Node (astronomy)

“ It is, accordingly, quite possible to photograph objects so faint as to be altogether beyond the power of any telescope to reveal—witness the chemical disclosure of the invisible nebula encircling Nova Persei—and we may thus eventually learn whether a blank space in the sky truly represents the end of the stellar universe in that direction, or whether farther and farther worlds roll and shine beyond, veiled in the obscurity of immeasurable distance. ”

“ Such were the earliest attempts to construct the reflecting telescope, that noble instrument which has since effected such splendid discoveries in astronomy. Looking back from the present advanced state of practical science, how great is the contrast between the loose specula of Gregory and the fine Gregorian telescopes of Hadley, Short, and Veitch,—between the humble six-inch tube of Newton and the gigantic instruments of Herschel and Ramage. ”

“ Stars which have bright lines in their spectra (§ 312) form another link connecting nebulae with stars. A good many converging lines of evidence thus point to a greater variety in the arrangement, size, and structure of the bodies with which the telescope makes us acquainted than seemed probable when sidereal astronomy was first seriously studied; they also indicate the probability that these bodies should be regarded as belonging to a single system, even if it be of almost inconceivable complexity, rather than to a number of perfectly distinct systems of a simpler type. ”

“ There the sun in every detail of disk and spectrum is photographed by day, and stars with their spectra and the nebulæ by night. Great streams of stars are discovered and the speed and direction of their drift ascertained. The marvels of the spiral nebulæ are unfolded, their multitudinous forms portrayed and deciphered.
And their distances? And the distances of the still more wonderful clusters? Far, inconceivably far beyond the Milky Way. And are they "island universes"? And can man, the measurer, measure the distance of the "mainland" beyond? ”

“ The lesson of patience which can be learned from the labors of these successors is no mean one. To this patience, supplemented by a skill which usually must be of a high order, we owe the later discoveries of the telescope, such as the finding of the eighth satellite of Saturn (by Bond and Lassell) , and of the two interior moons of Uranus (Lassell, 1847 and 1851) , and of the satellite of Neptune.
There is hardly an object in the whole heavens—planets, of course, excepted—which has been so thoroughly and faithfully studied as the Great Nebula in Orion. ”

“ In dealing with problems of structure and motion in the Galactic system, the 100-inch telescope offers especial advantages, because of its vast light-gathering power. Studies of radial velocities of the stars have hitherto been necessarily confined to the brighter stars, for the most part even to those visible to the naked eye. While some of these are very distant, most of the stars whose radial velocities are known belong to a very limited group, perhaps constituting a distinct cluster of which the sun is a member, but in any event of insignificant proportions when contrasted with the Galaxy. ”

“ Every one now knows that the Milky Way, that girdle of light which spans the evening sky, is formed of clouds of stars too minute to be seen by the unaided vision. It seems to form the base on which the universe is built and to bind all the stars into a system. It comprises by far the larger number of stars that the telescope has shown to exist. Those we see with the naked eye are almost equally scattered over the sky. But the number which the telescope shows us become more and more condensed in the Milky Way as telescope power is increased. ”

“ He commonly observes with a magnifying power of one hundred and fifty, and is sure that after four or five years he will have passed in review every object above our horizon. He showed me the book in which his observations up to this time are written, and I am astonished at the great number of them. Each sweep covers 2° 15′ in declination, and he lets each star pass at least three times through the field of his telescope, so that it is impossible that anything can escape him. He has already found about 900 double stars and almost as many nebulæ. ”

“ The vibrations and tremors of the atmosphere are the greatest of all obstacles to astronomical observation, and the search is always in order for regions of the world, in deserts or on high mountains, where the quietest atmosphere is to be found.
Quite another power of the telescope is dependent on its objective solely: its light-gathering power. Light by which we see a star or planet is admitted to the retina of the eye through an adjustable aperture called the pupil. In the dark or at night, the pupil expands to an average diameter of one-fourth of an inch. ”

“ Because when the nodes are in the quadratures the body P is perpetually attracted from the plane of its orbit; and because this attraction is made towards S in its passage from, the node C through the conjunction A to the node D; and to the contrary part in its passage from the node D through the opposition B to the node C; it is manifest that, in its motion from the node C, the body recedes continually from the former plane CD of its orbit till it comes to the next node ”

“ Holden's and Schaeberle's views of planetary nebulæ, Burnham's and Hussey's hair's-breadth star-splitting [Pg 434] operations, Keeler's measurements of nebular radial motion, Barnard's detections and prolonged pursuit of faint comets, his discovery of Jupiter's tiny moon, Campbell's spectroscopic determinations—all this could only have been accomplished, even by an exceptionally able and energetic staff, with the aid of an instrument of high power and quality. But there was another condition which should not be overlooked.
The best telescope may be crippled by a bad situation. ”

“ A photographic campaign including spiral nebulæ of various types will settle the question of internal motions. The large scale of the spiral nebulæ at the principal focus of the Hooker telescope, and the experience gained in the measurement of nebular nuclei for parallax determination, will help greatly in this research. A multiple-slit spectrograph, already applied at Mount Wilson, will be employed, not only on spiral nebulæ whose plane is directed toward us, but also on those whose plane lies at an angle sufficient to permit both components of motion to be measured by the two methods. ”

“ The Nubecula Major, as the larger object is called, is a congeries of clusters of stars, of irregular form, globular clusters and nebulæ of various magnitudes and degrees of condensation, among which is interspersed a large portion of irresolvable nebulous matter, which may be, and probably is, star-dust, but which the power of the twenty-feet telescope shows only as a general illumination of the field of view, forming a bright ground on which the other objects are scattered. ”

