Imágenes de página


thousand stars. The names of the constellations, their situation in the heavens, with other particulars, are best learned by studying the artificial representation of the heavens, a modern celestial globe.

The Galaxy or milky way must not be neglected ; it is one of the most remarkable appearances in the heavens ; it is a broad circle of a whitish hue, in some places it is double, but for the most part consists of a single path surrounding the whole celestial

The great Galileo discovered by the telescope, that the portion of the heavens which this circle passes through was every where filled with an infinite multitude of exceeding small stars, too small to be discovered by the naked eye, but by the combination of their light, diffusing a shining whiteness through the heavens. Mr. Brydone says, that when he was at the top of Mount Ætna, the milky way had the most beautiful effect, appearing like a pure flame that shot across the heavens.

The stars appear of a sensible magnitude to the naked eye, because the retina is not only affected by ' the rays of light which are emitted directly from them, but by many thousands more, which, falling upon our eye-lashes, and upon the visible aerial particles about us, are reflected into our eyes so strongly, as to excite vibrations, not only in those points of the retina where the real images of the stars are formed, but also in the other parts round about it.

This makes us imagine the stars to be much bigger, than they would be if we saw them only by the few rays which come directly from them to our eyes,


without being intermixed with others. Any one may be made sensible of this, by looking at a star of the first magnitude through a long narrow tube which, though it takes in as much of the sky as would hold a thousand of such stars, scarce renders that one visible.

The number of the stars almost infinitely exceeds what we have yet been speaking of. An ordinary telescope will discover, in several parts of the heavens, ten times as many stars as are visible to the naked eye. Hooke in his Micrographia says, that with a telescope of twelve feet he discovered seventyeight stars among the Pleiades, and with a more perfect telescope many more.

Galileo reckoned eighty in the space between the belt and the sword of Orion, and above five hundred more in another part of the same constellation, within the compass of one or two degrees square. Antonia Maria de Rheita counted in the same constellation above ten thousand stars. Future improvements in the telescopes may enable us to discover numberless stars, that are now invisible; and many more may be which are too remote to be seen through telescopes, even when they have received their ultimate improvement. Dr. Herschel, to whose ingenuity and assiduity the astronomical world is so much indebted, and whose enthusiastic ardor has revived the spirit of discoveries, of which we shall speak more largely in another part of this essay, has evinced what may be effected by improvements in the instruments of observation. In speaking here of his discoveries, I shall use the words of

M. De la Lande. *

“ In passing rapidly over the heavens with his new telescope, the universe increased under his eye; 44000 stars, seen in the space of a few degrees, seemed to indicate, that there were seventy-five millions in the heavens.” He has also shewn that many stars, which to the eye or through ordinary glasses appear single, do in fact consist of two or more The Galaxy or milky way owes its light entirely to the multitude of small stars, placed so close as not to be discoverable even by an ordinary telescope. The nebulæ, or small whitish specks, discerned by means of telescopes, owe their origin to the same cause ; former astrologers could only reckon 103, Dr. Herschel has discovered upwards of 1250 of these clusters, besides a species which he calls planetary nebulæ. But what are all those, when compared to those that fill the whole expanse, the boundless fields of ether! Indeed, the immensity of the world must contain such numbers, as would exceed the utmost stretch of the hu, man imagination: for who can say, how far the universe extends, or where are the limits of it? where the Creator stayed “his rapid wheels ;” or where he “fixed the golden compasses ?"

OF THE PLANETS, AS SEEN FROM THE SUN. Our solar observer having attained a competent knowledge of the fixed stars, will now apply him

* Memoires de l'Academie de Dijon, 1785. + In all the larger sort of telescopes, the apparent number of stars is found to be encreased, as the aperture of the tube is augmented.


self to consider the planets: these, as we have already observed, he will soon distinguish, by their motion, from the fixed stars; the stars always remaining in their places, but the planets will be seen passing by them with unequal velocities. Thus, on observing the earth, for instance, he will find it moving among the fixed stars, and approaching nearer and nearer to the more eastern ones; in a year's time it will complete its revolution, and return to the same place again.

He will find seven of these bodies revolving round the sun, to each of which he will assign a name, calling the swiftest Mercury, denominating the others, in order, according to their velocities, as Venus, then the Earth, and afterwards Mars, Jupiter, Saturn, and the Georgium Sidus.

Proceeding with attention in thus exploring and examining the heavens, he will perceive that the Earth is always accompanied by a small star, Jupiter by four, Saturn by seven, and the Georgium Sidus by two :* these sometimes precede, as others follow; now pass before, and then behind the planets they respectively attend. These small bodies he will call secondary planets, satellites, or moons.

The observer, by remarking the exact time when each planet passes over some fixed star, and the time they employ from their setting out to their return to the same star again, will find the times elapsing beLween each successive return of the same planet to

* Four more have lately been discovered by Dr: Herschel, and will be hereafter described. EDIT.

the same star, to be equal ; and he would say, that the several planets describe circles in several periods ; but that each of them always completes its own circle in the same space of time.

He will further observe, that there are certain bodies, which at their first appearance are small, obscure, ill-defined, and that move very slow, but which afterwards increase in magnitude, light, and velocity, until they arrive at a certain size, when they lose these properties, and diminish in the same manner as they before augmented, and at last disappear. To these bodies, which he will find in all the regions of the heavens, moving in different directions, he will give the name of comets.


Our observer will take notice, that the planets run successively through those constellations which he has denominated Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricornus, Aquarius, Pisces; and that they never move out of a certain space or zone of the heavens, which we call the zodiac.

He will find, by proceeding in his observation, that the orbits of the planets are not all in the same plane, but that they cross each other in different parts of the heavens; so that, if he makes the orbit of any one planet a standard, and considers it as having no obliquity, he would judge the paths of all the rest to be inclined to it; each planet having

« AnteriorContinuar »