In its action, the Moon is irregular
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In its action, as in most other things, the Moon is irregular. The north pole is tipped a little toward us at times, and occasionally we can see a bit farther to the east and west along its surfaces. Altogether about six-tenths of the Moon has been seen. In general the surface resembles the surface of the Earth. And as there is a science of geography for the Earth, there is likewise selenography for the Moon. This science has mapped out the mountains and remains of oceans, the craters of presumably extinct volcanoes, larger but lower than ours. It has given names to the most prominent features. No explorer who reaches the Moon will have the pleasure of naming discoveries for himself on this side of the Moon. He can have a picnic on the other side. But on all the visible area of the planet there seems to be no stir of life, no water in the oceans, no movement in the craters.
The Moon occults the stars which pass behind its back. We see them shining clearly as they approach the surface of the Moon; the next instant they are gone. There is no period when they grow less bright. In less than an hour they reappear as suddenly as they went. Had the Moon any fuzzy atmospheric edges, we should see them grow dim and fade before they finally vanished and their reappearance would similarly be gradual, as if emerging from a fog. Nothing of that sort happens. Their light is as clear, when they are near the edge of the Moon, as when they are far away. From this clarity astronomers have concluded that the Moon has no atmosphere, and absence of atmosphere means absence of life.
Without air to temper the heat of the Sun and the cold of interstellar space, the temperatures of day and night upon the Moon form a violent contrast. We experience no similar differences on Earth. Our most violent extremes, "from Greenland's icy mountains to India's coral strand," are all rather cozy by comparison with the dark and light portions of the Moon, and the Moon has no intermediate regions.
Lack of atmosphere produces other troubles, quite apart from temperature, and from the difficulty in breathing without air. Meteors have nothing to break their fall. Astronomers estimate that millions of meteors enter the Earth's atmosphere every day. The friction of the air rubbing against them as they fall produces an intense heat, and at night we see them as shooting stars. The vast number of small meteors burn up en route. Only a very few are great enough to reach the Earth; but the effects of these few are sufficient indications of meteoric effects. Were we not wrapped in our protective atmosphere, our Earth might be pitted with craters. There is no such protection on the Moon.
In a hotel in the Redwoods of Northern California, there is a sign, "Do not park your car under the trees; even a small twig falling from the height of two hundred feet may puncture the top or the cushions." Meteors fall from an indefinite height, and the additional danger involved is a problem not worth solving by personal experience.
The Moon's movements around the Earth are rapid and (as usual) irregular. It changes its position among the stars, going counterclockwise at the rate of about its own diameter (one-half degree) per hour and occults stars. Occultation of stars by the Moon forms the best available method for determining absolute longitude. The method also has the advantage of simplicity in the field. The observer needs a good portable telescope in addition to his usual equipment of surveyor's transit or sextant, watch and the inevitable Nautical Almanac in which occultations are listed. He finds his latitude and local time by altitude observations and then takes the time of the occultation of the star, or of its reappearance. With these data, and considerable calculation, he can deduce his longitude, without knowing with what meridian his watch is supposed to be keeping time. If you wish to make yourself really unpopular, halt your trip for a whole day to observe an occultation and then find that you have made an error in prediction and that it does not take place.
This movement of the Moon among the stars was among the earliest phenomena noticed. The Sun takes a year to complete its circuit, and the path is divided into twelve familiar signs of the zodiac; but the Moon takes only a month, and the long astronomical poem of the Hindu Vedas refers to a lunar day as the time from moonrise to moonrise, twenty-seven such days during a month, and twenty-seven divisions to the lunar zodiac, each represented by a constellation. These were the "alighting-places" of the Moon in the Arabic word, and the "wives of the Moon" in the Sanskrit--twenty-seven wives with whom the Moon could not play favorites. He was forced to divide his time equally among them. The Copts knew of the division, and the Chinese made a similar zodiac in very early times; all the epic poems of India are full of the story, but no hint of it ever seems to have reached Greece, and when Greek influence struck India in the third century A. D., the lunar zodiac bowed before the western Sun and gave way, as the Moon has always given way in the end.
The irregularity of the Moon's movements is not only usual, but also quite noticeable, with the result that, while the Moon's light may be ideal for lovers, mariners and astronomers will try to find their position by any other stellar body. Only if no other star or planet is available will they turn, with a sigh, to the Moon. They can easily determine the altitude of the Moon's upper or lower edge--but then the fun begins.
The abridged Nautical Almanac gives the position of the Moon for every two hours, as it would be seen from the center of the Earth; likewise the Moon's semi-diameter (which is the astronomer's way of saying radius--old habits die hard) and its horizontal parallax.
