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round Castor will be nearly 342 years and 2 months, in a retrograde direction.

Of the two stars which compose y Leonis, the smaller one revolves round the larger in an apparent elliptical orbit, and performs a retrograde revolution in 1200 years.

The beautiful double star, & Boötes, is composed of 2 stars, one of which is of a light red, and the other of a fine blue, having the appearance of a planet and its satellite. From observation Dr. Herschel concludes, that the orbit of the small star is elliptical, and performs its revolution, according to the order of the signs, in 1681 years.

The double star, ( Herculis is composed of a greater and lesser star; the former being of a beautiful bluish-whit and the latter of a fine ash colour. The smaller one revo. round the larger, nearly in the plane of the spectator. On the 11th of April, 1803, it was nearly occulted by the larger star.

The double star, & Serpentis, has, like & Boötes, undergone a considerable change in the angle of position, without any variation in the distance between the two stars. Dr. Her schel computes the period of the smaller star round the larger to be 375 years.

The double star, y Virginis, which has long been known to astronomers, is composed of two nearly equal stars; the smaller, according to Dr. Herschel, completes its revolution in about 708 years.

When we take into consideration the very small angle which the apparent distance of these double stars subtends, and the slow motion of the revolving stars, we must conclude that the period of their respective revolutions cannot be ascertained with any great degree of accuracy. Squire's Astronomy.

4. From former observations, it appears considerable changes have taken place among the fixed stars. Stars have disappeared, and new ones have appeared.

The most remarkable new star recorded in history, was that which appeared in 1572, in the chair of Cassiopeia. It was for a time brighter than Venus, and then seen at midday: it gradually diminished its lustre, and after 16 months disappeared. That the circumstances of this star were faithfully recorded we can have no doubt, since many different astronomers of eminence saw and described it. Cor nelius Gemma viewed that part of the heavens, the sk

being very clear, and saw it not. The next night it appeared with a splendour exceeding all the fixed stars, and scarcely less bright than Venus. Its colour was at first white and splendid, afterwards yellow, and in March, 1573, red and fiery like Mars or Aldebaran; in May of a pale livid colour, and then became fainter and fainter till it vanished.

Another new star, little less remarkable, appeared in Oct. 1604. It exceeded every fixed star in brightness, and even appeared larger than Jupiter. Kepler wrote a dissertation on it. Changes have also taken place in the lustre of the fixed stars; Aquilæ is now considered less bright than y. A small star near & Ursa Majoris is now probably more bright than formerly, from the circumstance of its being named Alcor, an Arabic word, which signifies sharp-sightedness in the person who could see it. It is now very visible.

Several stars also change their lustre periodically. o Ceti, in a period of 333 days, varies from the 2d to the 6th magnitude. The most striking of all is Algol, or Persei. Goodricke has with great care determined its periodical variations. Its greatest brightness is of the 2d, and least of the 4th magnitude; its period is only 2 days, 21 hours: it changes from the 2nd to the 4th magnitude in 35 hours, and back again in the same time, and so remains for the rest of the 2d. 21 hours. These singular appearances may be explained, by supposing the fixed star to be a body revolving on an axis, having parts of its surface not luminous.

The following are some of the most remarkable variable stars, viz :

New star of 1572 in Cassiopeia, which changes from the 1 to 0; that is, from the first magnitude to be invisible; period 150 years.

• Of the Whale, from 2 to 0; period 333 days. New star of 1604, in the east foot of Serpentarius, from 1 to 0; period not known.

Lyræ, from 3 to 5; period 6 days, 9 hours. New star of 1670, in the Swan's head, which has not peen seen since 1672.

η Antinoi, from 3 to 5; period 7 days, 4 hours, 15 minutes. x In the Swan's neck, from 5 to 0; period 369 days, 21 hours.

Another in the same constellation, near y in the breast; from 3 to 0; period 18 years.

d Chephei, from 3 to 5; period 5d. 8h. 37m.

Some stars, like ẞ in the Whale, have gradually in

creased in brilliancy; others, like d in the Great Bear, have been constantly diminishing in brightness.

5. The number of nebulæ is very considerable. Dr. Herschel has discovered above 2000: before his time only 103 were known. But far the greater part of these 2000 can be seen only with telescopes equal to his own.

He has given an account of several phenomena, which he calls nebulous stars, stars surrounded with a faint luminous atmosphere. He describes one observed Nov. 13, 1790, in the following manner. A most singular phenomenon: a star of the Sth magnitude, with a faint luminous atmosphere, of a circular form, and of about 3' diameter; the star is exactly in the centre, and the atmosphere is so diluted, faint, and equal throughout, that there can be no surmise of its consisting of stars; nor can there be a doubt of the evident connexion between the atmosphere and the star. Another star, not much less in brightness, and in the same field with the above, was perfectly free from any such appearance.

