Astronomy for Amateurs Part 14

CHAPTER VIII

THE EARTH

Our grand celestial journey lands us upon our own little planet, on this globe that gravitates between Mars and Venus (between War and Love), circulating like her brothers of the solar system, around the colossal Sun.

The Earth! The name evokes in us the image of Life, and calls up the theater of our activities, our ambitions, our joys and sorrows. Does it not, in fact, to ignorant eyes, represent the whole of the universe?

And yet, what is the Earth?

The Earth is a star in the Heavens. We learned this much in our first lesson. It is a globe of opaque material, similar to the planets Mercury, Venus, Mars, Jupiter, etc., as previously described. Isolated on all sides in s.p.a.ce, it revolves round the Sun, along a vast orbit that it accomplishes in a year. And while it thus glides along the lines of solar attraction, the terrestrial ball rotates rapidly upon itself in twenty-four hours.

These statements may appear dubious at first sight, and contradictory to the evidence of our senses.

Now that the surface of the Earth has been explored in all directions, there is no longer room to doubt that it is a globe, a sort of ball that we adhere to. A journey round the world is common enough to-day, and always yields the most complete evidence of the spherical nature of the Earth. On the other hand, the curvature of the seas is a no less certain proof. When a s.h.i.+p reaches the dark-blue line that appears to separate the sky from the ocean, it seems to be hanging on the horizon. Little by little, however, as it recedes, it drops below the horizon line; the tops of the masts being the last to disappear. The observer on board s.h.i.+p witnesses the same phenomenon. The low sh.o.r.es are first to disappear, while the high coasts and mountains are much longer visible.

The aspect of the Heavens gives another proof of the Earth's rotundity.

As one travels North or South, new stars rise higher and higher above the horizon in the one direction or the other, and those which s.h.i.+ne in the lat.i.tude one is leaving, gradually disappear. If the surface of the Earth were flat, the s.h.i.+ps on the sea would be visible as long as our sight could pierce the distance, and all the stars of the Heavens would be equally visible from the different quarters of the world.

Lastly, during the eclipses of the Moon, the shadow projected by the Earth upon our satellite is always round. This is another proof of the spherical nature of the terrestrial globe.

We described the Earth as an orb in the Heavens, similar to all the other planets of the great solar family. We see these sister planets of our world circulating under the starry vault, like luminous points whose brilliancy is sometimes dazzling. For us they are marvelous celestial birds hovering in the ether, upheld by invisible wings. The Earth is just the same. It is supported by nothing. Like the soap-bubble that a.s.sumes a lovely iridescence in the rays of the Sun, or, better, like the balloon rapidly cleaving the air, it is isolated from every kind of support.

Some minds have difficulty in conceiving this isolation, because they form a false notion of weight.

The astronomers of antiquity, who divined it, knew not how to prevent the Earth from falling. They asked anxiously what the strong bands capable of holding up this block of no inconsiderable weight could be.

At first they thought it floated on the waters like an island. Then they postulated solid pillars, or even supposed it might turn on pivots placed at the poles. But on what would all these imaginary supports have rested? All these fanciful foundations of the Earth had to be given up, and it was recognized as a globe, isolated in every part. This illusion of the ancients, which still obtains for a great many citizens of our globule, arises, as we said, from a false conception of weight.

Weight and attraction are one and the same force.

A body can only fall when it is attracted, drawn by a more important body. Now, in whatever direction we may wander upon the globe, our feet are always downward. _Down_ is therefore the _center_ of the Earth.

The terrestrial globe may be regarded as an immense ball of magnet, and its attraction holds us at its surface. We weigh toward the center. We may travel over this surface in all directions; our feet will always be below, whatever the direction of our steps. For us, "below" is the inside of our planet, and "above" is the immensity of the Heavens that extend above our heads, right round the globe.

This once understood, where could the Earth fall to? The question is an absurdity. "Below" being toward the center, it would have to fall out of itself.

Let us then picture the Earth as a vast sphere, detached from all that exists around it, in the infinity of the Heavens. A point diametrically opposed to another is called its _antipodes_. New Zealand is approximately the antipodes to France. Well, for the inhabitants of New Zealand and of France the top is reciprocally opposed, and the bottom, or the feet, are diametrically in opposition. And yet, for one as for the other, the bottom is the soil they are held to, and the top is s.p.a.ce above their heads.

