Imágenes de página
PDF
ePub

disappear, being effaced by his light, though but a few days before it was at a sufficient distance from him. That it is the sun which approaches the stars, and not the stars the sun, is plain, for this reason: the stars always rise and set every day at the same points of the horizon, opposite to the same terrestrial objects, and are always at the same distance from each other; whereas the sun is continually changing both the place of its rising and setting, and its distance from the stars.

The sun advances nearly one degree every day, moving from west to east; so that in 365 days we see the same star near the setting sun, as was observed to be near him on the same day in the preceding year. In other words, the sun has returned to the place from whence he set out, or made what we call his annual revolution.

We cannot indeed observe the sun's motion among the fixed stars, because he darkens the heavens by his splendour, and effaces the feeble light of those stars that are in his neighbourhood; but we can observe the instant of his coming to the meridian altitude; we can also compute what part of the starry heaven comes to the same meridian, at the same time, and with the same altitude. The sun must be at that point of the starry heavens thus discovered. Or we can observe that point in the heavens, which comes to the meridian at midnight, with a declination as far from the equator on one side, as the sun's is on the other side; and it is evident, the sun 'must be in that part of the heavens, which is diametrically op

[ocr errors][ocr errors]

posite to this point. By either of these methods we can ascertain a series of points in the heavens, through which the sun passes, forming a circle called the ecliptic.*

OF THE APPARENT PHENOMENA OF THE MOON.

The motion of the moon through the heavens, and her appearance therein, are still more remarkable than those of the sun; she engages the attention “ by the nightly changes in her circling orb”. At the new moon, or when she first becomes visible, she is seen in the western part of the heavens, at no great distance from the sun. She increases every night in size, and removes to a greater distance from the sun, till at last she appears in the eastern part of the horizon, when the sun is disappearing in the western; she then appears with a full round face, and we say it is full moon. After this, she gradually removes further and further eastward, till at last she seems to approach the sun as nearly in the east as she did before in the west, and rises a little before him in the morning; whereas in the first part of her course she sets in the west, long after him. All these different appearances happen in the space of a month,

, after which they re-commence in the same manner; 5 sometimes half-restoring day with her waxing brightness; sometimes waning into dimness, and scarcely scattering the nocturnal gloom.”

* The conformity of this definition of the ecliptic, with that giren in page 13, will be seen hereafter.

There is sometimes an irregularity in these appear-ances, particularly in harvest-time, when the moon appears for several days to be stationary in the heavens, and to preserve nearly the same distance from the in

consequence of which, she rises at that season of the year nearly at the same hour for several nights.

sun;

OF THE APPARENT MOTION OF THE STARS..

In contemplating the Stars, it is observed that some among them have the singular property of neither rising in the east, nor setting in the west; but seem to turn round one immoveable point, near which is placed a single star, called the pole, or polar star.

This point is more or less elevated, according to the part of the earth from which it is viewed. Thus to the inhabitants of Lapland it is much more vertical, or elevated above the horizon, than with us: we see it more elevated than the inhabitants of Spain: and these again see it more elevated than those of Barbary. By continually travelling southward, we should at last see the pole star depressed to the horizon, and the other pole would appear in the south part of the horizon, round which the stars in that part would revolve. There is, however, no star in the southern hemisphere so near the pole as that in the northern hemisphere. Supposing us still to tra .. vel southward, the north pole would entirely dis.. appear, and the whole hemisphere would seem to

1

turn round a single point in the south, as the northern hemisphere appears to turn round the polar

star.

The general appearance, therefore, of the starry heaven is that of a vast concave sphere turning round two fixed points (diametrically opposite to each other, the one in the north, the other in the south) once in twenty-four hours.

Hence it is that the stars, though they keep the same relative places with respect to each other, yet change their situation very sensibly with respect to the horizon; some rising above, others descending below it; some that were invisible, now becoming visible; while, on the other hand, many are disappearing. Some never descend below the horizon; although, as they turn round, they are sometimes nearer to, at others, further from it, describing whole circles about a point above it. If the observer turns himself round, he will find some stars rise only as it were to set again; many describing small -ares, and others larger ones.

THE APPEARANCES OF THE PLANETS.

[ocr errors]

Besides the fixed stars, there are other bodies in the heavens, which are continually changing their places, both with respect to the stars, and one another; these are called planets. They move among the signs of the zodiac, never departing far from the ecliptic. Their apparent motion is very irregular, confused, and perplexed; sometimes they appear as

going forwards, sometimes backwards, and at others are stationary.

Mercury emits a bright white light, but keeps so near the sun, and is so small, that he is

very

seldom visible; and when he does make his

appearance,

his motion towards the sun is so swift, that he can only be discerned for a short time. He appears a little after sun-set, and again a little before sun-rise.

Venus is the most beautiful star in the heavens, known by the names of the morning and evening star. She also, like Mercury, keeps near the sun, though she recedes from him much further, and, like him, is never seen in the eastern quarter of the heavens when the sun is in the western; but always either attends him in the evening, or gives notice of his approach in the morning.

Mars is of a red fiery colour, giving a much duller light than Venus, though he sometimes appears almost equal to her in size. He is not subject to the same limitations in his motions as Venus and Mercury, but appears sometimes very near the sun, at others at a greater distance from him, rising when the sun sets, or setting when he rises.

Jupiter and Saturn likewise often appear at great distances from the sun. The former shines with a bright light, the latter with a pale faint one. The motion of Saturn among the fixed stars is so slow, that unless carefully observed, and that for some time, he will not be thought to move at all.

The Georgium Sidus is the planet discovered by Dr. Herschel. It is reckoned to be twice the dis

« AnteriorContinuar »