6. As the earth moves in an orbit, nearly circular, round the sun, an observer on its surface in one situation, is nearer some stars by the diameter of the earth's orbit, than in another, and consequently the angular distances of those stars ought to appear greater. But the angular distances of the fixed stars, observed at different seasons of the year, always remain the same, even when observed with the most exquisite instruments. Hence, the diameter of the earth's orbit, which is about 190 millions of miles, bears no sensible proportion to their distance.

The greatest angle which the diameter of the Earth's orbit subtends at any fixed star, is called its annual parallax, and sometimes only parallax.

According to the observations of Dr. Bradley, the annual parallax of y Draconis, a star of the second magnitude, situate nearly in the solstitial colure, about 150 from the pole of the ecliptic, is imperceptible. The observations of Pond, the present astronomer at Greenwich, and those of Dr

Brinkley, of Trinity College, Dublin, agree also as to this star, in showing that the annual parallax is imperceptible. Now, admitting the annual parallax of the nearest fixed star, suppose Sirius, to be 2", then its distance from the Earth would be no less than 9,797,587,500,000 miles, or nearly ten million million of miles. But admitting the parallax of the nearest fixed star to be only 1", which is probably too great, then its distance will be nearly twenty million million of miles.

The parallax of any fixed star has been, till lately, thought imperceptible. Piazzi, from his observations made at Palermo, suspected a parallax of a few seconds in several stars. Dr. Brinkley, who has paid particular attention to this subject, says, that his observations made with the circle, eight feet in diameter, belonging to the observatory of Trinity College, Dublin, appeared to point out a parallax in several stars; and that the agreement of results, obtained by different sets of observations, seemed to leave no doubt on this head. However, observations made elsewhere do not confirm his results.

The distance of the fixed stars, proved by the motion of the Earth, is indeed wonderful, yet there is nothing contrary to reason and experience in admitting it. Why should we, as Dr. Brinkley justly observes, limit the bounds of the universe by the limits of our senses? We see enough in every department of nature, to deter us from rejecting any hypothesis, merely because it extends our ideas of the creation and divine Creator.

Our knowledge of the fixed stars must be much more circumscribed than of the planets, since the best telescopes do not magnify the fixed stars so as to submit their diameters to measurement; but it is well ascertained that the apparent diameter of the brightest of them is less than 1". The fixed stars, as we have seen, are at immeasurable distances from us, at distances compared with which the whole solar system is but a point. Their diameters are less than we can measure, yet their light is more intense than that of the planets. We conclude, therefore, that they are self-shining bodies, and according to a high degree of probability, like our Sun, the centre of planetary systems. Admitting this, the multitudes of fixed stars that may be discovered with the most inferior telescopes, show us an extent of the universe that our imagiuation can scarcely comprehend; but what is even this, compared to the extent that the discoveries and conjectures of Dr. Herschel point out? We cease to have distinct ideas, when we numerate by ordinary measures the distances of the fixed stars, and we require the aid of other circumstances to enable us to comprehend them. Thus, we compute that the nearest of the fixed stars is so far distant, that light, which moves at the rate of 192,900 miles in a second, will take above a year in coming from the star to the Earth; that the light of many telescopic stars may have been many hundred years in reaching us; and still farther, that, according to Dr. Herschel, the light of some of the nebulæ, just perceptible in his forty feet telescope, has been above a million of years on its passage. The limit of the distance of the nearest fixed star, may be considered as well ascertained; but any thing advanced with respect to the distances of the others, must be in a manner conjectural.

The brighter fixed stars have been supposed to be nearer to us than the rest. Besides their superior lustre leading) this conclusion, many of them were discovered to have small motions, called proper motions, that could only be explained by supposing them to arise from a real motion in the stars themselves, or in the Sun and solar system, or from a motion compounded of both these circumstances.

Now, whichever of these suppositions was adopted, it was reasonable to suppose, that the cause of the smaller stars not appearing to be affected, could only arise from the greater distance of those stars. However, it is now ascertained that some of the smaller stars appear to have proper motions, much greater than those of the brightest stars. Hence, conclusions deduced from the proper motions of the bright stars, respecting the relative distances of those stars, must tend to weaken conclusions that might be deduced from their brightness and apparent magnitudes.

There is a double star of the sixth magnitude, the 61st star of the Swan, which consists of two stars, within a few seconds of each other. Each of these stars are moving nearly at the same rate, that is, at the rate of 6" in a year. It is likely they are also moving about their common centre of gravity. At present they preserve nearly the same distance from each other. This proper motion is far greater than has been observed in any of the brighter stars, or indeed in any star. It might be supposed, on this account, that these stars (61 cygni) are nearer to us than the brighter stars. To ascertain this point, Dr. Brinkley has made observations of the zenith distances, at the opposite seasons, and he could not discover any sensible parallax parallax in these stars.

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