Lucid spots. Cloudy stars. are situate in that part of the heavens. This track appears single in some parts, in others double. 364. There are several little whitish spots in the heavens, which appear magnified, and more luminous when seen through telescopes; yet without any stars in them. One of these is in Andromeda's girdle, and was first observed A. D. 1612, by Simon Marius: it has some whitish rays near its middle, is liable to several changes, and is sometimes invisible. Another is near the ecliptic, between the head and bow of Sagittarius: it is small, but very luminous. A third is on the back of the Centaur, which is too far south to be seen in Britain. fourth, of a smaller size, is before Antinous's right foot, having a star in it which makes it appear more bright. A fifth is in the constellation of Hercules, between the stars and, which spot, though but small, is visible to the bare eye, if the sky be clear, and the Moon absent. A 365. Cloudy stars are so called from their misty appearance. They look like dim stars to the naked eye; but through a telescope they appear broad illuminated parts of the sky; in some of which is one star, in others more. Five of these are mentioned by Ptolemy. 1. One at the extremity of the right hand of Perseus. 2. One in the middle of the Crab. 3. One, unformed, near the sting of the Scorpion. 4. The eye of Sagittarius. 5. One in the head of Orion. In the first of these appear more stars through the telescope than in any of the rest, although 21 have been counted in the head of Orion, and above forty in that of the Crab. Two are visible in the eye of Sagittarius without a telescope, and several more with it. Flamstead observed a cloudy star in the bow of Sagittarius, containing many sinall stars: and the star d above Sagittarius's right shoulder is encompassed with several more. Both Cassini and Flamstead discovered one between the Great and Little Dog, which is very full of stars, Magella nic visible only by the telescope. The two whitish spots near the south pole, called the Magellanic clouds by sailors, which to the bare eye resemble part of the Milky Way, appear through telescopes to be a mixture of small clouds and stars. But the most re- clouds. markable of all the cloudy stars is that in the middle of Orion's sword, where seven stars (of which three are very close together) seem to shine through a cloud, very lucid near the middle, but faint and illdefined about the edges. It looks like a gap in the sky, through which one may see (as it were) part of a much brighter region. Although most of these spaces are but a few minutes of a degree in breadth, yet, since they are among the fixed stars, they must be spaces larger than what is occupied by our solar system; and in which there seems to be a perpetual uninterrupted day, among numberless worlds,which no human art ever can discover. -vens. 366. Several stars are mentioned by ancient astro- Changes nomers, which are not now to be found; and others in the hea are now visible to the bare eye, which are not recorded in the ancient catalogue. Hipparchus observed a new star about 120 years before CHRIST; but he has not mentioned in what part of the heavens it was seen, although it occasioned his making a catalogue of the stars; which is the most ancient that we have. The first new star that we have any good account New stars of, was discovered by Cornelius Gemma on the 8th of November, A. D. 1572, in the chair of Cassiopea. It surpassed Sirius in brightness and magnitude; and was seen for 16 months successively. At first it appeared bigger than Jupiter, to some eyes, by which it was seen at mid-day; afterwards it decayed gradually both in magnitude and lustre, until March 1573, when it became invisible. On the 13th of August 1596, David Fabricius. observed the Stella Mira, or wonderful star, in the. neck of the Whale; which has been since found to appear and disappear periodically seven times in six Cannot be comets. years, continuing in the greatest lustre for 15 days together; and is never quite extinguished. In the year 1600, William Jansenius discovered a changeable star in the neck of the Swan; which, in time, became so small as to be thought to disappear entirely, till the years 1657, 1658, and 1659, when it recovered its former lustre and magnitude, but soon decayed; and is now of the smallest size. In the year 1604, Kepler and many of his friends saw a new star near the heel of the right foot of Serpentarius, so bright and sparkling, that it exceeded any thing they had ever seen before; and took notice that it was every moment changing into some of the colours of the rainbow, except when it was near the horizon, at which time it was generally white. It surpassed Jupiter in magnitude, which was near it all the month of October, but easily distinguished from Jupiter by the steady light of that planet. It disappeared between October 1605, and the February following, and has not been seen since that time. In the year 1670, July 15, Hevelius discovered a new star, which in October was so decayed as to be scarce perceptible. In April following it regained its lustre, but wholly disappeared in August. In March 1672, it was seen again, but very small; and has not since been visible. In the year 1686, a new star was discovered by Kirch, which returns periodically in 404 days. In the year 1672, Cassini saw a star in the neck of the Bull, which he thought was not visible in Tycho's time; nor when Bayer made his figures. 