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transits of 1761 and 1769 ? If the distance of the Earth from the Sun be known, how can the distance of the other planets be ascertain. ed? Was there much interest felt in the transit of 1761 ? Where was it observed? What prevented the transit from being fully observed in some places ? From the various observations on the parallax, what did Mr. Short make the parallax ? From this, what would be the mean parallax? What effect had the observations on the transit of 1769 on the result of the preceding observations ?
The Fixed Stars.
The fixed stars are so denominated from their always retaining the same situation in relation to each other. We have seen, that the Earth is, at one season of the year, 190,000,000 miles distant from its situation at the opposite season ; yet these stars have no sensible parallax. The star which is north at one time, is north at any
other time. Most of the stars, indeed, appear to have diurnal revolution round the Earth; but this arises from the rotation of the Earth on its axis, and is no more than is caused by that rotation.
That the stars always retain the same apparent situation, must be owing to their immense and inconceivable distance. Let two persons be placed one rod distant from each other, east and west. An object, ten rods distant, which is due north from one, will easily be perceived not to be north of the other. But let the object be ten miles distant from these observers, and if it be north of one, it will scarcely be perceived not to be north of the other; the angle can be ascertained only by nice observation. Let this principle be applied to the fixed stars, and the student will be sensible, that their
distance is truly immense. We form very inadequate ideas of the Earth's distance from the Sun ;* of course of twice that distance. But this immense distance, 190,000,000 miles, makes no perceptible difference in the situation of the fixed stars, even when viewed with the nicest instruments. “ From what we know,” says Mr. Ferguson,“ of the immense distance of the stars, the nearest may be computed at 32,000,000,000,000 of miles from us, which is farther than a cannon ball would fly in 7,000,000 of years.'
From the distance of the stars it may be concluded, that they shine by their own native light; and not by the reflected rays of the Sun. For those rays, decreasing in number in any given space as the squares of the distances increase, cannot by reflected light make objects visible at a distance so inconceivably great.
The fixed stars are, without doubt, suns to other systems. Thus they are now considered by the unanimous consent of astronomers. They may be distinguished from the planets by the twinkling of their light. The diameter of a star appears much less viewed through a good telescope, than when seen without the aid of instruments.
Not more than 1000 stars are visible to the naked eye in either hemisphere. They seem, indeed, innumerable, when, in a clear evening, we turn our eyes towards the heavens. But, in attentive observation, most of those bright spots, which appeared to be stars, vanish. They are probably reflections from minute particles of various kinds continually floating in our atmosphere. The British catalogue contains not more than about 3,000 stars, in both hemispheres ; though it includes many not visible to the naked eye. By improved reflecting telescopes the number is found to be great beyond all conception. “Dr. Herschel says that, in the most crowded part of the milky way, he has had fields of view, that contained no less than 588 štars, and
* See the time it would require a courier to pass from the Earth to the Sun, Chap. I., Sec. VII.
these were continued for many minutes ; so that, in a quarter of an hour, he has seen 116,000 stars pass through the field of view of a telescope of only 15' aperture ; and at another time, in 41 minutes, he saw 258,000 stars, pass through the field of his telescope."*
Many stars appear single to the naked eye, which, on being viewed with a good telescope, are found to consist of two, three, or more stars. Some are denominated by Dr. Herschel insulated stars, because they seem removed from the attractive force of other stars. Such are our Sun, Arcturus, Capella, Sirius, and many others.
“A binary sidereal system, or double star, properly so called, is formed by two stars situated so near each other as to be kept together by their mutual attraction." It is, however, evident that stars may be situated, one nearly behind the other, so as to appear binary, though immensely distant.
The double star Epsilon, Boötes, is beautiful, composed of two stars, one light red, the other a fine. blue. Plate viii. Fig. 3, represents this star, as seen by telescopes of different magnifying powers.
The double star Zeta, in the constellation Hercules, is composed of two stars; the greater a beautiful bluish white, the less a fine ash color.
The star Delta, of the Swan, is binary, composed of two stars very unequal in their apparent magnitude; the larger white, the less reddish.
The pole-star is binary, composed of two stars of very unequal magnitude; the larger white, the less red. In Plate viïi. Fig. 4, is represented the treble star in the left fore foot of the constellation Monoceros, one of the most beautiful objects of the kind in the heavens.
The Beta, in the constellation Lyra, or the Harp, is quadruple, white, but three of them a little inclined to red.
* We are not told his manner of counting.
The Lambda, in Orion, is quadruple. More properly, it is a double star with two stars at a small distance. The double star is unequal ; the largest white, the smallest a pale rose color.
A catalogue of the principal double stars may be seen in Dr. Brewster's supplement to Ferguson. Its insertion here would far exceed the limits designed for this compend.
Several stars have appeared for a time in the heavens, and then disappeared. In ancient catalogues, stars are enumerated, which are not now to be seen, even by the powerful instruments of modern astronomy. Others are now visible, which seem not to have been noticed by the ancients.
A new star was discovered by Cornelius Gemma, in 1572, in the chair of Cassiopeia. It surpassed Sirius in brightness and magnitude. To some eyes it appeared larger than Jupiter ; and might be seen at mid-day. It afterwards gradually decreased, and, after sixteen months, entirely disappeared.
In 1596, the Stella Mira, or wonderful star, in the neck of the whale, was observed by Fabricius. It seemed alternately to vanish and reappear seven times in six years. During this time, however, it is said never to have been entirely extinct.
In 1600, a changeable star, in the neck of the Swan, was observed by Jansenius. The same was observed, and its place determined, by Kepler. It was seen by Riciolus, in 1616, 1621, and 1624. But, from 1640 to 1650, it was invisible. It had several instances of appearing and again vanishing, prior to the year 1715; when it re-appeared as a star of the sixth magnitude, its present appearance.
In 1604, a new star was discovered by Kepler and some of his friends near the head of Serpentarius. It exhibited a bright and sparkling appearance, beyond any they had before seen. Assuming the different colors of the rainbow, it appeared every moment changing, ex
cept near the horizon, where it generally appeared white. It was near Jupiter in October of that year,
and surpassed that planet in magnitude, but disappeared before the following February.
Several other stars have appeared, vanished, and reappeared ; some of them in regular succession. Such changeable stars may be suns, having extensive spots. Stars of this kind, by a regular rotation on their axes, may alternately present their dark and luminous sides. "Maupertuis is of opinion, that some stars, by their prodigious quick rotation on their axes, may not only assume the figure of oblate spheroids, but, by their great centrifugal force, arising from such rotation, they may become of the figure of millstones, or reduced to flat circular plates, so as to be quite invisible, when their edges are turned towards us ; as Saturn's ring is in such positions. But when any eccentric planets or comets go
flat star, in orbits much inclined to its equator, the attraction of the planets or comets, in their perihelia, must alter the inclinations of that star; on which account it will appear more or less large and luminous, as its broad side is more or less turned towards us. Ferguson.
The propriety of the term fired, as applied to the stars, seems rendered at least doubtful by the observations of modern astronomers. An advancement of the solar system, in absolute space, is now considered certain. It was observed by Halley and Cassini. The first explanation of it was given by Mayer. But, to point out the region in the heavens to which the solar system is advancing, was reserved to Dr. Herschel. " He has examined this subject with his usual success, and has certainly discovered the direction in which our system is gradually advancing. He found that the apparent proper motion of about forty-four stars out of fifty-six, is very nearly in the direction which would result from a motion of the Sun towards the constellation Hercules, or, more accurately, to a place in the heavens,