Letters on Astronomy Part 23

INSTRUMENTAL MEASUREMENTS.--When astronomical instruments were first employed to measure the angular distance between two points on the celestial sphere, it was not attempted to measure s.p.a.ces smaller than ten minutes--a s.p.a.ce equal to the third part of the breadth of the full moon. Tycho Brahe, however, carried his measures to sixty times that degree of minuteness, having devised means of determining angles no larger than ten seconds, or the one hundred and eightieth part of the breadth of the lunar disk. For many years past, astronomers have carried these measures to single seconds, or have determined s.p.a.ces no greater than the eighteen hundredth part of the diameter of the moon. This is considered the smallest arc which can be accurately measured directly on the limb of an instrument; but _differences_ between s.p.a.ces may be estimated to a far greater degree of accuracy than this, even to the hundredth part of a second--a s.p.a.ce less than that intercepted by a spider's web held before the eye.

DISCOVERY OF NEW PLANETS.--In my twenty-third letter (see page 286), I gave an account of the small planets called asteroids, which lie between the orbits of Mars and Jupiter. When that letter was written, no longer ago than 1840, only four of those bodies had been discovered, namely, Ceres, Pallas, Juno, and Vesta. Within a few years past, nineteen more have been added, making the number of the asteroids known at present twenty-three, and every year adds one or more to the list.[17] The idea first suggested by Olbers, one of the earliest discoverers of asteroids, that they are fragments of a large single planet once revolving between Mars and Jupiter, has gained credit since the discovery of so many additional bodies of the same cla.s.s, all, like the former, exceedingly small and irregular in their motions, although there are still great difficulties in tracing them to a common origin.

GREAT COMET OF 1843.--This is the most wonderful body that has appeared in the heavens in modern times; first, on account of its appearing, when first seen, in the broad light of noonday; and, secondly, on account of its approaching so near the sun as almost to graze his surface. It was first discovered, in New England, on the 28th of February, a little eastward of the sun, shining like a white cloud illuminated by the solar rays. It arrested the attention of many individuals from half past seven in the morning until three o'clock in the afternoon, when the sky became obscured by clouds. In Mexico, it was observed from nine in the morning until sunset. At a single station in South America, it was said to have been seen on the 27th of February, almost in contact with the sun. Early in March, it had receded so far to the eastward of that body as to be visible in the southwest after sunset, throwing upward a long train, which increased in length from night to night until it covered a s.p.a.ce of 40 degrees. Its position may be seen on a celestial globe adjusted to the lat.i.tude of New Haven (41 18') for the 20th of March, by tracing a line, or, rather, a broad band proceeding from the place of the sun towards the bright star Sirius, in the south, between the ears of the Hare and the feet of Orion.

The comet pa.s.sed its perihelion on the 27th of February, at which time it almost came in contact with the sun. To prevent its falling into the sun it was endued with a prodigious velocity; a velocity so great that, had it continued at the same rate as at the instant of perihelion pa.s.sage, it would have whirled round the sun in two hours and a half. It did, in fact, complete more than half its revolution around the sun in that short period, and it made more than three quarters of its circuit around the sun in one day. Its velocity, when nearest the sun, exceeded a million of miles per hour, and its tail, at its greatest elongation, was one hundred and eight millions of miles; a length more than sufficient to have reached from the sun to the earth. Its heat was estimated to be 47,000 times greater than that received by the earth from a vertical sun, and consequently it was more intense than that produced by the most powerful blowpipes, and sufficient to melt like wax the most infusible bodies. No doubt, when in the vicinity of the sun, the solid matter of the comet was first melted and then converted into vapor, which itself became red hot, or, more properly speaking, _white hot_. Much discussion has arisen among astronomers respecting the periodic time of this comet. Its most probable period is about 175 years.

DISTANCES OF THE STARS.--I have already mentioned (page 389) that the distance of at least one of the fixed stars has at length been determined, although at so great a distance that its annual parallax is only about one third of a second, implying a distance from the sun of nearly sixty millions of millions of miles. Of a distance so immense the mind can form no adequate conception. The most successful effort towards it is made by gradual and successive approximations. Let us, therefore, take the motion of a rail-way car as the most rapid with which we are familiar, and apply it first to the planetary s.p.a.ces, and then to the vast interval that separates these nether worlds from the fixed stars. A rail-way car, travelling constantly night and day at the rate of twenty miles per hour, would make 480 miles per day. At this rate, to travel around the earth on a great circle would require about 50 days, and 500 days to reach the moon. If we took our departure from the sun, and journeyed night and day, we should reach Mercury in a little more than 200 years, Venus in nearly 400, and the Earth in 547 years; but to reach Neptune, the outermost planet, would require 16,000 years. Great as appear the dimensions of the solar system, when we imagine ourselves thus borne along from world to world, yet this s.p.a.ce is small compared with that which separates us from the fixed stars; for to reach 61 Cygni it would take 324,000,000 years. But this is believed, for certain satisfactory reasons, to be one of the nearest of the stars. Several other stars whose parallax has been determined are at a much greater distance than 61 Cygni. The pole star is five times as far off; and the greater part of the stars are at distances inconceivably more remote.

Such, especially, are those which compose the faintest nebulae.