367. Many stars, beside those above-mentioned, have been observed to change their magnitudes; and as none of them could ever be perceived to have tails, it is plain they could not be comets; especially as they had no parallax, even when largest and brightest. It would seem that the periodical stars have vast clusters of dark spots, and very slow rotations on their axes; by which means, they must disappear when the side covered with spots is turned towards us. And as for those which break out all of a sudden with such lustre, it is by no means improbable that they are Suns whose fuel is almost spent, and again supplied by some of their comets falling upon them, and occasioning an uncommon blaze and splendour for some time: which indeed appears to be the greatest use of the cometary part of any system*. Some of the stars, particularly Arcturus, have been Some stars. observed to change their places above a minute of a their plachange degree with respect to others. But whether this be ces. owing toany real motion in the stars themselves, must require the observations of many ages to determine. If our solar system change its place with regard to absolute space, this must in process of time occasion an apparent change in the distances of the stars from each other and in such a case, the places of the nearest stars to us being more affected than those which are very remote, their relative positions must seem to alter, though the stars themselves were really immoveable. On the other hand, if our own system be at rest, and any of the stars in real motion, this must vary their positions; and the more so, the nearér they are to us, or the swifter their motions are; or the *M. Maupertuis, in his Dissertation on the figures of the Celestial Bodies (p. 91-93), is of opinion that some stars, by their prodigious quick rotations on their axes, may not only assume the figures of oblate spheroids, but that by the great centrifugal force arising from such rotations, they may become of the figures of mill-stones; or be reduced to flat circular planes, so thin as to be quite invisible when their edges are turned toward us; as Saturn's ring is in such positions. But when any eccentric planets or comets go round any flat star, in orbits much inclined to its equator, the attraction of the planets or comets in their perihelions must alter the inclination of the axis of that star; on which account it will appear more or less large and luminous, as its broad side is more or less turned toward us. And thus he imagines we may account for the apparent changes of magnitude and lustre in those stars, and likewise for their appearing and disappearing. The ecliptic less ob to the equator than for merly. more proper the direction of their motion is for our 368. The obliquity of the ecliptic to the equinoc lique now tial is found at present to be above the third part of a degree less than Ptolemy found it. And most of the observers after him found it do decrease gradually down to Tycho's time. If it be objected, that we cannot depend on the observations of the ancients, because of the incorrectness of their instruments; we have to answer, that both Tycho and Flamstead are allowed to have been very good observers; and yet we find that Flamstead makes this obliquity 2 mi. nutes of a degree less than Tycho did, about 100 years before him: and as Ptolemy was 1324 years before Tycho, so the gradual decrease answers nearly to the difference of time between these three astronomers. If we consider, that the Earth is not a perfect sphere, but an oblate spheroid, having its axis shorter than its equatorial diameter; and thatthe Sun and Moon are constantly acting obliquely upon the greater quantity of matter about the equator, pulling it as it were toward a nearer and nearer coincidence with the ecliptic; it will not appear improbable that these actions should gradually diminish the angle between those planes. Nor is it less probable that the mutual attraction of all the planets should have a tendency to bring their orbits to a coincidence; but this change is too small to become sensible in many ages. * * M. de la Grange has demonstrated, in the most satisfactory manner, that no permanent change can take place in the magnitudes, figures, or inclinations, of the planetary orbits; and that the periodical changes are confined within very narrow limits: the ecliptic therefore, will never coincide with the equator, nor change its inclination above 2 degrees. In short, the solar planetary sy stem oscillates, as it were, round a medium state, from which it never swerves very far. See note subjoined to p. 116. |