direction. Nothing can be more sublime, or better calculated to fill the mind with profound astonishment, than to contemplate the revolution of comets, while in that part of their orbits which comes within the sphere of the telescope. Some seem to come up from the immeasurable depths below the ecliptic, and, having doubled the heavens' mighty cape, again plunge downward with their fiery trains, "On the long travel of a thousand years." Others appear to come down from the zenith of the universe to double their perihelion about the Sun, and then reascend far above all human vision. Others are dashing through the solar system in all possible directions, and apparently without any undisturbed or undisturbing path prescribed by Him who guides and sustains them all. 541. Until within a few years, it was universally believed that the periods of their revolutions must necessarily be of prodigious length; but within a few years, two comets have been discovered, whose revolutions are performed, comparatively, within our own neighborhood. To distinguish them from the more remote, they are denominated the Comets of a short period. The first was discovered in the constellation Aquarius, by two French astronomers, in the year 1786. The same comet was again observed by Miss Caroline Herschel, in the constellation Cygnus, in 1795, and again in 1805. In 1818, Professor Encke determined the dimensions of its orbit, and the period of its sidereal revolution; for which reason it has been called "Encke's Comet." Map IX., Fig. 77. This comet performs its revolution around the Sun in about 3 years and 4 months, in an elliptical orbit which lies wholly within the orbit of Jupiter. Its mean distance from the Sun is 212,000,000 of miles; the eccentricity of its orbit is 179,000,000 of miles; consequently, is 358,000,000 of miles nearer the Sun in its perihelion, than it is in its aphelion. It was visible throughout the United States in 1825, when it presented a fine appearance. It was also observed at its next return in 1828; but its return to its perihelion on the 6th of May, 1832, was invisible in the United States, on account of its great southern declination. It has returned at regular periods since that time. 542. The second "comet of a short period," was observed in 1772; and was seen again in 1805. It was not until its reappearance in 1826, that astronomers were able to determine the elements of its orbit, and the exact period of its revolution. This was successfully accomplished by M. Biela of Josephstadt; hence it is called Biela's Comet. 541. What opinion respecting their periods? What distinction in comets founded on the lengths of their periods? History of "Encke's Comet?" Its period, orbit, mean distance, eccentricity of its orbit? 542. History of "Biela's Comet ?" Its diameter? According to observations made upon it in 1805, by the celebrated Dr. Olbers, its diameter, including its envelope, is 42,280 miles. It is a curious fact, that the path of Biela's Comet passes very near to that of the Earth; so near, that at the moment the center of the comet is at the point nearest to the Earth's path, the matter of the comet extends beyond that path, and includes a portion within it. Thus, if the Earth were at that point of its orbit which is nearest to the path of the comet, at the same moment that the comet should be at that point of its orbit which is nearest to the path of the Earth, the Earth would be enveloped in the nebulous atmosphere of the comet. With respect to the effect which might be produced upon our atmosphere by such a circumstance, it is impossible to offer anything but the most vague conjecture. Sir John Herschel was able to distinguish stars as minute as the 16th or 17th magnitude through the body of the comet! Hence it seems reasonable to infer, that the nebulous matter of which it is composed, must be infinitely more attenuated than our atmosphere; so that for every particle of cometary matter which we should inhale, we should inspire millions of particles of atmospheric air. 543 This is one of the comets that was to come into collision with the Earth, and to blot it out from the Solar System. In returning to its perihelion, November 26th, 1832, it was computed that it would cross the Earth's orbit at a distance of only 18,500 miles. It is evident that if the Earth had been in that part of her orbit at the same time with the comet, our atmosphere would have mingled with the atmosphere of the comet, and the two bodies, perhaps, have come in contact. But the comet passed the Earth's orbit on the 29th of October, in the 8th degree of Sagittarius, and the Earth did not arrive at that point until the 30th of November, which was 32 days afterwards. If we multiply the number of hours in 32 days, by 68,000 (the velocity of the Earth per hour), we shall find that the Earth was more than 52,000,000 miles behind the comet when it crossed her orbit. Its nearest approach to the Earth at any