The Music of the Spheres/Chapter 17

by Florence Armstrong Grondal

THE SOUTHERN PORTION OF THE MOON AT LAST QUARTER.
Photograph by Mount Wilson Observatory through the 100-inch Hooker telescope.

CHAPTER XVII

THE EARTH'S MOON

A Near View of a Satellite

Diameter—2163 miles

The moon's diameter is equal to one-fourth that of the earth. As viewed from a nearby planet—neighbor Venus, the planetoids, or neighbor Mars—the moon and the earth would appear as two close stars.

Both Jupiter and Saturn have moons larger than ours, but Jupiter, for instance, is over a thousand times larger than the earth, and Saturn ranks next to Jupiter. The moons of Mars are exceedingly tiny, and Mercury and Venus have none at all. Neptune has one moon, Uranus four, Jupiter nine and Saturn ten. Of all these moons, it is most natural for us to want complete information on the moon that lies nearest to us, the moon of the earth, which travels around us as we journey around the sun, at a distance of only 240,000 miles. With the aid of the greatest of telescopes, through which the accompanying photograph was taken, the moon can be brought within a clear range of 60 miles, and even closer. However, an opera-glass reduces the distance to one-half, and such a glass, or a very small telescope, is much better in every way for the amateur.

With the nearest planets millions of miles in the distance and the nearest stars millions of millions of miles away, the short distance of 240,000 miles to the moon seems almost no distance at all; in fact, the moon lies so close to us that it not only rolls through the shadow of the earth, but we may also see the various features, and even the shadows of those features, upon its surface.

The moon and the earth form a little system of their own and are bound together by a strong attraction.
EARTH-LIT NEW MOON.
Showing a slender crescent embracing the earth-lit night portion of the moon. Photograph through 40-inch refractor by Yerkes Observatory. This strong attraction, called the force of gravity, has resulted in a strange thing happening to both the earth and the moon, for the earth pulls so hard on the moon that it has long since destroyed the moon's axial rotation with reference to itself, and the moon pulls so hard on the earth that it moves the great fluid body of the ocean. Thus the moon has always the same face toward the earth for it cannot turn around and show us the other side, while the pulling of the earth by the moon causes the ocean's waters to rise and fall every 12 hours and 26 minutes, or twice a day. When the Greek and Roman travelers first ventured to the ocean, they could not understand why great stretches of beaches were left dry, and with equal regularity covered up; where did the water go, and why? It seemed a hopeless mystery to people who had lived around a landlocked sea where the tide, with few exceptions, rises but a few inches. When the sun and moon are in line with the earth they co-operate in their pulling and hauling at the sea. The moon is then either "new" or "full" and it is then that the highest tides occur.

"The silver Moon o'er briny seas presides,
And heaves huge ocean with alternate tides."
—Lucan's Pharsalia (Rowe's Trans.)

At exactly new moon (the moon is called new when it lies between the earth and sun), the moon comes above the horizon at the same time as the sun and sets with it. A few days after new moon we sometimes see what is called the old moon in the young moon's arms, the dimly lighted portion being illuminated by light reflected from the day side of the earth. This earthlight at the moon is at least 50 times greater than moonlight at the earth. Its somewhat ruddy color is caused by the sunlight having passed twice through our atmosphere. The bright crescent, which is the young moon's arms enclosing the old moon, is the sun-illuminated portion of the moon.

"I saw the new moon late yestere'en,
Wi' the auld moon in her arms."
—Old Scotch Ballad.

After "new moon" the moon appears a little to the east of the sun as a thin semicircle, the horns of which always point to the cast. The horns point to the east because the sun sets in the west and the illuminated part of the moon is on the side nearest to the sun, a fact sometimes forgotten by artists. The thin crescent of the moon gradually becomes broader as the moon moves away from the sun and in the course of four or five days increases to a semicircle.

"The Moon her monthly course had now begun,
And with increasing horns forsook the Sun;"
—Lucan's Pharsalia.

