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about one-third part of the whole quantity: At the horizon, in this climate, it is found to be about 33'. In climates nearer to the equator, where the air is purer,
the refraction is less; and in the colder climates, nearer to the pole, it increases exceedingly ; and, is a happy provision for lengthening the ap-pearance of the light, at those regions so remote from the sun. Gassendus relates that, some Hollanders who wintered in Nova Zembla, in lat. 75°, were surprized with a sight of the sun seventeen days before they expected him in the horizon. This difference was owing to the refraction of the atmosphere in that latitude. To the same cause, together with the peculiar obliquity of the moon's orbit to the ecliptic, some of these very northern regions are indebted for an uninterrupted light from the moon, much more than half the month ; and, sometimes, almost as long as it is capable of affording any light to other parts of the earth.
Through this refraction, we are favoured with the sight of the sun about three minutes and a quarter before it rises above the horizon; and also as much every evening after it sets below it; which, in one year, amounts to more than forty hours.
It is to this property of refraction that we are also indebted for that enjoyment of light from the sun, when he is below the horizon, which produces the morning and evening twilight. The sun's rays, in falling upon the higher part of the atmosphere, aré. reflected back to our eyes, and form a faint light, which gradually augments till it becomes day. It is
owing to this, that the sun illuminates the whole hemisphere at once : deprived of the atmosphere, he would have yielded no light, but when our eyes were directed towards him ; and even when he was in meridian splendor, the heavens would have
appeared dark, and as full of stars as on a fine winter's night. The
rays of light would have come to us in straight lines ; the appearance and disappearance of the sun would have been instantaneous; we should have had a sudden transition from the brighest sunshine to the most profound darkness, and from thick darkness to a blaze of light. Thus, by refraction, we are prepared gradually for the light of the sun, the duration of its light is prolonged, and the shades of darkness softened.
To it we must attribute another curious phenomenon, mentioned by Pliny; for he relates, that the moon had been eclipsed once in the west, at the same time that the sun appeared above the horizon in the east. Mæstlinus, in Kepler, speaks of another instance of the same kind, which fell under his own observation.
OF THE FIXED STARS.
No part of the universe gives such enlarged ideas of the structure and magnificence of the heavens, as the consideration of the number, magnitude, and distance of the fixed stars. We admire, indeed with propriety, the vast bulk of our own globe; but, when we consider how much it is surpassed by most
of the heavenly bodies, what a point it degenerates into, and how little more even the vast orbit in which it revolves would appear, when seen from some of the fixed stars, we begin to conceive more just ideas of the extent of the universe, and of the boundaries of creation.
The most conspicuous and brightest of the fixed stars of our horizon is Sirius. The earth, in moving round the sun, is 190 millions of miles nearer to this star in one part of its orbit, than in the opposite ; yet the magnitude of the star does not appear to be in the least altered, or its distance affected by it; so that the distance of the fixed stars is great beyond all computation. The unbounded space appears filled, at proper distances, with these stars ; each of which is probably a sun, with attendant planets rolling round it. In this view, what and how amazing, is the structure of the universe !
Though the fixed stars are the only marks by which astronomers are enabled to judge of the course of the moveable ones, and we have asserted their relative positions do not vary; yet this assertion must be confined within some limits ; for
many of them are found to undergo particular changes, and perhaps the whole are liable to some peculiar motion, which connects them with the universal system of created nature. Dr. Herschel even goes so far as to suppose, that there is not, in strictness of speaking, one fixed star in the heavens ; but that there is a general motion of all the starry systems; and, consequently, of the solar one, among the rest,
There are some stars, whose situation and place were heretofore known and marked with precision, that are no longer to be seen; new ones have also been discovered, that were unknown to the ancients, while numbers seem gradually to vanish. There are others which are found to have a periodical increase and decrease of magnitude ; and it is probable, that the instances of these changes, would have been more numerous, if the ancients had possessed the same accurate means of examining the heavens as are used at present.
New stars offer to the mind a phenomenon more surprising, and less inexplicable, than almost any other in the science of astronomy. I shall select a few instances of the more remarkable ones, for the instruction of the young pupil : a consideration of the changes that take place, at so immense a distance as the stars are known to be from him, may elevate his mind to consider the immensity of his power, who regulates and governs all these wide extended inotions ; “ who hath measured the waters in the hollow of his hand, and meted out heaven with a span.
It was a new star discovered by Hipparchus, the chief of the ancient astronomers, that induced him to compose a catalogue of the fixed stars, that future observers might learn from his labours, whether any of the known stars disappeared, or new ones were produced. The same motives engaged the illustrious Tycho Brahe, to form, with unremitting labour and assiduity, another new catalogue of the stars. Of new stars, the first of which we have a good
account, is that which was discovered in the constellation Cassiopea, in the month of November, of the year 1572;. a time when astronomy was sufficiently cultivated, to enable the astronomers to give the account with precision. It remained visible about sixteen months; during this time, it kept its place in the heavens, without the least variation. It had all the radiance of the fixed stars, and twinkled like them; and was, in all respects, like Sirius, excepting that it surpassed it in brightness and magnitude. It appeared larger than Jupiter, who was at that time in his perigree; and was scarce less bright than Venus.
It was not by degrees that it acquired this diameter, but shone forth at once of its full size and brightness, as if of instantaneous creation. It continued about three weeks in full and entire splendor, during which time it might be seen even at noonday, by those who had good eyes, and knew where to look for it. Before it had been seen a month, it became visibly smaller, and from thence continued diminishing in magnitude till March 1574, when it entirely disappeared. As it decreased in size, it varied in colour ; at first, its light was white, and extremely bright; it then became yellowish, afterwards of a suddy colour, like Mars; and finished with a pale livid white, resembling that of Saturn,
In August 1596, Fabricius observed a new star in the neck of the Whale. In 1637, Phocyllides Holwarda observed it again, and, not knowing that it had been seen before, took it for a new discovery: