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About 430 W. of this figure is El Melik, a star of the 3d magnitude, in the E. shoulder, and the principal one in this constellation. 10° S. W. of El Melik, is another star of the same magnitude, situated in the W. shoulder, called Sad es Saud.
Ancha of the 4th magnitude, is in the right side, 8° S. of El Melik. 9° E. of Ancha, is another star of the 4th magnitude, whose letter naine is Lambda.
Scheat, of the 3d magnitude, lying below the knee, is situated 830 S. of Lambda; and 14° S. of Scheat, the brilliant star Fomalhaut,* of between the 1st and 2d magnitudes, terininates the cascade in the mouth of the Southern Fish. This star is cominon to both these constellations, and is one of those from which the lunar distance is computed for ascertaining the longitude at sea. It culminates at 9 o'clock on the 22d of October.
Fomalhaut, Deneb Kaitos, and Alpha in the head of the Phoenix, make a large triangle, whose vertex is in Deneb Kaitos. Those two stars of the 4th magnitude, situated 4° S. of Sad es Saud, and nearly the same distance from Ancha, are in the tail of Capricorn. They are about 20 apart. The western one is called Deneb Algedi.
The rest of the stars in the cascade are quite small; they may be traced from the letter Y, in the urn, in a southeasterly direction towards the tail of Cetus, from which the cascade suddenly bends off near Scheat, in an opposite course, and finally disappears in the mouth of the Southern Fish, 30° S. of Y.
HISTORY.-This constellation is the famous Ganymede, a beautiful youth of Phrygia, son of Tros, king of Troy, or, according to Lucian, son of Dardanus. He was taken up to heaven by Jupiter as he was tending his father's flocks on Mount Ida, and became the cupbearer of the gods in place of Hebe. There are various opinions, however, among the ancients respecting its origin. Some sup. pose it represents Deucalion, who was placed among the stars after the celebrated deluge of Thessaly, 1500 years before the birth of our Saviour; while others think it designed to commeinorate Cecrops, who came from Egypt to Greece, founded Athens, established science, and introduced the arts of polished life.
The ancient Egyptians supposed the setting or disappearance of Aquarius, caused the Nile to rise, by the sinking of his urn in the water.-In the Zodiac of the Hebrews, Aquarius represents the tribe of Reuben.
PISCIS AUSTRALIS, VEL NOTIUS.
THE SOUTHERN FISH.-This constellation is directly S. of Aquarius, and is represented as a fish drinking the water which Aquarius pours from his urn. Its mean declination is 31° S. and its mean right ascension and time of passing the meridian are the same as those of Aquarius, and it is seen on the meridian at the same time; viz., on the 15th of October. It contains 24 visible stars, of which one is of the 1st magnitude or between the 1st and 2d, two are of the 3d, and five of the 4th. The first and most beautiful of all is Fomalhaut, situated in the mouth. This is 14° directly S. of Scheat in Aquarius, and may be seen passing the meridian low down in the southern hemisphere, on the 22d and 23d of October.
What is the name of the principal star in this constellation? What is its position? What star in the W. shoulder? Describe the situation of Ancha. What is the posi tion of Scheat and Fomalhaut? To what constellations is Fomalhaut common? Of what nautical importance is it? When does it culminate? With what other stars does it form a large triangle? How may you trace the stars in the cascade? Describe the situation and appearance of the Southern Fish. What are its mean right ascension and declination? When is it on the meridian? What is the whole number of its stars? What is the magnitude of its principal ones? What are the name and position of the most brilliant star in the constellation? When and where does it pass the meridian?
Its position in the heavens has been determined with the greatest possible accuracy, to enable navigators to find their longitude at sea.
The mode of doing this cannot be explained here. The problem is one of some difficulty. It consists in finding the angular distance between some star whose position is well known, and the moon when she is passing near it; also, the altitude of each, at the same instant, with good sextants. These data furnish the elements of a spherical triangle, the solution of which, after various intricate corrections, is made to result in the longitude of the given place.-See note to Arieties. In 1714, the British Parliament offered a reward of 10,000 pounds sterling, to any man who should discover a method of determining the longitude within 10, or 60 geographic miles of the truth; 15.000 pounds to the man who should find it within 40 miles, and 20,000 pounds, if found within 30 miles. These rewards in part have been since distributed among eminent mathematicians, in Europe, agreeably to the respective merits of their discoveries.