The semicircle then increases to an oval and by the time the moon is behind the earth with the earth between it and the sun, the oval has increased to a full circle. Each day the moon rises about 50 minutes later than on the preceding day and in the course of a month changes from a crescent—a quarter—a full shining face, to a quarter, a crescent and again a slender new moon. It has now traveled completely around the earth. Since the moon has always the same face toward the earth (if one will walk around a table with the eyes always toward the center, this journey will be well illustrated), it will be seen by us in all its phases before it completes a day and night, or, in other words, although the earth turns all sides to the sun in 24 hours, the moon must travel all the way around the earth, which takes almost 30 of our days, before it completes a day and night. This means a continuous stretch of 14¾ times 24 hours of daylight, which is about equal to 29 of our days, of daylight, followed by 14¾ times 24 hours of night, when the sun is below the horizon. It would be a strange experience to travel to this country where the day and night each last half a month.

If a journey to the weird, rough country of the moon were possible, one would experience even greater surprises than the length of the day and the night, for the sky, as seen from this airless sphere, would be strangely different from anything that we are accustomed to. Here the sun hangs like a big gold jewel edged with a tiny fringe of scarlet, the pearly petals of its delicate corona far outstretched against the star-filled sky. The brilliant ball of the earth, which would appear almost 13 times as large in area as the moon, rests immovable and ever present, its great globe turning completely around every 24 hours. Also, as the moon does to us, it would exhibit to an observer on the moon an interesting variety of phases, mammoth crescents, quarters and disks. Such a sky!—a huge white planet, a flower-like sun and multitudes of unwinking stars. And even more, for stretched from horizon to horizon of that black dome, even though the sun shines brightly, is the misty arch of the Milky Way glowing in a glory quite unknown to mortals on earth. All this would happen for the simple reason that the moon has no appreciable amount of atmosphere to diffuse the sun's rays, make the sky seem blue and render the stars invisible. This would also unveil many of the smaller stars that are not seen here without optical aid—and every star would "burn" but none would "twinkle." In the morning of that long lunar day of 354 hours, the yellow face of the sun would slowly push its gayly bedecked head above the black rim of the horizon unheralded by a dawn; neither would twilight trail behind it when it sank beyond our sight after the long day was done, for these phenomena are due entirely to the light-reflecting and refracting power of an atmosphere.

The absence of an atmosphere on the moon, or at least the presence of only a very rare one, is proved by the suddenness with which a star will disappear when the moon passes between the star and us. Also surface features would not otherwise stand forth with such clearness nor shadows be so sharp and inky-black. When the moon passes in front of the sun at the time of an eclipse, its outline shows perfectly sharp against the yellow disk, even showing, like jagged saw-teeth, the "mountains of eternal light" which tower near the southern pole. When Venus makes a transit across the sun its whole edge is illuminated instead of being merely sharply defined, for Venus is enclosed in a wrapping of atmosphere.

The spectroscope has proved that there is no water on the moon. If it is airless and waterless it consequently must be lifeless. But hold, a moment. Perhaps we go too fast with our this and that and therefores, for here come reports from Pickering, one of our most tireless observers, who declares that there is a considerable amount of dampness still lodged in the deep cavities of the circular mountains, and that this dampness ascends in mists when the sun rises. These mists do not seem to float freely but cling to ridges and higher levels and later dissolve there. The outlines of these cavities are slightly blurred until the mist is banished.

 

The Gray Land on the Moon

The curious gray areas which extend over two-fifths of the surface of the moon are the most conspicuous features to be seen with the unaided eye. These areas are comparatively smooth and level and invariably lower than the surrounding country.
THE GRAY LAND OF THE MOON. The rest of the moon is extremely rough, the southern hemisphere in particular presenting the most singular circumambient scenery of thousands of round pits fringed by lofty ramparts of circular mountains.