time, was about 51,000,000 of miles; its nearest approach to the Sun, was about 88,000,000 of miles. Its mean distance from the Sun, or half the longest axis of its orbit, is 337,000,000 of miles. Its eccentricity is 253,000,000 of miles; consequently, it is 507,000,000 of miles nearer the Sun in its perihelion than it is in its aphelion. The period of its sidereal revolution is 2460 days, or about 64 days. 544. Although the comets of Encke and Biela are objects of very great interest, yet their short periods, the limited space within which their motion is circumscribed, and consequently the very slight disturbance which they sustain from the attraction of the planets, render them of less interest to physical astronomy than those of longer periods. They do not, like them, rush from the invisible and inaccessible depths of space, and, after sweeping our system, depart to distances with the conception of which the imagination itself is confounded. They possess none of that grandeur which is connected with whatever appears to break through the fixed order of the universe. What curious fact stated? What result if the Earth were to be enveloped in the comet? 543. What mischief formerly anticipated from Biela's comet? Its return in 1832? How near a collision in distance and in time? Its nearest approach to the Earth? To the Sun? Its mean distance from him? Its eccentricity and period? 544. Why are the comets of short periods less interesting than others? For what comet is it reserved to afford grounds for the proudest triumphs of mathematical science? It is reserved for the comet of Halley alone to afford the proudest triumphs to those powers of calculation by which we are enabled to follow it in the depths of space, 2,000,000,000 of miles beyond the extreme verge of the solar system; and, notwithstanding the disturbances which render each succeeding period of its return different from the last, to foretell that return with precision. To be able to predict the very day and circumstances of the return of such a bodiless and eccentric wanderer, after the lapse of so many years, evinces a perfection of the astronomical calculus that may justly challenge our admiration. 545. "The re-appearance of Biela's comet," says Herschel, "whose return in 1832 was made the subject of elaborate calculations by mathematicians of the first eminence, did not disappoint the expectations of astronomers. It is hardly possible to imagine anything more striking than the appearance, after the lapse of nearly seven years, of such an all but imperceptible cloud or wisp of vapor, true, however, to its predicted sime and place, and obeying laws like those which regulate the planets." Herschel, whose Observatory was at Slough, England, observed the daily progress of this comet from the 24th of September, until its disappearance, compared its actual position from day to day, with its calculated position, and found them to agree within four or five minutes of time in right ascension, and within a few seconds of declination. Its position, then, as represented on a planisphere which the author prepared for his pupils, and afterwards published, was true to within a less space than one-third of its projected diameter. Like some others that have been observed, this comet has no luminous train by which it can be easily recognized by the naked eye, except when it is very near the Sun. This is the reason why it was not more generally observed at its late return. Although this comet is usually denominated "Biela's comet," yet it seems that M. Gambart, director of the Observatory at Marseilles, is equally entitled to the honor of identifying it with the comet of 1772, and of 1805. He discovered it only 10 days after Biela, and immediately set about calculating its elements from his own observations, which are thought to equal, if they do not surpass, in point of accuracy, those of every other astronomer. 546. Up to the beginning of the 17th century, no correct notions had been entertained in respect to the paths of comets. Kepler's first conjecture was that they moved in straight lines; but as that did not agree with observation, he next concluded that they were parabolic curves, having the Sun near the vertex, and running indefinitely into the regions of space at both extremities. There was nothing in the observations of the earlier astronomers to fix their identity, or to lead him to suspect that any one of them had ever been seen before; much less that they formed a part of the solar system, revolving about the Sun in elliptical orbits that returned into themselves. 547. This grand discovery was reserved for one of the most industrious and sagacious astronomers that ever lived-this was Dr. Halley, the cotemporary and friend of Newton. When the comet of 1682 made its appearance, he set himself about observing it with great care, and found there was a wonderful resem 545. Remarks on the re-appearance of Biela's comet? What remarkable calculation referred to? Form of this comet? Is it really Biela's comet? 