Recreations in Astronomy Part 10

Let us lift ourselves up a thousand miles from the earth. We see it as a ball hung upon nothing in empty s.p.a.ce. As the drop of falling water gathers itself [Page 142] into a sphere by its own inherent attraction, so the earth gathers itself into a ball. Noticing closely, we see forms of continents outlined in bright relief, and oceanic forms in darker surfaces. We see that its axis of revolution is nearly perpendicular to the line of light from the sun. One-half is always dark. The sunrise greets a new thousand miles every hour; the glories of [Page 143] the sunset follow over an equal s.p.a.ce, 180 behind. We are glad that the darkness never overtakes the morning.

[Ill.u.s.tration: Fig. 54.--Earth and Moon in s.p.a.ce.]

_The Aurora Borealis._

While east and west are gorgeous with sunrise and sunset, the north is often more glorious with its aurora borealis. We remember that all worlds have weird and inexplicable appendages. They are not limited to their solid surfaces or their circ.u.mambient air. The sun has its fiery flames, corona, zodiacal light, and perhaps a finer kind of atmosphere than we know. The earth is [Page 144]

not without its inexplicable surroundings. It has not only its gorgeous eastern sunrise, its glorious western sunset, high above its surface in the clouds, but it also has its more glorious northern dawn far above its clouds and air. The realm of this royal splendor is as yet an unconquered world waiting for its Alexander. There are certain observable facts, viz., it prevails mostly near the arctic circle rather than the pole; it takes on various forms--cloud-like, arched, straight; it streams like banners, waves like curtains in the wind, is inconstant; is either the cause or result of electric disturbance; it is often from four hundred to six hundred miles above the earth, while our air cannot be over one hundred miles.

It almost seems like a revelation to human eyes of those vast, changeable, panoramic pictures by which the inhabitants of heaven are taught.

[Ill.u.s.tration: Fig. 55.--The Aurora as Waving Curtains.]

Investigation has discovered far more mysteries than it has explained.

It is possible that the same cause that produces sun-spots produces aurora in all s.p.a.ce, visible in all worlds. If so, we shall see more abundant auroras at the next maximum of sun-spot, between 1880-84.

_The Delicate Balance of Forces._

A soap-bubble in the wind could hardly be more flexible in form and sensitive to influence than is the earth. On the morning of May 9th, 1876, the earth's crust at Peru gave a few great throbs upward, by the action of expansive gases within. The sea fled, and returned in great waves as the land rose and fell. Then these waves fled away over the great mobile surface, and in less than five hours they had covered a s.p.a.ce equal to half of Europe. The waves ran out to the Sandwich Islands, six [Page 145] thousand miles, at the rate of five hundred miles an hour, and arrived there thirty feet high. They not only sped on in straight radial lines, but, having run up the coast to California, were deflected away into the former series of waves, making the most complex undulations.

Similar beats of the great heart of the earth have sent its pulses as widely and rapidly on previous occasions.

The figure of the earth, even on the ocean, is irregular, in consequence of the greater preponderance of land--and hence greater density--in the northern hemisphere. These irregularities are often very perplexing in making exact geodetic measurements. The tendency of matter to fly from the centre by reason of revolution causes the equatorial diameter to be twenty-six, miles longer than the polar one. By this force the Mississippi River is enabled to run up a hill nearly three miles high at a very rapid rate. Its mouth is that distance farther from the centre of the earth than its source, when but for this rotation both points would be equally distant.

If the water became more dense, or if the world were to revolve faster, the oceans would rush to the equator, burying the tallest mountains and leaving polar regions bare. If the water should become lighter in an infinitesimal degree, or the world rotate more slowly, the poles would be submerged and the equator become an arid waste.

No balance, turning to 1/1000 of a grain, is more delicate than the poise of forces on the world. Laplace has given us proof that the period of the earth's axial rotation has not changed 1/100 of a second of time in two thousand years.

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_Tides._

But there is an outside influence that is constantly acting upon the earth, and to which it constantly responds. Two hundred and forty thousand miles from the earth is the moon, having 1/81 the ma.s.s of the world. Its attractive influence on the earth causes the movable and nearer portions to hurry away from the more stable and distant, and heap themselves up on that part of the earth nearest the moon. Gravitation is inversely as the square of the distance; hence the water on the surface of the earth is attracted more than the body of the earth, some parts of which are eight thousand miles farther off; hence the water rises on the side next the moon. But the earth, as a whole, is nearer the moon than the water on the opposite side, and being drawn more strongly, is taken away from the water, leaving it heaped up also on the side opposite to the moon.

A subsidiary cause of tides is found in the revolution of the earth and moon about their common centre of gravity. Revolution about an axis through the centre of a sphere enlarges the equator by centrifugal force. Revolution about an axis touching the surface of a flexible globe converts it into an egg-shaped body, with the longer axis perpendicular to the axis of revolution. In Fig. 56 the point of revolution is seen at the centre of gravity at G; hence, in the revolution of earth and moon as one, a strong centrifugal force is caused at D, and a less one at C. This gives greater height to the tides than the attraction of the moon alone could produce.