With the aid of a small telescope, the grayish spots on the moon somewhat resemble seas as they might appear at a great distance. Thus old astronomers, believing that they might at least be ancient sea-bottoms from which the water had evaporated, named them accordingly—although rather romantically. These names are still retained, disguised in Latin on the map of the moon and referred to in Latin by present day astronomers. Translated, however, they add a quaint glamour to the Gray Land and the eye wanders interestedly over "The Sea of Showers," "The Sea of Tranquillity," "The Sea of Clouds," "The Sea of Nectar," "The Gulf of Dews," "The Ocean of Storms," "The Bay of Rainbows," "The Marsh of Sleep" or "The Lake of Dreams."

Since the days of Galileo, improved telescopes and continued research have constantly increased our knowledge of the moon. The curious so-called "seas" are not, seemingly, old sea-bottoms; they are dark plains on the lunar surface much more likely to be great lava plains than sea-basins, according to the evidence that has been collected. Many scientists now regard these areas as a part of the surface where the thin crust became weakened and succumbed in an early stage of the moon's development; the lava then welled forth from the interior and flowed out in great molten sheets over the surrounding country. Since over 100,000 craters have also been observed on this side of the moon, all in fairly good condition, and since there has been no weathering, it is sometimes said that the moon presents a picture of the volcanic age petrified.

The gray patches are best seen when the moon is full. Under a telescope these areas appear darker with occasional traces of yellow or green tints. The colors may be due to some form of vegetation or masses of colored volcanic material.

 

Moon Mountains

The most prominent mountains on the moon are the Apennines. These extend in a continuous curve for 450 miles a little north of the center of the moon, forming an impressive "shoreline" for Mare Imbrium, the "Sea of Showers." Running northward, just above the Apennines, is the Caucasus range, and east of the Caucasus, lie the Alps. The Alps and Caucasus mountains are far less conspicuous than the Apennines, for not only do these latter mountains reach into a higher altitude but they also are crowned along their 450 miles with some 3000 steep and rugged peaks. The Alps are noted for a remarkable flat-bottomed valley which cuts in a straight line through these mountains for a distance of 80 miles. This valley is from 3½ to 6 miles wide and at least 11,000 feet in depth. Such a long, level strip bordered by gigantic mountains is as unique a formation as the mammoth craters with the towering rims.

The Leibnitz and the Doerfel mountains are on the southern part of the moon near the pole. These mountains are so situated that the light of the sun always shines from their summits, and even during an eclipse they are visible in profile. Some of the peaks on these "mountains of eternal light" are very high, their needle-like points reaching upward for 26,000 to 27,000 feet. It has been estimated that if our mountains on earth were comparatively as high as the mountains on the moon, our earthly mountains would rise to the height of about 15 miles! The height and sharpness of the moon mountains is due partly to the low surface gravity, which is only about ¹⁄₆th that of the earth, and to the fact that there is no leveling influence such as is caused by frosts and rains.

Isolated mountains rising up like a huge lump, or a pen point, from a flat surface, are commonly found on the moon. These often stand several miles high and are particularly interesting as seen in the northern hemisphere of the moon, when the long shadow of such a solitary mountain stretches in inky-blackness against the smooth gray plain. Several of these may be seen in Mare Imbrium just above the crater Plato, the loftiest of which is Pico, rising about 8000 feet. Isolated mountains are very frequently found in the center of craters. This would seem a strange formation close at hand—a large crater, or a ringed plain, miles in extent, with a tall slender mountain standing in its center.

The oddest and most common mountains, however, are the circular mountains which surround a plain or a large sunken cavity, like a mighty wall. Such mountains appear on almost every part of the moon's surface and in some places they appear in such profusion that their rings frequently touch and even overlap one another. The peaks of the circular mountains are often two or three miles in height and their shadows sometimes cover a large portion of the plain which they surround. These shadows are continually varying, for during the increase of the moon they are thrown in one direction and during the decrease, in the direction exactly opposite. During full moon they disappear altogether and the plains are filled with light, but lunar scenes are much more interesting when the sunlighted portions are accentuated by the long, queer shadows, for these shadows are black and clear-edged and stand out like silhouettes done in ebony, even at this distance of 240,000 miles. Because they have no light reflected into them from an atmosphere, these shadows are actually so densely black that if one stepped into such a shadow he would instantly be blotted from view.