HISTORY.-This constellation is supposed to have taken its name from the transformation of Venus into the shape of a fish when she fled, terrified at the horrible advances of the monster Tvphon, as we have related in the mythology of the Fishes.-(See Pisces.)
VARIABLE AND DOUBLE STARS-CLUSTERS-NEBULE.
1. VARIABLE STARS.-The periodical variations of brilliancy to which some of the fixed stars are subject, may be reckoned among the most remarkable of their phenomena. Several stars, formerly distinguished by their splendour, have entirely disappeared; others are now conspicuous which do not seem to have been visible to the ancient observers; and there are some which alternately appear and disappear, or, at least, of which the light undergoes great periodic changes. Some seem to become gradually more obscure, as Delta in the Great Bear; others, like Beta in the Whale, to be increasing in brilliancy. Some stars have all at once blazed forth with great splendour, and, after a gradual diminution of their light, again become extinct. The most remarkable instance of this kind is that of the star which appeared in 1572, in the time of Tycho Brahe. It suddenly shone forth, in the constellation Cassiopeia, with a splendour exceeding that of stars of the first magnitude, even of Jupiter and of Venus, at their least distances from the earth; and could be seen, with the naked eye, on the meridian, in full day! Its brilliancy gradually diminished from the time of its first appearance, and at the end of sixteen months, it entirely disappeared, and has
For what purpose has its position been very accurately determined? Describe the periodical variations of brilliancy to which some of the fixed stars are subject? Mention some of the most remarkable instances of such variations, and describe them particularly.
never been seen since. (See a more particular account of this phenomenon, page 40.)
Another instance of the same kind was observed in 1604, when a star of the first magnitude suddenly appeared in the right foot of Ophiuchus. It presented, like the former, all the phenomena of a prodigious flame, being, at first, of a dazzling white, then of a reddish yellow, and, lastly, of a leaden paleness; in which its light expired. These instances prove that the stars are subject to great physical revolutions.-Page 41. A great number of stars have been observed whose light seems to undergo a regular periodic increase and diminution. They are properly called Variable Stars. One in the Whale has a period of 334 days, and is remarkable for the magnitude of its variations. From being a star of the second magnitude, it becomes so dim as to be seen with difficulty through powerful telescopes. Some are remarkable for the shortness of the period of their variation. Algol has a period of between two and three days; Delta Cephei, of 5 days; Beta Lyræ, of 6 2-5 days; and Mu Antinoi, of 7 days.
The regular succession of these variations precludes the supposition of an actual destruction of the stars; neither can the variations be supposed to arise from a change of distance; for as the stars invariably retain their apparent places, it would be necessary to suppose that they approach to, and recede from the earth in straight lines, which is very improbable. The most probable supposition is, that the stars revolve, like the sun and planets, about an axis. "Such a motion,” says the elder Herschel, "may be as evidently proved, as the diurnal motion of the earth. Dark spots, or large portions of the surface, less luminous than the rest, turned alternately in certain directions, either towards or from us, will account for all the phenomena of periodical changes in the lustre of the stars, so satisfactorily, that we certainly need not look for any other cause."
2. DOUBLE STARS.-On examining the stars with telescopes of considerable power, many of them are found to be composed of two or more stars, placed contiguous to each other, or of which the distance subtends a very minute angle. This appearance is, probably, in many cases, owing solely to the optical effect of their position relative to the spectator; for it is evident that two stars will appear contiguous if they are
What are such stars denominated? Describe the variations of one in the Whale. What stars are remarkable for the shortness of the period of their variations? Why may we not suppose that the stars which disappear are actually destroyed? Why may not the variations arise from a change of distance? What is the most probable supposition in regard to their cause? How does Dr. Herschel explain these phenomena On examining the stars with a telescope of considerable power, what other peculiarity do we had To what is this appearance, in many cases, owing?
placed nearly in the same line of vision, although their real distance may be immeasurably great.
There are, however, many instances in which the angle of position of the two stars varies in such a manner as to indicate a revolution about each other and about a common centre. In this case they are said to form a Binary System performing to each other the office of sun and planet, and are connected together by laws of gravitation like those which prevail in the solar system. The recent observations of Sir John Herschel and Sir James South, have established the truth of this singular fact, beyond a doubt. Motions have been detected, so rapid as to become measurable within very short periods of time; and at certain epochs, the satellite or feebler star has been observed to disappear, either passing behind or before the primary, or approaching so near to it that its light has been absorbed by that of the other.
