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actly into the same situation, with respect to the stars, as it was on the same day of a former year,-on the same day of any year the same stars will always come to the meridian at the same time.
A siderial day, by which astronomers generally reckon, is 4 m. shorter than the solar one ; and a siderial year (that is, the time in which the earth completes a revolution in its orbit, as indicated by its return to the same point in the heavens) is 365 d. 6 h. 9 m. 9.6 s. long, ac. cording to solar time, but is 366 d. 6 h. 9 m. 9.6 s. reckoned in siderial time. Civil time is regulated by the equinoctial year, which is the interval between two returns of the sun to the same equinox. Its duration is 365 d. 5 h. 48 m. 49.7 s.
OF NEW, TEMPORARY, VARIABLE, AND DOUBLE STARS.
Several stars which are marked in the early catalogues, and some which have been observed by modern astronomers, have disappeared from the sky. On the other hand, there are some stars now in the heavens which have only recently become visible; these are termed new stars.
A kindred phenomenon is that of stars becoming suddenly visible, shining for a brief period with considerable splendour, and then disappearing altogether. These are temporary stars.
The earliest star of this kind on record is that which suddenly appeared in the year 125 B.C. It was observed by Hipparchus, who was in consequence induced to draw up a catalogue of the stars, the earliest on record. Another blazed forth in A.D. 389, remained three weeks as bright as Venus, and then disappeared altogether. There are records of similar appearances in the years 945, 1264, and 1572. The star of 1572 was observed by Tycho Brahe. His attention was drawn to it by observing one evening (Nov. 11) on his return from his observatory to his dwelling-house, a group of country people gazing at a star which he was sure did not exist half an hour before. It was then as brilliant as Sirius and continued to increase, till it surpassed Jupiter when brightest, and was even visible at mid-day. It began to diminish in Dec. of the same year; and in March, 1574, it had entirely disappeared. The stars of 945, 1264, and 1572 appeared in the same part of the heavens, between Cepheus and Cassiopeia. This circumstance, combined with the fact of their having appeared at nearly equal intervals, has suggested the idea that the appearances are owing to the revolution of the same star in an extremely eccentric orbit. This conjec
ture will be confirmed or overthrown in about 30 or 40 years, when, if correct, the star will re-appear.
There is a class of stars denominated variable, whose light undergoes a periodical increase and diminution.
One of the most remarkable of these is Algol in Medusa's head. It is usually visible as a star of the 2nd mag., and as such continues visible for 2 d. 14 hrs. when it suddenly begins to diminish in splendour, and in about 3} hrs. is reduced to the 4th mag. Its feeblest lustre lasts little more than 15 m., when it begins to increase, and in 31 hrs. more is restored to its usual brightness. Its full period is 2 d. 20 hrs. 48 m. Another of these stars is o Ceti, called also Mira, or the wonderful star. It goes through its phases in 331 d. 10 hrs. When brightest it is of the 2nd mag., and remains so for about a fortnight. It then decreases during 3 months, when it passes out of sight, continues invisible for 5 months, and then reappears. There are altogether about 20 stars ascertained to be variable, and upwards of 50 suspected to belong to the class.
Many stars are multiple, that is, they appear to be single when seen by the naked eye, but are found to consist of two or more when viewed through a telescope of sufficient power.
The number of these double stars, and the smallness of the interval between the stars so conjoined, forbid the idea of an accidental contiguity. This is entirely put out of the question by the discovery of the fact that many of these double stars revolve round a common centre. Fifty or sixty instances have been noticed of the two stars revolving about each other in regular orbits, and constituting what are termed binary stars, to distinguish them from double stars generally so called. Some of the most remarkable of these are Castor, whose constituents complete a revolution in 252 yrs.; m Coronæ in 43 yrs. ; & Ursæ in 58 yrs.; 61 Cygni in 452 yrs. ; y Virginis in 629 yrs.
Besides binary stars, systems consisting of three, four, and even five individuals, have been found to exist.
Many of the double stars exhibit the curious and beautiful phenomena of contrasted colours. In such instances the larger star is usually of a ruddy or orange hue while the smaller one appears blue or green. Persons possessed of ordinary telescopes will get an illustration of this by observing the star Albireo in the beak of the Swan.
The whole number of stars whose multiple character has been ascertained, cannot be less than 6,000.
OF NEBULÆ AND CLUSTERS OF STARS.