On account of the vividness of the shadows, moon-mountains are best observed during the first and last quarter of the moon. A good time to begin observing with a glass is when the moon is a narrow crescent in the western sky. The most prominent object in the narrow crescent is the Mare Crisium or the Sea of Crises, 380 miles long. South of Mare Crisium is Mare Fecunditatis, the Sea of Fecundity and Mare Nectaris, the Sea of Nectar. When these three "Seas" are visible with Mare Tranquillitatis just coming into view, the moon is between 5 and 6 days old, that is, 5 or 6 days after new moon. The Leibnitz mountains may then be seen near the southern pole. Watch carefully evening after evening as the light creeps over new regions and brings them into view. The best place to select for observation is the boundary line between the illuminated and unilluminated portions, called the "terminator," for the shadows that are cast make the irregular features stand out more distinctly and bring out the individual beauties of the lunar scenes. Since we are looking down upon the moon, a mountain peak, at this distance, resembles a point of light, and a mountain range whose base is still in the shadow, but whose peaks are lighted by the rays of the sun, a straight or curved row of lights. Besides beautifying the country of the moon and revealing to us the peculiar sharpness of the mountain peaks, the shadows have proved of great practical value to astronomers, for by measuring the length of their shadows, the heights of the mountains are calculated.

 

The Craters on the Moon

Craters are so numerous in some localities on the moon that one might walk for hundreds and hundreds of miles and step on nothing but crater rims. On the earth a crater is usually a rare object situated on the top of a mountain, but on the moon they are scattered all over the lunar plains, a state of landscape which permits of no short cuts. If, however, in the dim, distant past, life had ever developed on the moon, a moon-being would have been ¹⁄₆th as heavy and able to jump six times as high as an earth-being for the surface gravity of the moon is only about ¹⁄₆th that of the earth. Thus if a lunarian had no other mode of transportation, he might at least have been able to surmount many of the minor obstacles by jumping over them.

A map of the moon resembles a big mud-ball with all the little pebbles picked out. A crater only six miles in diameter
NORTHERN PORTION OF THE MOON.
Note the "Sea of Showers," the "Bay of Rainbows," Copernicus and Plato, the three craters north of the lunar "Apennines," the Caucasus Range, the Alps and the Valley of the Alps. Refer to the maps. Photograph by Mount Wilson Observatory through the 100-inch Hooker telescope. may be easily seen in a small telescope, although there are thousands less than a mile in diameter which may be seen in a large telescope. Craters from 16 miles to 60 miles in diameter are common and a few even measure as much as 100 to 150 miles. These craters have deeply sunken floors and are usually surrounded by gigantic mountain walls which rise like a fringe of rock above the general level of the region outside. Near the center of many of the craters with the largest brims, one and sometimes two or three sharp-peaked mountains tower, casting a long, weird shadow over the crater's floor. The surrounding brim is almost invariaably higher than the central mountain, yet many of these central mountains are a mile or so high.

Probably the deepest of the moon's craters is Theophilus, on the edge of the circular "sea" Mare Nectaris, and it is best seen while the moon is still a slender crescent. The encircling walls of this crater rise from 14,000 to 18,000 feet above its floor, the floor itself being 10,000 feet below the general level of the moon outside the wall. The walls of this amazing cavity, in the main, slope gently to the outer plain but rise very steeply from the depths of the inner floor which measures 64 miles across. Three lofty cone-shaped mountains rest in the center of this floor, their sharp, black shadows stretching to the left. One of these mountains rises 6000 feet, yet its summit is some 4000 feet lower than the level of the outside plain! Imagine standing on the rim of Theophilus and watching the darkness flow into its huge bowl until the mountain peaks in the center look like sunny isles in a dense, black sea! This is a real crater, for the word "crater" means "cup."