DISCOVERY OF THE PLANET NEPTUNE.--From the earliest ages down to the year 1781, the solar system was supposed to terminate with the planet Saturn, at the distance of nine hundred millions of miles from the sun; but the discovery of Ura.n.u.s added another world, and doubled the dimensions of the solar system. It seemed improbable that any more planets should exist at a distance still more remote, since such a body could hardly receive any of the vivifying influences of the central luminary. Still, certain irregularities to which the Ura.n.u.s was subject, led to the suspicion that there exists a planet beyond it, which, by its attractions, caused these irregularities. Impressed with this belief, two young astronomers of great genius, Le Verrier, of France, and Adams, of England, applied themselves to the task of finding the hidden planet.

The direction in which the disturbed body was moved afforded some clue to the part of the heavens where the disturbing body lay concealed; the kind of action it excited at different times indicated that it was beyond Ura.n.u.s, and not this side of that planet; and the magnitude of the forces it exerted gave some intimation of its size and ma.s.s. The law of distances from the sun which the superior planets observe (Saturn being nearly twice the distance of Jupiter, and Ura.n.u.s twice that of Saturn), led both these astronomers to a.s.sume that the body sought was nearly double the distance of Ura.n.u.s from the sun. With these few and imperfect data, as so many leading-strings proceeding from the planet Ura.n.u.s, they felt their way into the abysses of s.p.a.ce by the aid of two sure guides--the law of gravitation and the higher geometry. Both astronomers arrived at nearly the same results, although they wrought independently of each other, and each, indeed, without the knowledge of the other. Le Verrier was the first to make public his conclusions, which he communicated to the French Academy at their sitting, August 31, 1846. They saw that there existed, at nearly double the distance of Ura.n.u.s from the sun, a planet larger than that body; that it lay near a certain star seen at that season in the southwest, in the evening sky; that, on account of its immense distance, it was invisible to the naked eye, and could be distinctly seen with a perceptible disk only by the most powerful telescopes; being no brighter than a star of the ninth magnitude, and subtending an angle of only three seconds. Le Verrier communicated these results to Dr. Galle, of Berlin, with the request that he would search for the stranger with his powerful telescope, pointing out the exact spot in the heavens where it would be found. On the same evening, Dr. Galle directed his instrument to that part of the heavens, and immediately the planet presented itself to view, within one degree of the very spot a.s.signed to it by Le Verrier. Subsequent investigations have shown that its apparent size is within half a second of that which the same sagacious mind foresaw, and that its diameter is nearly equal to that of Ura.n.u.s, being 31,000, while Ura.n.u.s is 35,000 miles.[18] The distance from the sun is less than was predicted, being only about 3000, instead of 3600 millions of miles; and its periodic time is 164-1/2, instead of 217 years, as was supposed by Le Verrier.

One satellite only has yet been discovered, and this was first seen by Professor Bond with the great telescope of Harvard University.

RECENT TELESCOPIC DISCOVERIES.--The great reflecting telescope of Lord Rosse, and the powerful refracting telescopes of Pulkova and Cambridge, have opened new fields of discovery to the delighted astronomer. A new satellite has been added to Saturn, first revealed to the Cambridge instrument, making the entire number of moons that adorn the nocturnal sky of that remarkable planet no less than eight. Still more wonderful things have been disclosed among the remotest _Nebulae_. A number of these objects before placed among the irresolvable nebulae, and supposed to consist not of stars, but of mere nebulous matter, have been resolved into stars; others, of which we before saw only a part, have revealed themselves under new and strange forms, one resembling an animal with huge branching arms, and hence called the _crab_ nebula; another imitating a scroll or vortex, and called the _whirlpool_ nebula; and other figures, which to ordinary telescopes appear only as dim specks on the confines of creation, are presented to these wonderful instruments as glorious firmaments of stars.

In the year 1833, Sir John Herschel left England for the Cape of Good Hope, furnished with powerful instruments for observing the stars and nebulae of the southern hemisphere, which had never been examined in a manner suited to disclose their full glories. This great astronomer and benefactor to science devoted five years of the most a.s.siduous toil in observing and delineating the astronomical objects of that portion of the heavens. He had before extended the catalogue of nebulae begun by his ill.u.s.trious father, Sir William Herschel, to the number of 2307; and beginning at that point, he swelled the number, by his labors at the Cape of Good Hope, to 4015. He extended also the list of double stars from 3346 to 5449, and showed that the luminous spots near the South Pole, known to sailors by the name of the "Magellan Clouds," consist of an a.s.semblage of several hundred brilliant nebulae.

The United States have contributed their full share to the recent progress of astronomy. Powerful telescopes have been imported, made by the first European artists, and numerous others, of scarcely inferior workmanship and power, have been produced by artists of our own. The American astronomers have also been the first to bring the electric telegraph into use in astronomical observations; electric clocks have been so constructed as to beat simultaneously at places distant many hundred miles from each other, and thus to furnish means of determining the difference of longitude between places with an astonishing degree of accuracy; and facilities for recording observations on the stars have been devised which render the work vastly more rapid as well as more accurate than before. Indeed, the inventive genius for which Americans have been distinguished in all the useful arts seems now destined to be equally conspicuous in promoting the researches of science.

FOOTNOTES:

[17] The names of all the asteroids known at present are as follows:

1. Ceres. 9. Metis. 17. Psyche.

2. Pallas. 10. Hygeia. 18. Melpomene.

3. Juno. 11. Parthenope. 19. Fortuna.

4. Vesta. 12. Victoria. 20. Ma.s.salia.

5. Astraea. 13. Egeria. 21. Lutetia.

6. Hebe. 14. Irene. 22. Calliope.

7. Iris. 15. Eunomia. 23. Un-named.

8. Flora. 16. Thetis.

[18] Sir John Herschel, however, states its diameter at 41,500 miles