The most astonishing as well as most mysterious objects in the heavens are nebulæ, which are misty patches of light, and are scattered in considerable numbers through space. When viewed through powerful telescopes some of them are found to consist of clusters of stars; these are termed resolvable nebulæ ; in other cases the diffused luminosity continues unbroken; these nebulæ are said not to be resolvable.
On a clear winter's night a faint light is observed to envelope the six or seven stars which form the Pleiades; a common telescope will show that it is occasioned by a cluster of stars too minute to be individually recognized by the naked eye. A cluster in the sword handle of Perseus affords a similar and very beautiful exhibition; and the luminous spot in Cancer, called Præsepe or the bee-hive, may with equal facility be resolved. In the constellation Hercules, midway between the stars on and & is a nebula which in favourable circumstances may be seen by the naked eye. Viewed through an ordinary telescope it appears exactly like a small round comet without a tail, but when very high powers are applied to it, an appearance like that figured in the cut is presented. “ It would be a vain task (says Sir John Herschel) to count the stars in one of these globular clusters. They are not to be reckoned by hundreds. On a rough calculation it would appear that many of them must contain at least ten or twenty thousand stars compacted and wedged together in an area not more than a tenth part of that covered by the moon.” And yet, in all probability, the individuals of such a group are suns like our own, and their mutual distances equal to those which separate our sun from the nearest fixed star! The works of God baffle our comprehension-how mysteriously great and glorious is the Creator! “ Canst thou by searching find out God? canst thou find out the Almighty unto perfection ? "
Many, however, of the nebulæ (and such only are properly so called) cannot be resolved. A remarkable nebula of this kind is in the constellation Andromeda, near the star y. It is visible, under favourable circumstances, to the naked eye, and when seen through a telescope has the appearance of a Candle shining through a horn (see cut.) It has in it a few small stars; but they are obviously casual, and the nebulæ itself offers not the slightest appearance to give ground for a suspicion of its consisting of stars." A telescope of very moderate powers directed to the middle star of the three forming the sword of Orion will reveal a very beautiful nebula. Between the stars B and y Lyræ (see fig. of Lyra) is a nebula that has the appearance of a ring, That is easily seen.
The number of nebulæ already catalogued is between two and three thousand.
It is considered by many that our sun partakes of the nebulous character. The peculiar appearance called the zodiacal light has led to thus conjecture. It is easily seen after almost any clear sunset in tropical climates, but in this country it can only be distinguished in the evening about the months of April and May, or at the opposite season before sunrise. It consists of a long train of faint light of a conical form surrounding the sun and following generally the course of the ecliptic. It extends beyond the orbit of Venus.
PROBLEM I. To find the Right Ascension and Declination of any Star.
Bring the star to the brass meridian ; the degree of the meridian over it is the declination, and the degree of the equinoctial under the meridian the right ascension.
Declination on the celestial globe is the same thing as latitude on the terrestrial, and right ascension the same thing as longitude.
The right ascension may otherwise be found by elevating the globe for a right sphere, (viz. bringing the poles to coincide with the horizon,) and bringing the star to the eastern horizon ; the point of the equinoctial that comes to the horizon at the same time will be the right ascension. It may be expressed either in degrees or hours.
The use of the declination is principally to find the latitude of any place by the altitude of the stars.
1. What are the right ascension and declination of Sirius ? Ans. Rt. as. 99° 0', or 6 hrs. 36 m. Dec. 16° 27' S.
2. Required the rt. as. and dec. of the pole star. Ans. Rt. as. 13° 0', or 0 h. 52 m. Dec. 88° 14' N.
Required the right ascension and declination of
3. Andromeda's Girdle, Mirach, B. 4. Ram's Following Horn, a. 5. Whale's Jaw, Menkar, a. 6. Medusa's Head, Algol, B. 7. Perseus' Side, Algenib, a. 8. Brightest of the Seven Stars. 9. Bull's Eye, Aldebaran, a. 10. Auriga's Shoulder, Capella, a. 11. Orion's Foot, Rigel, B. 12. Bull's N. Horn, B. 13. Orion's Left Shoulder, Bellatrix, y. 14. Orion's Girdle, s. 15. Orion's Right Shoulder, Betelguese, a. 16. First Twin, Castor, a. 17. Little Dog, Procyon, a. 18. Second Twin, Pollux, ß. 19. Boötes, Arcturus, a. 20. Lyra, a.
PROBLEM II. Having the Right Ascension and Declination of a Star, to
find it on the Globe. Bring the right ascension, marked on the equinoctial, to the brass meridian ; then, under the given declination marked on the meridian, will be the star required.
EXAMPLES.—Required the stars whose right ascension and declination are as follow :