Oldalképek
PDF
ePub

to give light to hundreds of worlds that revolve about it, but which can no more be seen by us, on account of their great distance, than the solar planets can be seen from the fixed stars: yet,' he adds, 'as in this unexplored, and, perhaps, unexplorable, abyss of space, it is no necessary condition that the planets should be of the same magnitudes as those belonging to our system, it is not impossible but that planetary bodies may be discovered among the double and triple stars.' . · Though we meet, in Dr. Herschel's papers, with the terms condensation, clusters, &c. we are by no means from thence to infer that any of the celestial bodies in our nebula are nearer to one another than we are to Sirius, whose distance is supposed not to be less than 400,000 times that of the Sun from us, or 38 millions of millions of miles. The whole extent of the nebula being in some places nearly 500 times as great, must be such, that the light of a star placed at its extreme boundary, supposing it to travel with the velocity of 12 millions of miles every minute, must have taken nearly 3000 years to reach us. Dr. Herschel does not think that our nebula is the most considerable in the universe. "As we are used,” he says, 'to call the appearance of the heaven, where it is surrounded with a bright zone, the milky way, it may not be amiss to point out some other very remarkable nebulæ, which cannot well be less, but are probably much larger, than our own system; and, being also", extended, the inhabitants of the planets that attend the stars which compose them, must perceive the same phenomena: for which reason they may also be called milky-ways, for the sake of distinction. [This curious subject we may resume hereafter.) '.. The Sun's rising and setting is given for the three following days :Tuesday, 1st, Sun rises 12 m. past 7. Sun sets at 48 m. past 4 Friday, 11th, - - 29 . . 7 . • 31 . : Monday, 21st . · 44 · · 7 · • 16 · 4

Equation of Time. The following table will serve to adjust the clock or watch to true or equal time, by subtracting a certain number of minutes and seconds from apparent time:

m. S. Tuesday, 1st, from the time shown by the dial SUBTRACT 16 14 Sundays - 6th, - - - - - . - - - • - 16 11 Friday, 11th, clo: - - • • • • • • • : 15 48 Wednes. 16th, - - - - - - - - - - - - - 15 3 Monday, 21st, - - - - - - • • • • • 13 57 Saturday, 26th, • - . • - - - - - .'. - 12 32

The Sun enters the sign Sagittarius at 24 m. after 9 in the evening of the 22d.

The Moon enters its last quarter at 4 m. past 7 on the 4th: the change or new Moon occurs at - 15 m. past 1 in the morning of the 12th. This enters its first quarter at 16 m. past 5 in the morning of the 20th; and the Moon is at the full 52 m. past 11 in the forenoon of the 27th.

The time of the Moon's rising for the first five days after the full on the 28th of October, and also after that on the 27th of November, is as follows: "? Oct. and Nov.

Nov. and Dec. Oct, soth, . - 11 m. after 6 Nov. 28th, - - 18 m. after 3

31st, • • 46 ... 6. 29th, . . 11 ... 6 Nov. 1st, • • 32 - - 7

30th, - - 16 - .. 7 2d, . - 30 . - 8 Dec. 1st; - - 29 --- 8

3d, - - 36 : 9 2d, . - 46 ... 9 On the 25th, the Moon will eclipse the star u Ceti: the immersion will take place 49 m. past 4 in the afternoon, and the emersion 46 m. past 5: when the eclipse begins, the star will be nearly one minute north of the Moon's centre, and at the end it will be 2' south. On the 9th, the planet Jupiter will eclipse w me; and on the 25th, Mars will eclipse a me.

Mercury's inferior conjunction will be at 8 o'clock in the evening of the 27th, The Georgian planet will be in conjunction with the Sun at 2 in the afternoon on the 25th. :

There will be an eclipse of the first satellite of Ju.

[ocr errors]
[ocr errors]

piter visible at Greenwich 1 m. past 5 in the morning of the 5th day; and one of the second satellite on the 21st, at 56 m. past 4 in the morning. This time, in both cases, refers to the immersions or commencement of the eclipse.

VIEW OF THE SOLAR SYSTEM.