The most remarkable instance of a regular revolution of this sort, is that of Mizar, in the tail of the Great Bear; in which the angular motion is 6 degrees and 24 minutes of a great circle, annually; so that the two stars complete a revolution about one another in the space of 584 years. About eleven twelfths of a complete circuit have been already described since its discovery in 1781, the same year in which the planet Herschel was discovered.
A double star in Ophiuchus presents a similar phenomenon, and the satellite has a motion in its orbit still more rapid. Castor, in the Twins,* Gamma Virginis, Zeta in the Crab, Zi Bootis, Delta Serpentis, and that remarkable double star 61 Cygni, together with several others, amounting to 40 in number,† exhibit the same evidence of a revolution about each other and about a common centre. But it is to be remembered that these are not the revolutions of bodies of a planetary nature around a solar centre, but of sun around suneach, perhaps, accompanied by its train of planets, and their satellites, closely shrouded from our view by the splendour of their respective suns, and crowded into a space bearing hardly a greater proportion to the enormous interval which separates them, than the distances of the satellites of our plan
* Page 67.
+ Herschel's Astronomy, page 391.
Are there, however, any instances where one star revolves with another around a common centre? When two stars are thus situated, what system are they said to form? Why is it thus denominated? What modern astronomers of great celebrity have established the truth of this theory? What rates of motion did they detect in these binary systems? What other interesting phenomena, indicating a mutual revolution, did they discover? What is the most remarkable instance of this fact? Mention some other instances. Are these revolving stars of a planetary nature? Of what nature are they?
ets from their primaries, bear to their distances from the su itself.
The examination of double stars was first undertaken by the late Sir Willian Herschel, with a view to the question of parallax. His attention was, however soon arrested by the new and unexpected phenomena which these bodies pre sented. Sir William observed of them, in all, 2400. Sir James South and Her schel have given a catalogue of 380 in the Transactions of the Royal Society, fo. 1824, and South added 458, in 1826. Sir John Herschel, in addition to the above published an account of 1000, before he left England for the Cape of Good Hope, where he is, at the time we write, pushing his discoveries in the southern hemisphere with great perseverance and success. Professor Struve, with the great Dorpat telescope, has given a catalogue of 3,063 of the most remarkable of these stars.
The object of these catalogues is not merely to fix the place of the star within such limits as will enable us easily to discover it at any future time, but also to record a description of the appearance, position, and mutual distances, of the individual stars composing the system, in order that subsequent observers may have the means of detecting their connected motions, or any changes which they may exhibit. Professor Struve has also taken notice of 52 triple stars, among which No. 11 of the Unicorn, Zeta of Cancer, and Zi of the Balance, appear to be ternary systems in motion. Quadruple and quintuple stars have likewise been observed, which also appear to revolve about a common centre of gravity; in short, every region of the heavens furnishes examples of these curious phe
Colour of the Stars.-Many of the double stars exhibit the curious and beautiful phenomenon of contrasted colours, or complimentary tints. In such instances, the larger star is usually of a ruddy or orange hue, while the smaller one appears blue or green, probably in virtue of that general law of optics, which provides, that when the retina is under the influence of excitement by any bright, coloured light, feebler lights, which seen alone would produce no sensation but that of whiteness, shall for the time appear coloured with the tint complimentary to that of the brighter. Thus, a yellow colour predominating in the light of the brighter star, that of the less bright one, in the same field of view, will appear blue; while, if the tint of the brighter star verge to crimson, that of the other will exhibit a tendency to green-or even appear a vivid green. The former contrast is beautifully exhibited by Iota, in Cancer; the latter by Almaach, in Andromeda—both fine double stars. If, however, the coloured star be much the less bright of the two, it will not materially affect the other. Thus, for instance, Eta Cassiopeia exhibits the beautiful combination of a large white star, and a small one of a rich ruddy purple.
It is not easy to conceive what variety of illumination two suns—a red and a green, or a yellow and a blue one-must afford to a planet revolving about either; and what charming
What beautiful and curious phenomenon has been observed, as it regards the colour of double stars? Explain how these colours are usually contrasted. Mention an example of this phenomenon. How, if the coloured star be much the less bright of the two, will the other be affected? Give an instance. What may be the effect of such a variety of color in solar light?