Most of the large craters on the moon, however, are not so "cup-like," but, to pursue the simile still farther, are more like platters and saucers and individual butter plates, for their diameters range from five times to more than fifty times their depth.
THE LUNAR CRATER COPERNICUS, FIFTY MILES IN DIAMETER.
Photograph by Mount Wilson Observatory through the 100-inch Hooker telescope. Chacornac mentions a walled-plain called Schickard, on the southeast portion of the moon, which is 134 miles in breadth, and, although encircled by a mountain wall which in one place is nearly 10,000 feet high, a spectator centrally located on the floor of the crater would think himself on a boundless desert, for this encircling wall, due to the curvature of the surface of the moon, would lie entirely beneath his horizon.

One of the most magnificent craters on the moon is the great crater Copernicus. Copernicus lies in an isolated position a little southeast of the range of the Apennines, on the darkened space called Mare Imbrium. Being not far from the center of the lunar disk, it is an easy object for the eye to find and to use as a guiding point from which to locate other craters. A circular range of mountains much terraced on the inner side surrounds the great plain of Copernicus, while in the very center of its 56 miles of floor stands a solitary mountain with an altitude of 11,000 feet. It is believed by many astronomers that such terracing as is shown in the interior of Copernicus, is mainly due "to the repeated alternate rise, partial congelation, and subsequent retreat" of a great flow of lava which rose up from the floor during a period long past. A number of other craters also show such terracing. The bright appearance of Copernicus is caused by an interesting system of bright streaks which radiate from the circular rim of its mountains. Tycho, near the southern pole, has also such a system of radiating streaks, but so enlarged and intensified are the rays of Tycho's system that they run for hundreds of miles and dominate the whole scene when the moon is full.

Aristarchus, northeast of Copernicus, is the brightest single point on the moon. The peak in the center of its 29 miles of brilliant interior shines so brightly that it has often been seen on the dark side after new moon. Indeed, when Sir William Herschel first observed it through a telescope he mistook it for a volcano in action. The walls of Aristarchus are also conspicuously terraced and rise about 4000 feet above its floor.

South of Aristarchus and east of Copernicus may be seen Kepler, a 22-mile crater noted for its extended system of glistening streaks. Its surrounding wall, like that of Tycho and Copernicus, seems to be covered with the same shining substance. The wall of Kepler is very low but its crater is about 10,000 feet below the exterior plain.

Southeast of the two bright craters, Kepler and Copernicus, and almost at the edge of the moon, lies the darkest and also one of the largest of the moon's craters, or walled-plains, as the large craters are now more frequently called. This huge crater, Grimaldi, extends north and south for a distance of 148 miles and covers an area of 14,000 square miles. It is very noticeably dark in comparison to all surrounding objects.

Gassendi, a little southwest of Grimaldi, contains a great variety of detail both on its rim and on its floor. Its plain is 54 miles in diameter and includes a number of central peaks.

Again starting from Copernicus and letting the eye travel northwest along the Apennine mountains almost to the Caucasus, one arrives at a group of three beautiful craters which rest conspicuously on the level floor of Mare Imbrium. The largest of these craters is named Archimedes after the most celebrated of ancient mathematicians. It is to be noted that lunar craters have been given the names of great men—Copernicus, Tycho, Kepler, great astronomers; Aristotle, Plato, great philosophers,—a very fine recognition of scholarly worth. Archimedes, 52 miles in diameter, has no interior mountain, but the shadows of its tall rim are imposing as they fall across its plain.
Some Interesting Craters on the Moon. Aristillus is about 34 miles in diameter, and 10,000 feet deep. In the center of this deep cavity rises a massive many-peaked mountain whose base is often lost in the darkness of the crater's shadows. Autolycus, south of Aristillus and somewhat smaller, is 23 miles in diameter with a floor somewhat depressed below the country which surrounds it.

