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Moon Maps and Bias
Observe naked-eye lunar surface markings on the waxing gibbous Moon around sunset or on the waning gibbous Moon around sunrise. Draw your own map of these markings, without reference to any other map. Get other people to perform the same task at the same times you do. Finally, categorize the participants according to how much telescopic observation of the Moon or how much viewing of Moon photographs they have previously done, and compare their results.
Drawing your own map of the Moon is a fine introduction to the discipline of careful observation. It is also good as a first lunar activity because later you may become biased from looking at professional Moon maps in preparation for more detailed naked-eye observations of the Moon (such as in Activity 5). Of course, you already may have looked at enough photographs or maps of the Moon or viewed it enough in the past through telescopes to be somewhat biased But if that is so, you are at least less biased now than you are likely to become.
William Gilbert's moon map (adapted by Doug Myers). Made before Gilbert's death in 1603, this Is the only known pretelescopic moon map and the wonder is why it is so Inaccurate.
The best method of testing the question of bias is the second part of this activity: to have people with different levels of lunar experience attempt naked-eye moon maps at the same times you do.
Of course, comparing the results of these other mapmakers will throw light on additional interesting questions—for instance, how much more detail on the Moon can be detected by people with especially sharp eyesight? But that inquiry urgently raises an important complication for all who would perform this activity: How does one keep the variables of previous familiarity with the Moon, sharpness of vision, and other factors separate from one another?
You will need to define classifications such as "moderate experience" or "sharp eyesight" as precisely and as near to quantitatively as possible. Some parts of the definitions will be obvious, of course. Has the person ever looked through a telescope at the Moon? Does he or she own a telescope? Does he or she own any astronomy books? Further ideas for improving the definitions with which you begin can lead to much better end results.
There are some other kinds of variables that, fortunately, can be eliminated Each person should observe and sketch for the same amount of time—perhaps 15 minutes. Even more importantly, each person should observe and sketch at the same time and in the same county or small geographical region (so weather is likely to be the same). The ideal time is when the sky is neither too similar to the Moon's brightness (as in bright daylight) nor too different (as in full night)—in other words, around sunrise or sunset Joseph Ashbrook suggested that late morning twilight (with waning gibbous Moon) was the best time. See what you think.
1. At what exact time do you have greatest visibility of lunar details? At what time in twilight does visibility improve or worsen the quickest? How much does haze and the Moon's angular altitude affect when these times occur?
2. How much does previous experience with the Moon bias one's drawing of a naked-eye Moon map? How much does the sharpness of one's vision affect the accuracy and amount of detail on such a map?
Features at Each Lunar Phase
Study and draw the Moon's surface features on each day of its entire cycle of phases from one New Moon until the next.
In this activity, the observer takes advantage of the many different lighting angles that features of the Moon undergo at different lunar phases. Some of these features are spectacular only around certain phases, and not even visible at others.
How many days of phases make up the whole cycle? As viewed against the background of stars, the Moon takes about 27 1/3 days to complete one orbit of the Earth—a sidereal month. But during that time, Earth and Moon are also moving onward in their orbit of the Sun. As a result, a period of roughly 29 ½ days elapses before Earth, Moon, and Sun are in the same position relative to one another and the same lunar phase recurs. The 29 ½ days between any phase and its next recurrence is called the synodic month.
But where in the cycle of phases should we begin? For a number of reasons, New Moon is the best place to start Astronomers measure the Moon's "age" each month from the moment of New Moon, the moment of seeming birth. Indeed, the synodic month from one New Moon to the next has a special name. It is called a lunation.
Galileo moon map (adapted by Doug Myers). This was the second of two drawings Galileo made on November 30, 1609, his first two recorded telescopic observations (the magnification was 20 times).
Think of the lunation as a series of nightly pictures forming an eternal book that presents all we can ever see of our Moon. That concise and perfect book—one ideal form of organization in time of what the Moon offers—is what we want to produce if we wish to know the Moon.
Actually, of course, a lunation does not offer us quite the perfect series of nightly pictures that our sense of order seeks. For one thing, there are not exactly 29 days in a lunation. Also, for at least a day or two around New Moon—and this is variable—the Moon is too slightly illuminated or too near the Sun to view. There is one final important departure from perfection. The best time to look for naked-eye lunar surface features each night—around sunrise or sunset—is not likely to be precisely a whole number of days past the moment of New Moon.
Does the exact age of the Moon matter? Does one-quarter (or three-eighths) of a day make a significant difference in what is illuminated? Sometimes it does, even for nakedeye observers. The reason for this is the special visibility of features at the terminator—the line of sunrise or sunset on the lunar surface. Near this line, shadows of rugged lunar topography are longest, the features etched in sharpest relief. High-altitude features (mountains and crater rims) catch the rising sun earlier (and the setting sun later) than low-altitude features (the plains of the lunar "seas" and crater floors). The naked-eye jaggedness of the terminator is most often due to the mountainous edge of a lunar sea ("sea" or mare, plural maria) being in sunlight while when the much lower plain of the sea itself is not The terminator on the Moon moves at an amazingly slow rate—only about 9 miles per hour, even at the Moon's equator. But multiply that number by 6 hours and you see that one-fourth of a day can indeed bring to light the noticeable start of a huge new lunar feature.