The computer has to find the true position at the moment of observation, which is never by any chance that at the even hour given in the table, and he has to calculate the change. He then has to "augment for the semi-diameter" (it seems impossible to get away from that word) to what it would be if seen from the surface of the Earth instead of from the center, and then he has to reduce the horizontal parallax to what it should be for the altitude observed, so that he can find what the observation would have been if it had been taken at the center of the Earth--and then he is just beginning his work and is fully convinced that lovers should be allowed to keep their Moon and that it should not be allowed to become a general nuisance and pest to innocent mariners.
If mariners need the time only roughly--say within thirty minutes--the situation is quite different. With a little practice and consideration they can determine it whenever the Moon is above the horizon. The appearance of the Moon gives away her age at a single glance. There are seven days between the new Moon and, the first quarter when one half of the Moon's surface is visible. At quarter, the Moon is ninety degrees from the Sun and its position rather than its altitude gives the clue. The Moon is near the south at six o'clock in the evening near the time of the equinoxes when the Sun has just set.
The full Moon rises as the Sun sets, and as it wanes it rises about one hour later every evening. A crescent Moon in the east is a waning Moon but, if you see it, you will know that you have slept badly or have been out too late. It is a sure sign. The horns are always pointed away from the Sun, not toward it, as in some pictures.
The age of the Moon can be told in another way. Take a hand mirror and hold it so that the surface is pointing to just below the Moon and the image of the Moon is reflected back and forth between the upper and lower surfaces of the glass. Then count the number of reflections that it gives. For each day of its age, the Moon is supposed to give one reflection. In test it seemed to give one too few. There were only four reflections when the Moon was five days old. The number of the cocktails taken prior to the test is not to be included.
The light of the Moon has been used for photography but it is not satisfactory. The exposures take many minutes, while views exposed in sunlight are taken in hundredths of a second. "Moonlight photographs" so much vaunted in photographers' windows are much more easily taken in sunlight by anyone who knows the simple trick, and the results are far more pleasing than if they had been taken the "truthful way."
Moonlight has a commercial value, in the same way that daylight-saving time has. In rural England a rule was made that the street lights in small villages and hamlets should not be lit in fine weather for two nights before the Moon was full. As it waned the rising time was soon too late in winter to make it serviceable for "light-saving" purposes.
The Moon is of service to mankind in other ways than providing light at night. It is the major factor in creating the tides. This is entirely due to the force of gravity (which as every schoolboy knows is directly proportional to the masses of the two attracting bodies and inversely proportional to the square of the distance between them).
The greatest attractive effort of the Moon's mass is just on the surface directly below it, next on the main body of the Earth, and least on the surface of the Earth on the other side from the Moon.
The most fluid part of the Earth is the water in the oceans, so the Moon tends to raise the water just under it with the greatest force and it also pulls the solid part of the Earth away from the ocean on the opposite side.
As the Earth rotates on its axis, one part after another is subjected to this maximum pull and the tide flows, following under the Moon. Various features of the Earth, shallow water and narrow passages, retard the flowing tide, and make it lag.
The effect of the Sun must also be considered. It acts in exactly the same way, but, in spite of its greater mass, its much greater distance makes its effects much smaller than those of the Moon, as the tide-raising forces are inversely proportional to the cube of the distance.
The greatest tides are to be expected when the attractive forces of the Sun and Moon act together. This happens at new Moon, and also with the same effect at full Moon, for each of our luminaries is then trying to pull the water away from the Earth on opposite sides. These are called the "spring tides."
The smallest tides are when the attracting forces of the Sun and Moon are at right angles to each other, and tend to counteract. The results are the "neap tides" which have the least changes in level.
Very ingenious machines, combining mechanically the many harmonic motions which enter into the creation of tides, have been invented, largely due to the genius of Lord Kelvin. They can be used to predict the tides at various ports for many years in advance. One of the factors involved is the "lunar day" the period during which the Earth makes a complete revolution on its axis in regard to the Moon.
Absolute longitude and determination of tides are two uses for the Moon which have remained. The rest have gradually seeped away. The old lunar month has been replaced by an artificial month that corresponds to the solar calendar, and, while in parts of India, astronomers still divide the month into the "light half" and the "dark half," the western world hardly knows which is which. Astronomers today know a good deal more about the Moon's motion than was ever known in ancient times, yet the shepherd on the hills three thousand years ago probably knew far better when the Moon would be full than the average man in an office building today. Such artificial lights as early man possessed were powerless against the vast dark of a moonless night. When the Moon was new, no traveler dared venture forth, no sheep could be herded, no lost trails found until morning. Artificial light has done away with that.
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