Astronomy for Amateurs Part 11

What is the nature of these vast concentric circles that surround the planet with a luminous halo? They are composed of an innumerable number of particles, of a quant.i.ty of cosmic fragments, which are swept off in a rapid revolution, and gravitate round the planet at variable speed and distance. The nearer particles must accomplish their revolution in 5 hours, 50 minutes, and the most distant in about 12 hours, 5 minutes, to prevent them from being merged in the surface of Saturn: their own centrifugal force sustains them in s.p.a.ce.

[Ill.u.s.tration: FIG. 48. Varying perspective of Saturn's Rings, as seen from the Earth.]

With a good gla.s.s the effect of these rings is most striking, and one can not refrain from emotion on contemplating this marvel, whereby one of the brothers of our terrestrial country is crowned with a golden diadem. Its aspects vary with its perspective relative to the Earth, as may be seen from the subjoined figure (Fig. 48).

We must not quit the Saturnian province without mentioning the eight satellites that form his splendid suite:

Names. Distance from the planet. Time of revolution.

Kilometers. Miles. Days. Hours. Minutes.

1. Mimas 207,000 128,340 22 37 2. Enceladus 257,600 159,712 1 8 53 3. Tethys 328,800 203,856 1 21 18 4. Dione 421,200 261,144 2 17 41 5. Rhea 588,400 364,808 4 12 25 6. t.i.tan 1,364,000 845,680 15 22 41 7. Hyperion 1,650,000 1,023,000 21 6 39 8. j.a.phet 3,964,000 2,457,680 79 7 54

Here is a marvelous system, with, what is more, eight different kinds of months for the inhabitants of Saturn; eight moons with constantly varying phases juggling above the rings!

Now we shall cross at a bound the 1,400 million kilometers (868,000,000 miles) that separate us from the last station but one of the immense solar system.

URa.n.u.s

On March 13, 1781, William Herschel, a Hanoverian astronomer who had emigrated to England, having abandoned the study of music to devote himself to the sublime science of the Heavens, was observing the vast fields with their constellations of golden stars, when he perceived a luminous point that appeared to him to exceed that of the other celestial luminaries in diameter. He replaced the magnification of his telescope by more powerful eye-pieces, and found that the apparent diameter of the orb increased proportionately with the amplification of the power, which does not happen in the case of stars at infinite distance. His observations on the following evenings enabled him to note the slow and imperceptible movement of this star upon the celestial sphere, and left him in no further doubt: there was no star, but some much nearer orb, in all probability a comet, for the great astronomer dared not predict the discovery of a new planet. And it was thus, under the name of cometary orb, that the seventh child of the Sun was announced. The astronomers sought to determine the motions of the new arrival, to discover for it an elliptical orbit such as most comets have. But their efforts were vain, and after several months' study the conclusion was reached that here was a new planet, throwing back the limits of the solar system to a point far beyond that of the Saturnian frontier, as admitted from antiquity.

This new world received the name of Ura.n.u.s, father of Saturn, his nearest neighbor in the solar empire. Ura.n.u.s shines in the firmament as a small star of sixth magnitude, invisible to the unaided eye for normal sight, at a distance of 2,831,000,000 kilometers (1,755,000,000 miles) from the Sun. Smaller than Jupiter and Saturn, this planet is yet larger than Mercury, Venus, Mars, and the Earth together, thus presenting proportions that claim our respect and admiration.

His diameter may be taken at about 55,000 kilometers (34,200 miles), that is, rather more than four times the breadth of the terrestrial diameter. Sixty-nine times more voluminous than the Earth, and seventeen times more extensive in surface, this new world is much less than our own in density. The matter of which it is composed is nearly five times lighter than that of our globe.

Spectral a.n.a.lysis shows that this distant planet is surrounded with an atmosphere very different from that which we breathe, enclosing gases that do not exist in ours.

The Uranian globe courses over the fields of infinity in a vast orbit seventeen times larger than our own, and its revolution lasts 36,688 days, _i.e._, 84 years, 8 days. It travels slowly and sadly under the pale and languishing rays of the Sun, which sends it nearly three hundred times less of light and heat than we receive. At this distance the solar disk would present a diameter seventeen times smaller than that which we admire, and a surface three hundred times less vast. A dull world indeed! And what an interminable year! The idle people who are in the habit of being bored must find time even longer upon Ura.n.u.s than upon our little Earth, where the days pa.s.s so rapidly. And if matters are arranged there as here, a babe of a year old, beginning to babble in its nurse's arms, would already have lived as long as an old man of eighty-four in this world.

But what most seriously complicates the Calendar of the Uranians is the fact that the four moons which accompany the planet accomplish their revolution in four different kinds of months, in two, four, eight, and thirteen days, as is shown in the following table:

Distance from the planet. Time of revolution.

Kilometers. Miles. Days. Hours. Minutes.

1. Ariel 196,000 121,520 2 12 29 2. Umbriel 276,000 171,120 4 3 27 3. t.i.tania 450,000 279,000 8 16 56 4. Oberon 600,000 372,000 13 11 7

The most curious fact is that these satellites do not rotate like those of the other planets. While the moons of the Earth, Mars, Jupiter, and Saturn accomplish their revolution from east to west, the satellites of Ura.n.u.s rotate in a plane almost perpendicular to the ecliptic, and it is doubtless the same for the rotation of the planet.

If we had to quit the Earth, and fixate ourselves upon another world, we should prefer Mars to Ura.n.u.s, where everything must be so different from terrestrial arrangements? But who knows? Perhaps, after all, this planet might afford us some agreeable surprises. _Il ne faut jurer de rien._

NEPTUNE

And here we reach the frontier of the Solar System, as actually known to us. In landing on the world of Neptune, which circles through the Heavens in eternal twilight at a distance of more than four milliard kilometers (2,480,000,000 miles) from the common center of attraction of the planetary orbs, we once again admire the prodigies of science.