So the practitioner of this activity really should note the time of the lunar observation and calculate to the hour the exact age of the Moon. (Just check any almanac for the precise time of the last New Moon, and figure out how much time has elapsed between then and the observation.)
What other factors cause trouble for our goal of a perfect series of pictures that make up, once and for all, the entire panorama of the lunation? One such factor is libration. This phenomenon is explained and studied in Activity 6. You can ignore libration in your first attempt to draw the features throughout an entire lunation, but you will soon need to learn more about it.
The Moon offers very special sights when you can see it at less than 1½ days old or more than 28 days old These sights are discussed in Activity 12.
1. At what ages of the Moon do you first and last notice a surface feature? At which ages (if any) do you see one crescent point longer?
2. At what ages of the Moon do you see the most and the least jaggedness of the terminator? What specific features cause the jaggedness? (You may need to use optical aid or refer to Moon maps in books.)
Shades of Bright and Dark on the Moon
Make a chart showing levels of brightness (from brightest to darkest) on the naked-eye Moon. Do this at different phases, noticing the changes in each feature's or each area's brightness caused by the changing angle of illumination from the Sun. Rate the brightness of regions on a l-to-5 scale or even on a 1-to-10 scale (0 = shadow; 1 = darkest illuminated regions; 10 = brightest regions).
Most of us think of the naked-eye Moon as a bright disk with some dark markings. A careful look shows instead a marvelous range of subtle shadings—and brightenings (that is, areas somewhat brighter than the typically rather bright highlands of the Moon). How can we have overlooked some of these spectacular bright areas? They are only prominent when viewed at the best time of our day (near sunrise or sunset) and at the best time of the Moon's (near the Moon's noon—the phases when the Sun is high in the lunar sky over the areas).
Telescopic observers of the Moon refer to a scale of "degrees" of brightness ranging from 1 to 10 (with 0 for shadow). The floors of the craters Grimaldi and Riccioli are judged darkest of illuminated areas, rating a 1; parts of the crater Aristarchus are the brightest on the Moon, rating a 10.
But the naked-eye study of the Moon's gradations of brightness turns up some new information. The smaller features, visible in telescopes, blend to varying extents to produce quite different appearances of bright and dark. Looking at the Moon with the naked eye gives us new insight into the largest scale distribution of features—in this particular activity, the distribution of the brighter "lunarite" and darker "lunabase" with all that they may mean.
At certain lunar phases, the Sun shines high in the sky over particular young craters and over the bright dust that is strewn out from them in long streaks called rays. Whereas such a crater and its vicinity seem quite ordinary at other times, the high Sun lights up its ray system into startling prominence. The relatively small craters Stevinus and Byrgius do not receive much attention, even in many detailed telescopic guides to the Moon. But you will find that the two bright areas of their ray systems are virtually as prominent to the naked eye as the dark maria at the right times (Earth sunset with waxing gibbous Moon for the Stevinus area, Earth sunrise with waning gibbous Moon for the Byrgius area). The Stevinus region looks almost like a bright twin of the famous dark Mare Crisium somewhat north of it.
1. What bright areas other than those of Stevinus and Byrgius can you identify? At what age of the Moon is each most prominent?
2. What are the brightest and darkest features you see at each phase? Which of the maria seems darkest?
3. Where do you see the most numerous delicate patches of shading on the naked-eye Moon?
Patterns in the Distribution of Lunar Features
Organize the Moon's naked-eye features into as many patterns or systems as possible. Examine patterns and systems already known, but also invent at least a few of your own. Decide which are most useful for memorizing the Moon's layout and which may hold significance for our understanding of the Moon's nature.
Telescopic and spacecraft studies of the Moon have revealed many interesting patterns in the distribution of lunar features. Most remarkable is the extreme dissimilarity in the amount of maria on the near and far sides. Other patterns, maybe of coincidence rather than significance, include the three great hemisphere-spanning "crater-chains" on the Earth-facing side of the Moon and J.E. Spurr's "grid system" of faults and ridges. But what organizations of features might naked-eye study of the Moon reveal?
Naked-eye study lends itself to the recognition of very large-scale patterns. Our previous two activities can be helpful in identifying these patterns in the distribution of brightness or reflectivity (Activity 3) and in topography (Activity 2, noting the irregularities of the Moon's terminator at various phases caused by elevation differences).
Excerpted from Seeing the Sky by Fred Schaaf, Doug Myers. Copyright © 2012 Fred Schaaf. Excerpted by permission of Dover Publications, Inc..
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