« السابقةمتابعة »
sent instance, the position of the planet will be very unfavorable for witnessing this interesting phenomenon, -Saturn being so near the solar rays.
This invisibility occurs when the plane of the ring passes through the sun, or the earth, or between them ; in the first case, the sun shines only upon the edge of the ring, which is too thin (less than 1" in thickness) to reflect sufficient light to render it visible; in the second case, the edge only being opposed to us, it is not visible for the same reason ; in the third case, the dark side of the ring is exposed to us, and, therefore, the edge being the only luminous part which is towards the earth, it is invisible on the same account as before.
The ring, however, has been seen, when the earth was on the unenlightened side, wbich must either have been from the light reflected from the edge, or else the reflection of the light of the globe of Saturn upon the dark side of the ring,--analogous to the secondary light of the moon.
The ring may be visible when the sun is elevated 3 above its plane, or three days before its plane passes through the sun, and when the earth is elevated 2' 20" above the plane, or one day from the earth passing it.
That the ring is exceedingly thin, is evident from the satellites being seen to pass behind and before the ring, in such a manner, that they served as an excellent micrometer to measure its thickness by. The following observations, by Herschel, are very interesting :
July 18, 1789, at 16h, 41m. 9s. sidereal time, the third satellite seemed to hang upon the following arm, declining a little towards the north, and was seen gradually to advance upon it towards the body of Saturn;
but the ring was not so thick as the lucid point. July 23rd, at 19h. 41m. 8s. the fourth satellite was a very little preceding the ring, but the ring appeared to be less than half the thickness of the satellite. July 27th, at 20b. 15m. 12s. the fourth satellite was about the middle, upon the following arm of the ring, and towards the south; and the second at the farther end, towards the north ; but the arm was thinner than either. August 29th, at 22h. 12m. 25s. the fifth satellite was upon the ring, near the end of the preceding arm, and the thickness of the arm seemed to be about one-third or onefourth of the diameter of the satellite, which, in the situation it then was, he took to be less than one second in diameter. At the same time, the first appeared at a little distance following the fifth, in the shape of a bead upon a thread, projecting on both sides of the same arm; hence, the arm is thinner than the first, which is considerably smaller than the second, and a little less than the third. October 16, this celebrated astronomer followed the first and second satellites up to the very disc of the planet; and the ring was then so extremely faint, as not to obstruct his seeing them gradually approach the disc. These observations are sufficient to show the extreme thinness of the ring. But Herschel further observes, that there may be a refraction through an atmosphere of the ring, by which the satellites may be lifted up and depressed, so as to become visible on both sides of the ring, even though the ring should be equal in thickness to the smallest satellite, which may amount to a thousand miles.
In October, 1714, when the plane of the ring very nearly passed through the earth, and was approaching
it, it was observed, that while the arms were decreasing, both in length and breadth, the eastern arm appeared a little larger than the other for three or four nights, and yet it vanished first, for after two nights interruption, by clouds, the western arm alone was seen. In the same year, also, the arms appeared twice as short as usual. In 1774, both the arms appeared completely detached from the planet, and the eastern one larger than the other. From these observations, it is natural to conclude that there are irregularities on the surface of the ring, and that the disappearance of the arms arises from a curvature in its surface.
The disappearances and re-appearances of the ring succeed each other very exactly, and in the same order during each sidereal revolution of Saturn; the following are the epochs in which these phenomena have been observed :
1715, February 5th.......... Re-appeared.
1819, March 1st ....... ... Disappeared. After this month, it will be the northern plane of the ring that is visible; in the year 1838 it will attain its greatest ellipticity, and in the year 1847 again become invisible.
Some philosophers have considered the ring of Saturn as a sort of Aurora Borealis, or shining fluid ; others that this luminous zone is the tail of a comet, which the attraction of Saturn has compelled to circulate round
him: for these, and similar opinions, there is no foundation, as it is probable that the ring is as dense if not more so than the planet; the light of the ring appears sufficiently bright when the disc of Saturn is scarcely perceptible.
Uranus on the 1st, about half a degree north of , in Capricornus. This planet, when examined under the most favorable circumstances, exhibits a beautiful planetary disc, without the slightest appearance of any ring, either perpendicular or horizontal.
Sphere of the Fixed Stars. Direction of the zodiac on the 1st day, at 8 in the evening.
Aries and Pisces rising Aquarius S. E. by E. Capricornus S.S. E. Sagittarius on the meridian. Scorpio S.W. by S. Libra S.W. by W. Virgo setting.
TELESCOPIC OBJECTS. Sagittarius. This zodiacal constellation is remarkable for its nebulæ, some of which are of a singular description. North of the bow of Sagittarius is a train of faint light with stars. Between the bow of Sagittarius and right foot of Ophiuchus, is a mass of stars of the 8th and 9tb magnitudes, surrounded by nebulosity; a similar appearance is also near the star 11. Near 25 is a small nebula. Near the extremity of the bow of Sagittarius, in the Via Lactea, is great nebulosity, containing several stars, the light divided into several parts. The star 9 is encircled with a faint ligbt. Near 1 is a round nebula. Below the left arm of Sagittarius are two nebulæ, one of which is like a small