On the "shore-line" of Mare Imbrium, north of Archimedes, Aristillus and Autolycus, and not far from the "Valley of the Alps," lies Plato, one of the most easily found craters on the moon. Plato is a very huge oblong crater, 60 miles in diameter and containing an area of 2,700 miles. Its floor is exceptionally dark and flat and forms a striking contrast with the bright border of mountains which surround it. Quite a number of minute craterlets have been distinguished on its floor.

The southern hemisphere of the moon, particularly as one approaches the region about the southern pole, is pitted with such a bewildering number of both large and small craters that astronomers say that it is relief to the eye to study the comparatively smooth, shaded portions of the globe after having gazed for any length of time at the crowded mass of detail in this wild and rugged region. The two craters in the southern hemisphere that are perhaps the easiest for the amateur to locate are Clavius, an enormous crater near the mountains at the southern pole, and Tycho, just below it, with its wonderful streaks, or rays.

Clavius is a crater of gorgeous depth and has an area of 16,500 miles. Around its walls and on its floor are many secondary craters, while from its center rise tall mountain peaks, one of which reaches 24,000 feet above the bottom of one of its included craters.

Tycho is called "The Metropolitan Crater of the Moon," because its brilliant rays stand forth so prominently when the moon is full that all other craters are lost in obscurity. This magnificent crater measures 54 miles from rim to rim with its floor over 3 miles below the highest ridge of the massive mountains which surround it. These mountains are diversified by a series of terraces on their interior slopes while a peak 6000 feet in height rises in the center of its floor. The ray system of Tycho is wholly invisible at the time the sun rises or sets, which is the time when most of the objects on the moon are seen at their best. However when the sun has attained an altitude of 30 degrees, they mysteriously make their appearance, extending for hundreds and some even for several thousands of miles. These rays are never irregular because of interposed obstructions but stretch straight and true in a most remarkable way over mountains, pits and plains. So numerous are these bright rays about the brilliant crater of Tycho that at full moon they may even be located as a patch of light to the unaided eye, although an opera-glass, which brings the moon down to 120,000 miles instead of 240,000, greatly improves the seeing. William H. Pickering, in his book on "Mars," gives some of the latest information concerning Tycho:

"The white radiating lines or bands are seen to be due to numerous minute craterlets each giving out a triangular white streamer producing the general effect of a white band. It is probable that this observed regular distribution of the craterlets is due to their lying along invisible cracks radiating from the inner crater. It is much the same as the great volcanoes of the Andes, which stretch in a straight line for over 2000 miles between Peru and the strait of Magellan. The Alaskan volcanoes lie upon a uniformly curved line of nearly equal length. . . . This line formation is generally considered by geologists to be due to subterranean lines of weakness or cracks in the earth's crust."

The moon is considered to be a matured globe, evolutionally old, and it has even been referred to as being nearly dead. Lacking, air, water, life and even the diversion of sound, it surely is at least in but a semi-conscious stage, as worlds go. Mars is also called an old world, but Mars has air, its snow-caps melt in sheets of water, and we view the colors which mark its seasons, across a distance of 50,000,000 miles. The moon looks as it is, seared and old. There have been many theories advanced to explain the abundance of its craters and their peculiar construction, but some modified form of the volcanic action is the only tenable one. No one theory has as yet met with universal approval.

A great telescope with a 100-inch lens has lately been erected at Mount Wilson, near Pasadena, California. The telescopes which have mirrors, like this one, are called reflecting telescopes. The largest reflecting telescope before the one now mounted on Mount Wilson became available, was the splendid instrument at Little Saanich, Victoria, B.C., which has a 72-inch mirror. A telescope in which the light rays pass through a series of lenses fixed in a long tube pointed directly at the object is called a refracting telescope. The Yerkes telescope at the University of Chicago is an unusually fine 40-inch refractor.

With the greatest of telescopes the moon may be brought to within 60 miles of the earth and a lunar object a mile or so in diameter is easily discernible. The imagination is entranced. All success to astronomers!