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[Ill.u.s.tration: Fig. 56.]

If the earth had no axial revolution, the attractive point where the tide rises would be carried around the earth once in twenty-seven days by the moon's revolution about the earth. But since the earth revolves on its axis, it presents a new section to the moon's attraction every hour. If the moon were stationary, that would bring two high tides in exactly twenty-four hours; but as the moon goes forward, we need nearly twenty-five hours for two tides.

The attractive influence of the sun also gives us a tide four-tenths as great as that of the moon. When these two influences of the sun and moon combine, as they do, in conjunction--when both bodies are on one side of the earth; or in opposition, sun and moon being on opposite sides of the earth--we have spring or increased tides.

When the moon is in its first or third quarter, _i. e._, when a line from the moon to the earth makes a right angle with one from the sun to the earth, these influences antagonize one another, and we have the neap or low tides.

It is easy to see that if, when the moon was drawing its usual tide, the sun drew four-tenths of the water in a tide at right angles with it, the moon's tide must be by so much lower. Because of the inertia of the water [Page 148] it does not yield instantly to the moon's influence, and the crest of the tide is some hours behind the advancing moon.

The amount of tide in various places is affected by almost innumerable influences, as distance of moon at its apogee or perigee; its position north, south, or at the equator; distance of earth from sun at perihelion and aphelion; the position of islands; the trend of continents, etc. All eastern sh.o.r.es have far greater tides than western. As the earth rolls to the east it leaves the tide-crest under the moon to impinge on eastern sh.o.r.es, hence the tides of from seventy-five to one hundred feet in the Bay of Fundy. Lakes and most seas are too small to have perceptible tides. The spring-tides in the Mediterranean Sea are only about three inches.

This constant ebb and flow of the great sea is a grand provision for its purification. Even the wind is sent to the sea to be cleansed.

The sea washes every sh.o.r.e, purifies every cove, bay, and river twice every twenty-four hours. All putrescible matter liable to breed a pestilence is carried far from sh.o.r.e and sunk under fathoms of the never-stagnant sea. The distant moon lends its mighty power to carry the burdens of commerce. She takes all the loads that can be floated on her flowing tides, and cheerfully carries them in opposite directions in successive journeys.

It must be conceded that the profoundest study has not mastered the whole philosophy of tides. There are certain facts which are apparent, but for an explanation of their true theory such men as Laplace, Newton, and Airy have labored in vain. There are plenty of other worlds still to conquer.

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[Ill.u.s.tration: Fig. 57.--Lunar Day.]

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THE MOON.

New moon, [Symbol]; first quarter, [Symbol]; full moon, [Symbol]; last quarter, [Symbol].

EXTREME DISTANCE FROM THE EARTH, 259,600 MILES; LEAST, 221,000 MILES; MEAN, 240,000 MILES. DIAMETER, 2164.6 MILES [2153, LOCKYER].

REVOLUTION ABOUT THE EARTH, 29-1/2 DAYS. AXIAL REVOLUTION, SAME TIME.

When the astronomer Herschel was observing the southern sky from the Cape of Good Hope, the most clever hoax was perpetrated that ever was palmed upon a credulous public. Some new and wonderful instruments were carefully described as having been used by that astronomer, whereby he was enabled to bring the moon so close that he could see thereon trees, houses, animals, and men-like human beings. He could even discern their movements, and gestures that indicated a peaceful race. The extent of the hoax will be perceived when it is stated that no telescope that we are now able to make reveals the moon more clearly than it would appear to the naked eye if it was one hundred or one hundred and fifty miles away.

The distance at which a man can be seen by the unaided eye varies according to circ.u.mstances of position, background, light, and eye, but it is much inside of five miles.

Since, however, the moon is our nearest neighbor, a member of our own family in fact, it is a most interesting object of study.

A glance at its familiar face reveals its unequal illumination.

All ages and races have seen a man in the moon. All lovers have sworn by its constancy, and only part of them have kept their oaths.

Every twenty-nine or thirty days we see a silver crescent in the west, and are glad if it comes over the right shoulder--so [Page 152] much tribute does habit pay to superst.i.tion. The next night it is thirteen degrees farther east from the sun. We note the stars it occults, or pa.s.ses by, and leaves behind as it broadens its disk, till it rises full-orbed in the east when the sun sinks in the west.

It is easy to see that the moon goes around the earth from west to east. Afterward it rises later and smaller each night, till at length, lost from sight, it rises about the same time as the sun, and soon becomes the welcome crescent new moon again.

The same peculiarities are always evident in the visible face of the moon; hence we know that it always presents the same side to the earth. Obviously it must make just one axial to one orbital revolution. Hold any body before you at arm's-length, revolve it one-quarter around you until exactly overhead. If it has not revolved on an axis between the hands, another quarter of the surface is visible; but if in going up it is turned a quarter over, by the hands holding it steady, the same side is visible. Three causes enable us to see a little more than half the moon's surface: 1. The speed with which it traverses the ellipse of its...o...b..t is variable.