546. Former knowledge of the orbits of comets? 547. What grand discovery, and by whom? Process blance between it and three other comets that he found recorded, the comets of 1456, of 1531, and 1607. The times of their appearance had been nearly at equal and regular intervals; their perihelion distances were nearly the same; and he finally proved them to be one and the same comet, performing its circuit around the Sun in a period varying a little from 76 years. It is, therefore, called Halley's comet. (Map IX., Fig. 78.) The orbit of Halley's comet extends outward about 120,000,000 of miles beyond the orbit of Neptune, as represented in the following cut This is the same comet that filled the eastern world with so much consternation in 1456, as stated on page 253, and became an object of such abhorrence to the Church of Rome. The periodic times of the three comets just described, are as follow: Encke's, 1212 days. Halley's comet, true to its predicted time and place, is now (Oct. 1835) visible in the evening sky. But we behold none of those phenomena which threw our ancestors of the middle ages into agonies of superstitious terror. We see not the cometa horrenda magnitudinis, as it appeared in 1305, nor that tail of enormous length which, in 1456, extended over two-thirds of the interval between the horizon and the zenith, nor even a star as brilliant as was the same comet in 1682, with its tail of 30°. Its mean distance from the Sun is 1,713,700,000 miles; the eccentricity of its orbit is 1,658,000,000 miles; consequently it is 3,316,000,000 miles farther from the Sun in its aphelion than it is in its perihelion. In the latter case its distance from the sun is only 55,700,000 miles; but in the former it is 3,371,700,000 miles. Therefore, though its aphelion distance be great, its mean distance is less than that of Uranus; and great as is the aphelion distance, it is but a very small fraction less than one-five thousandth part of that distance from the Sun, beyond which the very nearest of the fixed stars must be situated; and, as the determination of their distance is negative and not positive, the nearest of them may be at twice or ten times that distance. of the discovery? Aphelion distance of Halley's comet? What former visit to our system referred to? Periods of the three comets just described? Appearance of Halley's comet in 1835? Its mean distance from the Sun? How compare with that of Uranus? How does his greatest distance compare with that of the Fixed Stars? 548. The orbit of Encke's comet is wholly within the orbit of Jupiter, while that of Biela's extends but a short distance beyond it. The aphelion distance of Halley's comet is 3,400,000,000 of miles, or 550,000,000 of miles beyond the orbit of Neptune. And even this is, in reality, a comet of short period compared with many that belong to our system. OBBITS OF SEVERAL COMETS. ENCKE'S HALLEY'S 76 YRS BIELA'S 6AYBS. 3 YRS. 549. The comet of 1819 was remarkable for its straight wedgeshaped appearance-not altogether unlike a shuttle-cock. It exhibited none of that curvature in its form which is an almost universal characteristic of cometary bodies. Map IX., Fig. 79. 550. The comet of 1843 was one of the most magnificent of modern times (See Map IX.,. Fig. 80). It was more than 60° in length. In the Southern Hemisphere it was so brilliant as to throw a very strong light upon the Earth. As its distance from the Sun varied, its color varied, from pale orange to rose red," and then to white. "It passed its perihelion on the 27th of February, at which time it almost grazed the surface of the Sun, approaching nearer to that luminary than any comet hitherto observed. Its motions at this time were astonishingly swift, and its brilliancy such as to induce the belief that it was at a white heat through its whole extent. Its period is supposed to be 21 years; consequently this must be its eighth return since 1668; and it will visit our sphere again in 1865." At the time of the appearance of this comet, Rev. Mr. Miller and others were earnestly warning the people of the United States, that the world was to be burned up on the 284 of April following; and the appearance of the comet was regarded by many as an indication that the end of all things was at hand. 551. The number of comets which have been observed since the Christian era, amounts to 700. Scarcely a year has passed without the observation of one or two. And since multitudes of them must escape observation, by reason of their traversing that part of the heavens which is above the horizon in the day 548. Where are the orbits of Encke's and Biela's comets situated? What said of Hal ley's comet? 549. Comet of 1819? 550. That of 1843? Its length? Britancy? What variation in its color? Its perihelion passage? Heat? Its period? Next appearance? Incident of its last appearance? 551. Number of comets? Why so few seen? |