Page images
PDF
EPUB

spread over a sheet of white paper, when viewed through a good telescope, and in a good state of the air. When they are viewed in an unfavourable state of the air, these small specks appear confused, and like a darkish penumbra surrounding the central spot, as they are generally described by the European astronomers. But the purity of the American air has discovered to us, that, what they call a penumbra, is really composed of an infinitude of very small, distinct, specks, each as intensely black as the central spot.

Various hypotheses have been framed by ingenious astronomers to account for the phenomena of the solar spots, but as they are mere hypotheses, unsupported by proofs of any kind, I shall not trouble you with the recital of any of them.

A faint ring of light has been observed to surround the sun, in the time of a total eclipse, and to follow him; which makes it probable, that he has an atmosphere, which is the cause of that appearance.

Although the sun appears to pursue the same course among the fixed stars from year to year, yet he appears to us to have a much greater meridian altitude at one time than at another; but this arises entirely from the motion of the earth and the situation of her axis; as we shall hereafter see.

MERCURY.

MERCURY is the nearest of our planets to the sun which we have yet discovered; but being seldom seen, and no spots appearing upon his face, it is not known whether he has any rotation on his axis, or how he may be situate. But, if we reason from analogy, it is probable that he has the regular returns of day and night, of summer and winter, and all the other

seasons of the year. We know that his orbit is inclined to the ecliptic in an angle of 6o, 59′, 20′′, which may make an alteration of the sun's meridian altitude if his axis be inclined to the ecliptic. In his course round the sun he moves at the rate of 95,000 miles every hour, which is near 200 times as quick as a cannon ball. As he is but 36,841,468 miles distant from the sun, and we are distant 95,173,117 miles from the source of light and day, he must receive near seven times as much light from the sun as we receive: the quantity of light being as the squares of the distances inversely. And although his heat from the sun should exceed ours in the same proportion, yet this would furnish no argument against his being inhabited, as it is as easy for the Almighty to suit the constitutions of his inhabitants to that degree of heat, as it was to accommodate ours to the temperature of our earth. Mercury appears to us, through a good telescope, when viewed at different times, with all the phases of the moon, save only that he never appears round and full; because when he should appear in this manner, he is so near to the sun as to be lost in his superior splendour. As his enlightened side is always turned towards the sun, this proves that he shines not by any inherent light of his own, but by the light of the sun, reflected from his surface. And that he moves round the sun, in an orbit, included within the orbit of the earth, or at least in an orbit of less radius, is plain, from his never having been seen in opposition to the sun, nor even more than about 23° degrees from him, either to the east or west of him, which is styled his greatest elongation. And besides this, he is sometimes seen on the face of the sun like a round dark spot, where he never could be seen, if he did not revolve

in an orbit included by the orbit of the earth, or if he had any inherent light. If he shone with an inherent light he could not be distinguished from the bright disk of the sun, when upon it, and could not be seen like a dark spot upon his face.

These particulars might be easily exemplified in the following manner. Let any person carry a ball suspended by a string round a lighted candle, with a slow and regular motion; and let a spectator, placed at the distance of three or four yards, view the part of the surface that is enlightened by the direct rays of the candle, and he will then observe it, in different positions, to assume the different phases, of the moon. When the ball is between the spectator's eye and the 'candle, it will appear dark, as no part of the enlightened disk is turned towards his eye; but when it is opposite to the candle and beyond it, the enlightened disk or hemisphere is turned both to him and to the candle, and therefore it will in this position appear like the full moon. Nearly in the middle between these positions on cach side, only one half of the enlightened surface is then turned to the spectator; and therefore the ball will appear like the moon in her quadratures. But in every other position between these, it will appear more or less horned or gibbous. If this experiment be made with a flat plate, it can never be made to appear horned or gibbous, in any position whatever. If the spectator remove to a considerable distance, he will observe the following appearances. The ball will scem to him to vibrate backwards and forwards on each side of the candle in a right line, moving quickest when it is between his eye and the candle, and slower when beyond it; and at the greatest elongations on either side, it will appear stationary for

[ocr errors]

a small time; spending more time in passing from one of these stations to the other, while it passes beyond the candle, than while it passes between the spectator and the candle; provided still, that the eye, the candle and the ball be kept in the same plane; and that the ball move with a uniform and equable velocity. The ball will appear to move from the right hand to the left in less or more time than in the contrary direction, according as it is between the eye and the candle, or beyond it; although in every revolution it will appear to recede to the same distance, on each side of it.

But if the ball be carried round the candle in an ellipsis, instead of a circle, and the candle be placed in one of the foci, the stationary points, at which the progressive and retrograde motions succeed each other, will be at different distances from the candle, unless the eye and the candle both lie in one of the axes. Now all these appearances take place in the motion of Mercury round the sun, and are precisely such as they should be, upon the supposition of its being an opake globe revolving round the sun in an orbit included within the orbit of the earth. Hence we conclude, that this is the figure and position of his orbit.

When Mercury is between the sun and the earth, he is said to be in his inferior conjunction, and in his superior conjunction, when he is beyond the sun. In each of these positions he must be in every revolution, so that he has two conjunctions with the sun and no opposition in each apparent revolution, and this apparent time would be the same with his true periodical time, if the earth were at rest. But, as the earth is moving the same way, when he has completed his period round the sun as viewed from a fixed star or the center of the sun, counted from the time of his inferior

conjunction, the earth will have moved near a quarter of her orbit forward, so that it will require an additional time to overtake it and be again in its inferior conjunction. This may be illustrated by the conjunction of the two hands of a watch or clock, which always requires more time than a complete revolution of the quicker.

If the orbit of Mercury were coincident with the ecliptic, Mercury, at every inferior conjunction, would pass over the face of the sun, and be seen upon it. But his orbit is inclined to it, in an angle of near seven degrees, and cuts it in the 14th degrees of Taurus and Scorpio, in which places the earth is about the 4th of May and the 6th of November. Should an inferior conjunction of Mercury, therefore, happen at either of these times, when he is near to his nodes, and the earth is in or near to the plane of his orbit, Mercury will then appear like a round black spot on the face of the sun, and pass over it from east to west in a few hours. This is denominated a transit of Mercury. As the periodical times of the earth and Mercury are nearly commensurate, and Mercury's motion is so quick, these transits frequently happen. We cannot see Mercury on the sun, at his next transit, on the 4th of May, in the year 1786, as it will be over, before the sun rises. But we may expect to see him transit, on the 5th of November, 1789, at 8h, 8', 8", in the morning.

Mercury is seldom seen by us, because he is generally hid in the rays of the sun, except when near his greatest elongation from the sun, when a line from the earth to him becomes a tangent to his orbit, and for a few evenings or mornings about that time.

« PreviousContinue »