Of Saturn. Saturn is a very conspicuous planet, though never so brilliant as Jupiter. The period of his revolution is 10,759 days, or almost 30 years. He moves from west to east nearly in the plane of the ecliptic, and exhibits many irregularities similar to those of Jupiter and Mars. He becomes retrograde both before and after his opposition, when at the distance of about 109° from the Sun. His retrograde motion continues 139 days, and, during its continuance, he describes an arc of about 6o. · Saturn, when viewed through a good telescope, makes a more remarkable appearance than any of the other planets. Galileo first discovered his uncommon shape, which he thought to be like two globes, one on each side a larger one. Having viewed him for two years, he was surprised to see him become quite round without these appendages, and then afterwards to assume them as before. These adjoining globes were what are now called the ansæ of his ring, the true shape of which was first discovered by Huygens, about forty years after Galileo. From the discoveries made by him and other astronomers, it appears that this planet is surrounded by a broad thin ring, the edge of which reflects little or none of the Sun's light to us, but the planes of the ring reflect the light in the same manner that the planet itself does; and, if we suppose the diameter of Saturn to be divided into three equal parts, the diameter of the ring is about seven of those parts. The ring is detached from the body of Saturn in such a manner, that the distance between the innermost part of the ring and the body is equal to its breadth. Both the outward and inward rim of the ring is projected into an ellipse, more' or less oblong according to the different degrees of ob- liquity with which it is viewed. Sometimes the eye of the observer is in the plane of the ring, and then it becomes invisible; either because the outward edge is not-fitted to reflect the Sun's light, or because it is too thin to be seen at such a distance. As the plane of this ring keeps always parallel to itself, it disappears twice in every revolution of the planet; that is, once in about 15 years, and he sometimes appears quite round for nine months together. At other times, the distance between the body of the planet and the ring is very perceptible, so much so, that a star has been, seen through the opening. When Saturn appears round, if our eye be in the plane of the ring, it will seem as a dark line across the middle of the planet's disk; and, if the eye be elevated above the plane of the ring, a shadowy belt will be visible, which is caused by the shadow of the ring, as well as by the interposition of part of it between the eye and the planet.

The shadow of the ring is broadest when the Sun is most elevated, but its obscure parts appear broadest when the eye is most elevated above the plane of it. · When it seems to be double, the ring next the body of the planet appears brightest; when the ring appears of an elliptical form, the parts about the ends of the largest axis are called ansæ, as has been already mentioned. These, a little before and after the disappear. ing of the ring, are of unequal magnitude: the largest ansa is longer visible before the planet's round phase, and it appears again sooner than the other. On the 1st of October, 1714, the largest ansa was on the east side, and on the 12th on the west side of the disk of the planet, which makes it probable that the ring has a rotation round an axis. Dr. Herschel has shown that it revolves in its own plane in about 104 hours,

The obseryations of this philosopher have added greatly to our knowledge of Saturn's ring. According to, him, there is one single, dark, considerable broadline, belt or zone, which he has constantly found on the north side of the ring. As this dark belt is subject to no change whatever, is is probably owing: to some permanent construction of the surface of the ring: this construction cannot be owing to the shadow of a chain of mountains, since it is visible all round on the ring, for there could be no shade at the ends of the ring: a similar argument will apply against the opinion of very'extended caverns. It is pretty evident that this dark zone is contained between two concentric circles, for all the phenomena correspond with the projection of such a zone. , The nature of the ring Dr. Herschel thinks no less solid than that of Saturn itself, and it is observed to cast a strong shadow upon the planet. The light of the ring is also generally brighter than that of the planet; for the ring appears sufficiently bright when the telescope affords scarcely light enough for Saturn. The doctor concludes that the edge of the ring is not flat, but spherical or spheroidical. The dimensions of the ring, or of the two rings with the space between them, Dr. Herschel has given as follows:

Miles. Inner diameter of the smaller ring -.. . • 146,345 • Outside diameter of ditto -..

. - 184,393 Inner diameter of the larger ring -..

· 190,248 - Outside diameter of ditto ••-•-•••••• 204,883

Breadth of the inner ring - ..... ... 20,000 Breadth of the outer ring ........-•.-•-. 7,200

Breadth of the vacant space, or dark zone -... 2,839 • The conjectures relative to the nature of this ring have been various. Some persons have imagined that the diameter of the planet Saturn was once equal to the present diameter of the outward ring, and that it was hollow; the present body being contained within the former surface, in some such a manner as a kernel is contained within its shell. They sup

« ElőzőTovább »