Ura.n.u.s was discovered with the telescope, Neptune by calculation. In addition to the solar influence, the worlds exert a mutual attraction upon each other that slightly deranges the harmony ordered by the Sun.

The stronger act upon the weaker, and the colossal Jupiter alone causes many of the perturbations in our great solar family. Now during regular observations of the position of Ura.n.u.s in s.p.a.ce, some inexplicable irregularities were soon perceived. The astronomers having full faith in the universality of the law of attraction, could not do otherwise than attribute these irregularities to the influence of some unknown planet situated even farther off. But at what distance?

A very simple proportion, known as Bode's law, has been observed, which indicates approximately the relative distances of the planets from the Sun. It is as follows: Starting from 0, write the number 3, and double successively,

0 3 6 12 24 48 96 192 384.

Then, add the number 4 to each of the preceding figures, which gives the following series:

4 7 10 16 28 52 100 196 388.

Now it is a very curious fact that if the distance between the Earth and the Sun be represented by 10, the figure 4 represents the orbit of Mercury, 7 that of Venus, 16 of Mars; the figure 28 stands for the medium distance of the minor planets; the distances of Jupiter, Saturn, and Ura.n.u.s agree with 52, 100, and 196.

The immortal French mathematician Le Verrier, who pursued the solution of the Uranian problem, supposed naturally that the disturbing planet must be at the distance of 388, and made his calculations accordingly.

Its direction in the Heavens was indicated by the form of the disturbances; the orbit of Ura.n.u.s bulging, as it were, on the side of the disturbing factor.

On August 31, 1846, Le Verrier announced the position of the ultra-Uranian planet, and on September 23d following, a German astronomer, Galle, at the Observatory of Berlin, who had just received this intelligence, pointed his telescope toward the quarter of the Heavens designated, and, in fact, attested the presence of the new orb.

Without quitting his study table, Le Verrier, by the sole use of mathematics, had detected, and, as it were, touched at pen's point the mysterious stranger.

Only, it is proved by observation and calculation that it is less remote than was expected from the preceding law, for it gravitates at a distance of 300, given that from the Earth to the Sun as 10.

This planet was called Neptune, G.o.d of the seas, son of Saturn, brother of Jupiter. The name is well chosen, since the King of the Ocean lives in darkness in the depths of the sea, and Le Verrier's...o...b..is also plunged in the semi-obscurity of the depths of the celestial element.

But it was primarily selected to do justice to an English astronomer, Adams, who had simultaneously made the same calculations as Le Verrier, and obtained the same results--without publishing them. His work remained in the records of the Greenwich Observatory.

The English command the seas, and wherever they dip their finger into the water and find it salt, they feel themselves "at home," and know that "Neptune's trident is the scepter of the world," hence this complimentary nomenclature.

Neptune is separated by a distance of four milliards, four hundred million kilometers from the solar center.

At such a distance, thirty times greater than that which exists between the Sun and our world, Neptune receives nine hundred times less light and heat than ourselves; _i.e._, Spitzbergen and the polar regions of our globe are furnaces compared with what must be the Neptunian temperature. Absolutely invisible to the unaided eye, this world presents in the telescope the aspect of a star of the eighth magnitude.

With powerful magnifications it is possible to measure its disk, which appears to be slightly tinged with blue. Its diameter is four times larger than our own, and measures about 48,000 kilometers (29,900 miles), its surface is sixteen times vaster than that of the Earth, and to attain its volume we should have to put together fifty-five globes similar to our own. Weight at its surface must be about the same as here, but its medium density is only 1/3 that of the Earth.

It gravitates slowly, dragging itself along an orbit thirty times vaster than that of our globe, and its revolution takes 164 years, 281 days, _i.e._, 164 years, 9 months. A single year of Neptune thus covers several generations of terrestrial life. Existence must, indeed, be strange in that tortoise-footed world!

While in their rotation period, Mercury accomplishes 47 kilometers (29-3/8 miles) per second, and the Earth 29-1/2 (18-1/8 miles), Neptune rolls along his immense orbit at a rate of only 5-1/2 kilometers (about 3-1/4 miles) per second.

The vast distance that separates us prevents our distinguishing any details of his surface, but spectral a.n.a.lysis reveals the presence of an absorbent atmosphere in which are gases unknown to the air of our planet, and of which the chemical composition resembles that of the atmosphere of Ura.n.u.s.

One satellite has been discovered for Neptune. It has a considerable inclination, and rotates from east to west.

And here we have reached the goal of our interplanetary journey. After visiting the vast provinces of the solar republic, we feel yet greater admiration and grat.i.tude toward the luminary that governs, warms, and illuminates the worlds of his system.

In conclusion, let us again insist that the Earth,--a splendid orb as viewed from Mercury, Venus, and Mars,--begins to disappear from Jupiter, where she becomes no more than a tiny spark oscillating from side to side of the Sun, and occasionally pa.s.sing in front of him as a small black dot. From Saturn the visibility of our planet is even more reduced. As to Ura.n.u.s and Neptune, we are invisible there, at least to eyes constructed like our own. We do not possess in the Universe the importance with which we would endow ourselves.

Neptune up to the present guards the portals of our celestial system; we will leave him to watch over the distant frontier; but before returning to the Earth, we must glance at certain eccentric orbs, at the mad, capricious comets, which imprint their airy flight upon the realms of s.p.a.ce.

CHAPTER VII

THE COMETS

SHOOTING STARS, BOLIDES, URANOLITHS OR METEORIC STONES