the Moon's largest craters. The craters tell you that the Moon was the victim of some cosmic target practice. You can see that the Moon was a pretty violent place when the planets were forming. Each one of those craters is the result of an impact that was so violent it dwarfs any atomic weapons we have on Earth.
The next question one might ask is why doesn't the Earth show the same kind of scars. Well, it does have a few, like Meteor Crater in Arizona. But most of the impact craters on the Earth were erased by the forces of wind and rain and the shifting continents, including mountain-building events and volcanism.
But the Moon is pretty much untouched for the last 3 billion years, so it's kind of like a museum for that violent era.
If you have a good telescope, you can start to look the details of craters. Some of the big craters have walls that look terraced like a staircase, and this is because right after the crater was blasted out of the Moon's crust, the rock that formed the walls of the crater was fractured by the impact and slumped down, or collapsed, to form a series of stair steps.
Many of these large craters have flat floors covered with fairly dark rock that look somewhat like seas or ponds. This is actually impact melt, which is material that had been melted by the giant impact and then rained back down into the crater and cooled.
If you look in the middle of many of these large craters, you'll see some very large mountains sticking up. It's hard to tell how big these things are when you're looking through a telescope, but you can rest assured that these mountains are thousands of feet high. These are called central peaks, and they formed when the crust rebounded after the impact.
There are craters all over the Moon in various stages of preservation. Many are much more beaten up, and have craters on top of craters.
But the youngest craters have what look like rays of bright material coming from them. These rays are made of ejecta that were blasted out of the crust when the crater formed and sprayed across the landscape. The best example is the crater Tycho, which is in the southern highlands. It's one of the best-preserved large impact craters on the Moon. You can see its ray pattern across much of the Moon's near side with binoculars, or sometimes even with the naked eye if you've got good eyes.
Finally, the dark lava seas are themselves filling in giant impact craters that are called impact basins. These basins can be hundreds of miles (kilometers) across. Mare Imbrium is one such giant impact basin. You can still see its round shape.
The basins are places where absolutely gigantic impacts took place. If they had been much bigger, they could have broken the Moon apart.
Through a telescope or binoculars, you'll also see that the Moon has a grayish-tan color. It's a very subtle shade. And it is exactly the color that the astronauts described when they went there. So you can actually get a little sense of what they saw by looking at the Moon yourself.
To the unaided eye, the Moon's surface appears mottled with irregular patches of gray amid splashes of white. These darker patches are known as maria, Latin for seas, since that is how they appeared to early skywatchers. We know them now as areas where lava pooled on the lunar surface billions of years ago, probably after an asteroid punched through the thin lunar crust.
Together, the light and dark regions arrange themselves into the popular face of the "Man in the Moon." They were interpreted as a "rabbit in the Moon" by the Maya and Aztecs of ancient Mexico, as well as the Mimbres Indians of the southwestern United States.
The Moon preserves a record of the pummeling it took 4.1 to 3.8 billion years ago. Earth got nailed by even more space rocks back then, but the planet swallowed the evidence by recycling it into the interior over the eons.
As sublime as the Moon appears to the eye, a pair of binoculars turns this marbled world into a breathtaking wonder. One of the best times to look at the Moon with any optical aid is during its crescent and quarter phases. At that time, the low, slanting angle of the rising Sun creates deep dark shadows in valleys and crater floors that contrast sharply with bright mountain ranges and crater rims. When the Moon is full, it is essentially "noontime" there, with the Sun overhead. Though this geometry makes the Moon appear very bright as seen from Earth, it lowers the contrast of the lunar features.
With a small telescope, a number of prominent lunar features may be observed at first or last quarter. Some of these may also be seen when the Moon is full, but as mentioned, their contrast will be significantly reduced. Moreover, the Moon is so bright that even at first or last quarter it can dazzle the eye (never mind its glaring appearance at full phase!).
Let's begin by considering the Moon's prominent features at first quarter.
Near the upper eastern (right) limb (as the Moon appears to the naked eye and in binoculars), is the round, dark basin known as Mare Crisium. This region can be seen standing in the full glare of the Sun during the early crescent stages. Our perspective of Crisium on the curve of the Moon's limb often gives it an oval appearance. To the west (left) lies Mare Tranquillitatis, the region of the first manned lunar landing. (By the way, none of the Apollo landing bases can be detected in telescopes from Earth.) Above and slightly to its left is another round basin, Mare Serenitatis, which stands partly in shadow. Serenitatis is rimmed on its western (or left) edge by great mountain chains.
Along the terminator, which is the dividing line between the lit and unlit regions of the Moon, many craters stand out in sharp light and shadow. Most prominent is the trio Theophilus, Cyrillus, and Catharina, all lying just below center. Theophilus and Cyrillus are actually overlapping and look like a figure 8. Catharina lies a little below them. Another prominent crater, Aristoteles, lies along the upper terminator, near the Moon's North Pole. Near first quarter it has a bright west rim, with the floor in deep shadow.
Lined up neatly along the terminator at first quarter, just below the lunar equator, are three prominent walled plains. Ptolemaeus lies furthest north (uppermost) and has a number of craters and pits within it. Adjoined to Ptolemaeus to the south, and slightly smaller, is Alphonsus, site of the robotic Ranger 9 landing in March 1965. Finally, just below Alphonsus, is Arzachel, a very conspicuous crater with terraced clefts along its rim and a prominent central peak.
At last quarter, the western (or left) half of the Moon is lit. One of the more prominent features is Oceanus Procellarum in the upper left quadrant, a sprawling expanse of dark basalt punctuated by the white chalk-like streak of Aristarchus, a very recent impact crater. Observers with telescopes have reported unusual brightness and color changes in this feature, which are considered to be attributable to gas emissions from within the crust.
Further east (right) is Mare Imbrium, a large, round basin that makes up one of the "eyes" of the Man in the Moon. Two bright ray craters, Copernicus and Kepler, lie near the equator in this section. The rays are produced by material ejected when an asteroid or comet struck the surface, splashing out bright fresh material from below. Mare Nubium lies directly below, or south, of Imbrium in the Moon's southern hemisphere. Mare Humorum, one of the smallest of the maria, lies to the west, or left.
Most of the Moon's southern hemisphere is rugged and pockmarked with craters, including the bright ray crater Tycho. To its south lies Clavius, one of the most dominant walled plains in this sector of the Moon. Its rim and floor are pitted with a variety of craters.
You don't have to wait until the Moon is full to appreciate our closest celestial neighbor. As lifeless as it appears in binoculars or a telescope, the shadow show at quarter phase imparts a dynamic quality to the lunar craggy surface. All at once, the Moon ceases to be a beautiful bauble floating over Earth's landscape and becomes a world unto its own.
http://www.space.com/spacewatch/moon_guide-2.html