“ But although these sometimes wander to distances considerably beyond the orbit of Neptune, it is probable that the extent of the void which separates our system from the nearest star is hundreds of times the distance of the farthest point to which a comet ever recedes. ”

“ Only a small part of the spectrum of a heavenly body, for instance, can be distinctly seen at one time; and a focal adjustment of half an inch is required in passing from the observation of a planetary nebula to that of its stellar nucleus. A refracting telescope loses, besides, one of its chief advantages over a reflector when its size is increased beyond a certain limit. ”

“ Now, let us remark what the telescope says the milky-way is—​an immense number, myriads upon myriads, of worlds! What a glorious view does this unfold to us of that God who has not only made the heavens, but us also!
But beside stars of various magnitudes, revealed by the telescope, there are other objects, called nebulæ, from their cloud-like appearance. These are of various sizes and forms, some being without defined shape, some being circular, some long and pointed, and one bearing a resemblance to a dumb bell. ”

“ More striking still is the motion of the planetary nebulæ, in excess of 25 kilometers per second, while type A stars move 11 kilometers, type G 15 kilometers, and type M 17 kilometers per second. Can the law connecting speed of motion and spectral type be so general that the planetary nebula is to be regarded as the final evolutionary stage? ”

“ His views on the nature of nebulæ underwent successive changes. At first he supposed all nebulæ to be but aggregations of stars. The logic was simple. To the naked [Pg 207] eye there are many groups of stars which appear nebulous. Praesepe is, perhaps, the best example. The slightest telescopic power applied to such groups alters the nebulous appearance, and shows that it comes from the combined and confused light of discrete stars. Other groups which remain nebulous in a seven-foot telescope, become stellar in a ten-foot. ”

“ The thousands of stars which spangle the heavens are all part of one "system;" but it has been found by the aid of the telescope that this system is but one out of many. Our system of stars occupies a space somewhat in the form of a thick lens or much-flattened sphere, but others are of very different forms, and some have but little regularity of form at all; our sun is one of the stars of this system. It is not known whether there are planets revolving round the other stars, the distance being far too great for any telescope to render them visible. ”

“ As the Earth moves from D to C, through the side AB of it’s Orbit, it is constantly meeting the light of Jupiter’s Satellites sooner, which occasions an apparent acceleration of their Eclipses: and as it moves through the other half H of it’s Orbit, from C to D, it is receding from their light, which occasions an apparent retardation of their Eclipses, because their light is then longer ere it overtakes the Earth. ”

“ Moving the star to ten times its present distance would increase the surface of the sphere which it illuminates a hundred-fold. We cannot move any special star, but we can examine stars of all brightnesses, and thus (presumably) of all distances.
Herschel's argument was, then, as follows: Since with such a telescope one can see a star ten times as far off as is possible to the naked eye, this telescope has the power of penetrating into space ten times farther than the eye alone. ”

“ All the matter composing all the bodies of the sun, planets, and their satellites, once existed in an exceedingly diffused state; rarer than any gas with which we are acquainted, filling a space larger than the orbit of Neptune. Gravitation gradually contracted this matter into a condensing globe of immense extent. Some parts would naturally be denser than others, and in the course of contraction a rotary motion, it is affirmed, would be engendered. Rotation would flatten the globe somewhat in the line of its axis. ”

“ For, when the right Ascension and Declination of any star is known, from astronomical tables, the telescope may be immediately adjusted to point to it, which will infallibly prevent his mistaking one star for another. In this way, likewise, the precise position of the planet Mercury, Uranus, Vesta, Juno, Ceres, Pallas—a small comet, a nebula, a double star, or any other celestial body not easily distinguishable by the naked eye, may be readily pointed out, when its right Ascension and Declination are known to a near approximation. ”

“ A section of the sidereal system along any great circle can be constructed from the data furnished by the gauges in the following way:
The solar system is within the mass of stars. From this point lines are drawn along the different directions in which the gauging telescope was pointed. On these lines are laid off lengths proportional to the cube roots of the number of stars in each gauge. The irregular line joining the terminal points will be approximately the bounding curve of the stellar system in the great circle chosen. Within this line the space is nearly uniformly filled with stars. ”

“ Accuracy in determining the distances of stars was just beginning to be reached, the clusters were obviously of all degrees of closeness following to the verge of irresolvability, and it was but natural to jump to the conclusion that the mystery of the nebulæ consisted in nothing but their vaster distance than that of clusters, and it was believed that all nebulæ would prove resolvable into stars whenever telescopes of sufficiently great power could be constructed. ”

“ The truth probably is that, as in ascending a mountain, we find the trees, which may be very dense at its base, thin out gradually as we approach the summit, where there may be few or none, so we might find the stars to thin out could we fly to the distant regions of space. The practical question is whether, in such a flight, we should find this sooner by going in the direction of the axis of our system than by directing our course towards the Milky Way. If a point is at length reached beyond which there are but few scattered stars, such a point would, for us, mark the boundary of our system. ”

“ He determined its orbit. He found that the solar system itself is the center of the machine-planet's orbit. It has neither aphelion nor perihelion, nor does its ecliptic ever shift. It is always perfect. ”

“ It yields to the first in the accuracy of the positions of the stars, but it is probably even superior to it in facilities for the correct representation of the nebula and stars considered as one mass. A piece of glass is ruled carefully into squares (see Figs. 6 and 7) and this is placed in the focus of the telescope so as to be plainly visible; the telescope is then directed upon the nebula, and a clock-work motion is applied to the telescope so that it follows the apparent motion of the nebula from east to west accurately. ”