It sometimes gets ahead of us, sometimes behind, and we see farther around the front or back part. 2. The axis is a little inclined to the plane of its...o...b..t, and its...o...b..t a little inclined to ours; hence we see a little over its north pole, and then again over the south pole. 3. The earth being larger, its inhabitants see a little more than half-way around a smaller body. These causes combined enable us to see 576/1000 of the moon's surface. Our eyes will never see the other side of the moon. If, now, being solid, her axial revolution could [Page 153] be increased enough to make one more revolution in two or three years, that difference between her axial and orbital revolution would give the future inhabitants of the earth a view of the entire circ.u.mference of the moon. Yet if the moon were once in a fluid state, or had oceans on the surface, the enormous tide caused by the earth would produce friction enough, as they moved over the surface, to gradually r.e.t.a.r.d the axial revolution till the two tidal elevations remained fixed toward and opposite the earth, and then the axial and orbital revolutions would correspond, as at present. In fact, we can prove that the form of the moon is protuberant toward the earth. Its centre of gravity is thirty-three miles beyond its centre of magnitude, which is the same in effect as if a mountain of that enormous height rose on the earth side. Hence any fluid, as water or air, would flow round to the other side.

The moon's day, caused by the sun's light, is 29-1/2 times as long as ours. The sun s.h.i.+nes unintermittingly for fifteen days, raising a temperature as fervid as boiling water. Then darkness and frightful cold for the same time succeed, except on that half where the earth acts as a moon. The earth presents the same phases--crescent, full, and gibbous--to the moon as the moon does to us, and for the same causes. Lord Rosse has been enabled, by his six-foot reflector, to measure the difference of heat on the moon under the full blaze of its noonday and midnight. He finds it to be no less than five hundred degrees. People not enjoying extremes of temperature should shun a lunar residence. The moon gives us only 1/6180000 as much light as the sun. A sky full of moons would scarcely make daylight.

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[Ill.u.s.tration: Fig. 58.--View of the Moon near the Third Quarter.

From a Photograph by Professor Henry Draper.]

There are no indications of air or water on the moon. When it occults a star it instantly shuts off the light and as instantly reveals it again. An atmosphere would gradually diminish and reveal the light, and by refraction [Page 155] cause the star to be hidden in much less time than the solid body of the moon would need to pa.s.s over it. If the moon ever had air and water, as it probably did, they are now absorbed in the porous lava of its substance.

_Telescopic Appearance._

[Ill.u.s.tration: Fig. 59.--Illumination of Craters and Peaks.]

Probably no one ever saw the moon by means of a good telescope without a feeling of admiration and awe. Except at full-moon, we can see where the daylight struggles with the dark along the line of the moon's sunrise or sunset. This line is called the terminator.

It is broken in the extreme, because the surface is as rough as possible. In consequence of the small gravitation of the moon, utter absence of the expansive power of ice s.h.i.+vering the cliffs, or the levelling power of rains, precipices can stand in perpendicularity, mountains shoot up like needles, and cavities three miles deep remain unfilled. The light of the sun falling on the rough body of the moon, shown in section (Fig. 59), illuminates the whole cavity at _a_, part of the one at _b_, casts a long shadow from the mountain at _c_, and touches the tip of the one at _d_, which appears to a distant observer as a point of light beyond the terminator, As the moon revolves the conical cavity, _a_ is illuminated on the forward side only; the light creeps down the backward side of cavity _b_ to the bottom; mountain _c_. comes directly under the sun and casts no shadow, and mountain _d_ casts its long shadow over the plain. Knowing the time of revolution, and observing the change of [Page 156] illumination, we can easily measure the height of mountain and depth of crater. An apple, with excavations and added prominences, revolved on its axis toward the light of a candle, admirably ill.u.s.trates the crescent light that fills either side of the cavities and the shadows of the mountains on the plain.

Notice in Fig. 58 the crescent forms to the right, showing cavities in abundance.

[Ill.u.s.tration: Fig. 60.--Lunar Crater "Copernicus," after Secchi.]

The selenography of one side of the moon is much better known to us than the geography of the earth. Our maps of the moon are far more perfect than those of the earth; and the photographs of lunar objects by Messrs. Draper and De la Rue are wonderfully perfect, [Page 157] and the drawings of Padre Secchi equally so (Fig. 60).

The least change recognizable from the earth must be speedily detected. There are frequently reports of discoveries of volcanoes on the moon, but they prove to be illusions. The moon will probably look the same to observers a thousand years hence as it does to-day.

This little orb, that is only 1/81 of the ma.s.s of the earth, has twenty-eight mountains that are higher than Mont Blanc, that "monarch of mountains," in Europe.

_Eclipses._

[Ill.u.s.tration: Fig. 61.--Eclipses; Shadows of Earth and Moon.]