The Earth turns on itself in twenty-four hours. Whatever is above us, _e.g._, at midday, we call high; twelve hours later, at midnight, we give the same qualification to the part of s.p.a.ce that was under our feet at noon. What is in the sky, and over our heads, at a given hour, is under our feet, and yet always in the sky, twelve hours later. Our position, in relation to the s.p.a.ce that surrounds us, changes from hour to hour, and "top" and "bottom" vary also, relatively to our position.

Our planet is thus a ball, slightly flattened at the poles (by about 1/292). Its diameter, at the equator, is 12,742 kilometers (7,926 miles); from one pole to the other is a little less, owing to the flattening of the polar caps. The difference is some 43 kilometers (about 27 miles).

Its circ.u.mference is 40,000 kilometers (24,900 miles). This ball is surrounded by an aerial envelope, the atmosphere, the height of which can not be less than 300 kilometers (186 miles), according to the observations made on certain shooting stars.

We all know that this layer of air, at the bottom of which we live, is a beautiful azure blue that seems to separate us from the sidereal abyss, spreading over our heads in a kind of vault that is often filled with clouds, and giving the illusion of resting far off on the circle of the horizon. But this is only an illusion. In reality, there is neither vault nor horizon; s.p.a.ce is open in all directions. If the atmosphere did not exist, or if it were completely transparent, we should see the stars by day as by night, for they are continually round us, at noon as at midnight, and we can see them in the full daylight, with the help of astronomical instruments. In fact, certain stars (the radiant Venus and the dazzling Jupiter) pierce the veil of the atmosphere, and are visible with the unaided eye in full daylight.

The terrestrial surface is 510,000,000 square kilometers (200,000,000 square miles). The waters of the ocean cover three-quarters of this surface, _i.e._, 383,200,000 square kilometers (150,000,000 square miles), and the continents only occupy 136,600,000 square kilometers (55,000 square miles). France represents about the thousandth part of the total superficies of the globe.

Despite the asperities of mountain ranges, and the abysses hollowed out by the waters, the terrestrial globe is fairly regular, and in relation to its volume its surface is smoother than that of an orange. The highest summits of the Himalaya, the profoundest depths of the somber ocean, do not attain to the millionth part of its diameter.

In weight, the Earth is five and a half times heavier than would be a globe of water of the same dimensions. That is to say:

6,957,930,000,000,000,000,000,000 kilograms (6,833,000,000,000,000,000,000 tons).

The atmospheric atmosphere with which it is surrounded represents.

6,263,000,000,000,000,000 kilograms (6,151,000,000,000,000 tons).

Each of us carries an average weight of some 17,000 kilograms (16 tons) upon his shoulders. Perhaps some one will ask how it is that we are not crushed by this weight, which is out of all proportion with our strength, but to which, nevertheless, we appear insensible. It is because the aerial fluid enclosed within our bodies exerts a pressure equal and opposite to the external atmospheric pressure, and these pressures are at equilibrium.

The Earth is characterized by no essential or particular differences relatively to the other worlds of our system. Like Venus of the limpid rays, like the dazzling Jupiter, like all the planets, she courses through s.p.a.ce, carrying into Infinitude our hopes and destinies. Bigger than Mercury, Venus, and Mars, she presents a very modest figure in comparison with the enormous Jupiter, the strange system of Saturn, of Ura.n.u.s, and even of Neptune. For us her greatest interest is that she serves as our residence, and if she were not our habitation we should scarcely notice her. Dark in herself, she burns at a distance like a star, returning to s.p.a.ce the light she receives from the Sun. At the distance of our satellite, she s.h.i.+nes like an enormous moon, fourteen times larger and more luminous than our gentle Phoebe. Observed from Mercury or Venus, she embellishes the midnight sky with her sparkling purity as Jupiter does for us. Seen from Mars, she is a brilliant morning and evening star, presenting phases similar to those which Mars and Venus show from here. From Jupiter, the terrestrial globe is little more than an insignificant point, nearly always swallowed up in the solar rays. As to the Saturnians, Uranians, and Neptunians, if such people exist, they probably ignore our existence altogether. And in all likelihood it is the same for the rest of the universe.

