Astronomical Occurrences.

Construction of the Heavens.

THERE are spots in the heavens called Nebulæ, some of which consist of clusters of telescopic stars; others appear as luminous spots of different forms. The most considerable is one in the midway between the two stars on the blade of Orion's sword, marked by Bayer; discovered in the year 1656, by the celebrated Huygens: it contains only seven stars, and the centre part is a bright spot upon a dark ground, and appears like an opening into brighter regions beyond. Dr. Halley, and others, have discovered nebulæ in different parts of the heavens. In the Connoissance des Temps for 1783 and 1784, there is a catalogue of 103 nebulæ observed by Messier and Mechain. But to Dr. Herschel we are indebted for catalogues of 2000 nebulæ and clusters of stars which he himself discovered. Some of them form a round compact system; others are more irregular, of various forms; and some are long and narrow. The globular systems of stars appear thicker in the middle than they would do if the stars were all at equal distances from each other; they are, therefore, condensed towards the centre. That stars should be thus accidentally disposed is too improbable a supposition to be admitted; he supposes, therefore, that they are brought together by their mutual attractions, and that the gradual condensation towards the centre is a proof of a central power of such a kind. The doctor observes, also, that there are some additional circumstances in the appearance of extended clusters, and nebulæ, that very much favour the idea of a power lodged in the brightest part; for though the form of them be not globular, it is plain that there is a tendency to sphericity. As the stars in the same nebulæ must be very nearly all at the same relative distances from us, and as they appear nearly of the same size, their real magnitudes must be nearly equal. Granting, therefore, that these nebula and

clusters of stars are formed by mutual attraction, Dr. Herschel concludes that we may form some judgment of their relative age by the disposition of their component parts, those being the oldest which are most compressed. He offers powerful arguments to prove that the milky way is the nebulæ, of which our Sun is one of the component parts. See Phil. Trans., vols. lxxvi and lxxix.

This illustrious astronomer has also discovered other phenomena in the heavens, which he calls nebulous stars; that is, stars surrounded with a faint, luminous atmosphere of large extent. Those which have been thus styled by other astronomers, he says, ought not to have been so called; for, on examination, they have proved to be either mere clusters of stars plainly to be distinguished by his large telescopes, or such nebulous appearances as might be occasioned by a multitude of stars at a vast distance. The milky way consists entirely of stars, and he says, 'I have been led on by degrees from the most evident conge ries of stars to other groups in which the lucid points were smaller, but still very plainly to be seen; and from them to such wherein they could barely be suspected, until I arrived, at last, to spots in which no trace of a star was to be discerned. But, then, the gradation to these latter were by such connected steps, as left no room for doubt but that all these phenomena were equally occasioned by stars variously dispersed in the immense expanse of the universe.'

In the same paper from which we have extracted the foregoing passage is given an account of some nebulous stars, one of which is thus described;

Nov. 13, 1790. A most singular phenomenon! a star of the eighth magnitude, with a faint luminous atmosphere of a circular form, and about 3' in diameter. The star is perfectly in the centre, and the atmosphere is so diluted, faint and equal throughout, that there can be no surmise of its consisting of stars, nor can there be a doubt of the evident connection

between the atmosphere and the star. Another star not much less in brightness, and in the same field of view with the above, was perfectly free from any such appearance.' Hence the doctor draws the following consequences: Granting the connection between the star and the surrounding nebulosity, if it consist of stars very remote which gives the nebulous appearance, the central star, which is visible, must be immensely greater than the rest; or, if the central star be no bigger than common, how extremely small and compressed must be those other luminous points which occasion the nebulosity! As, by the former supposition, the luminous central point must far exceed the standard of what we call a star; so, in the latter, the shining matter about the centre will be too small to come under the same denomination; we, therefore, either have a central, body which is not a star, or a star which is involved in a shining fluid, of a nature; totally unknown to us. This last opinion Dr. Herschel adopts. Light reflected from the star could not be seen at this distance: besides, the outward parts are nearly as bright as those near the star. Moreover, a cluster of stars will not so completely account for the milkiness or soft tint of the light of these nebulæ, as a self-luminous fluid. What a field of novelty,' says Dr. Herschel, is here opened to our conceptions! A shining fluid, of a brightness sufficient to reach us from the regions of a star of the 8th, 9th, 10th, 11th, or 12th magnitude, and of an extent so considerable as to take up 3, 4, 5, or 6 minutes in diameter.' He conjectures that this shining fluid may be composed of the light perpetually emitted from millions of stars. See Phil. Trans., vol. lxxxi.

According to the theory of Dr. Herschel, the uni verse consists of nebulæ, or innumerable collections of innumerable stars, each individual of which is a Sun not only equal, but much superior, to ours, at least if the assertion of Mr. Nicholson has any weight; for he tells us, that each individual Sun is destined

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,
Monday, 21st ·

29

7

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

Sunday, 6th,

16 11

Friday, 11th,

15 48

Wednes. 16th,

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:

Nov. and Dec.

T

- 46

6

20,

30

Dec. 1st,

3d,

31st, Nov. 1st,

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 m; and on the 25th, Mars will eclipse a m.

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