We must cherish no illusions as to the importance of our natal world. It is true that the Earth is not wanting in charm, with its verdant plains enameled in the delicious tones of a robust and varied vegetation, its plants and flowers, its spring-time and its birds, its limpid rivers winding through the meadows, its mountains covered with forests, its vast and profound seas animated with an infinite variety of living creatures. The spectacle of Nature is magnificent, superb, admirable and marvelous, and we imagine that this Earth fills the universe, and suffices for it. The Sun, the Moon, the stars, the boundless Heavens, seem to have been created for us, to charm our eyes and thoughts, to illumine our days, and shed a gentle radiance upon our nights. This is an agreeable illusion of our senses. If our Humanity were extinguished, the other worlds of the Heavens, Venus, Mars, etc., would none the less continue to gravitate in the Heavens along with our defunct planet, and the close of human life (for which everything seems to us to have been created) would not even be perceived by those other worlds, that nevertheless are our neighbors. There would be no revolution, no cataclysm. The stars would go on s.h.i.+ning in the firmament, just as they do to-day, shedding their divine light over the immensity of the Heavens. Nothing would be changed in the general aspect of the Universe.

The Earth is only a modest atom, lost in the innumerable army of the worlds and suns that people the universe.

Every morning the Sun rises in the East, setting fire with his ardent rays to the sky, which is dazzling with his splendor. He ascends through s.p.a.ce, reaches a culminating point at noon, and then descends toward the West, to sink at night into the purple of the sunset.

And then the stars, grand lighthouses of the Heavens, in their turn incandesce. They too rise in the East, ascend the vault of Heaven, and then descend to the West, and vanish. All the orbs, Sun, Moon, planets, stars, appear to revolve round us in twenty-four hours.

This journey of the orbs around us is only an illusion of the senses.

Whether the Earth be at rest, and the sky animated with a rotary movement round her, or whether, on the contrary, the stars are fixed, and the Earth in motion, in either case, for us appearances are the same. If the Earth turns, carrying all that pertains to it in its motion--the seas, the atmosphere, the clouds, and ourselves,--we are unable to perceive it, because all the objects that surround us keep their respective positions among themselves. Hence we must resort to logic, and reason out the two hypotheses.

For the accomplishment of this rapid journey of the Sun and stars around the Earth, it would be necessary that all the orbs of the sky should be in some way attached to a vault, or to circles, as was formerly supposed. This conception is childish. The peoples of antiquity had no notion of the size of the universe, and their error is almost excusable.

The distance separating Heaven from the Infernal Regions has been measured, according to Hesiod, by Vulcan's anvil, which fell from the skies to the Earth in nine days and nine nights, and it would have taken as long again to continue its journey from the surface of the Earth to the bowels of Hades.

To-day we have a more exact notion of the grandeur of the Universe. We know that millions and trillions of miles separate the stars from one another. And by representing these distances, we can form some idea of the difficulty there would be in admitting the rotation of the universe round the Earth.

The distance from here to the Sun is 149,000,000 kilometers (93,000,000 miles). In order to turn in twenty-four hours round the Earth, that orb would have to fly through s.p.a.ce at a velocity of more than 10,000 kilometers (6,200 miles) a second.

Yes! the Sun, splendid orb, source of our existence and of that of all the planets, a colossal globe, over a million times more voluminous than the Earth, and 324 thousand times heavier, would have to accomplish this immense revolution in order to turn round the minute point that is our lilliputian world!

This in itself would suffice to convince us of the want of logic in such an argument. But the Sun is not alone in the Heavens. We should have to suppose that all the planets and all the stars were engaged in the same fantastic motions.

Jupiter is about five times as far off as the Sun; his velocity would have to be 53,000 kilometers (32,860 miles) per second.

Neptune, thirty times farther off, would have to execute 320,000 kilometers (198,000 miles) per second.

The nearest star, [alpha] of the Centaur, situated at a distance 275,000 times that of the Sun, would have to run, to fly through s.p.a.ce, at a rate of 2,941,000,000 kilometers (1,823,420,000 miles) per second.

All the other stars are incomparably farther off, at infinity.

And this fantastic rotation would all be accomplished round a minute point!

To put the problem in this way is to solve it. Unless we deny the astronomic measures, and the most convincing geometric operations, the Earth's diurnal motion of rotation is a certainty.

To suppose that the stars revolve round the Earth is to suppose, as one author humorously suggests, that in order to roast a pheasant the chimney, the kitchen, the house, and all the countryside must needs turn round it.