Imágenes de página



the sun.

As the centre of the system is the only place from which the motion of the planets can be truely seen, let us suppose an observer placed in the centre of

In this situation he will see at one view all the heavens, which will appear to him perfectly spherical, the stars being so many lucid points in the concave surface of the sphere, whose centre is the sun, or, in the present instance, the eye of the observer:

. Our spectator will not, however, immediately conclude from appearances, either that the heavens are really spherical, or that the sun is in the centre of that sphere, or that the stars are all at an equal distance from him ; having been previously taught by experience and observation, that while he remains in the same place, he cannot judge properly of the distance of the surrounding objects, at least of those which are placed beyond the ordinary reach of view. When objects are removed beyond the distances we are accustomed to, the principles by which we form our general judgment fail us; and we can only tell which is nearest, or which is furthest, either by our own motion, or that of the objects. • To illustrate this, let us suppose a number of lamps to be placed irregularly, at different distances from the eye, in a dark night. Now, if in this case we suppose

the darkness to be so complete, that no intermediate objects could be seen, no difference in

colour discerned, nor any convergence towards the point of sight be perceived; our judgment could not assist us in distinguishing the distance of one from the other, and they would therefore all seem to be at an equal distance from the spectator.

For the same reason, the sun and moon, the stars and planets, appear to be all at an equal distance from us, though it is highly probable, that some of the stars are many millions of times nearer to us than others. The sun is demonstrated to be nearer than any of the stars. The moon and some of the planets are known by ocular proof to be nearer to us than the sun, because they sometimes come between it and our eye, and hide the whole, or a great part of his disc, from our view. They all, however, appear equally distant, and as if placed in the surface of a sphere, whereof our eye is the centre. In whatever place, therefore, the spectator resides, whether it be on this earth, in the sun, or in the regions of saturn, he will consider that place as the middle point of the universe, and the centre of the world; for it will be to him the centre of a spherical surface, in which all distant bodies seem to be placed.

These things being rendered plain, the pupil may proceed to consider the observations of the solar spectator ; to whom, as we have already observed, the heavens will appear as the surface of a concave sphere, concentrical to his eye: in this surface he will discover an innumerable host of fixt stars, which will for some time engage his attention, before he discovers that they may be distinguished into two

kinds ; the one, dispersed through the whole heavens, differing in their degree of brightness, but remaining always at the same relative distance from each other ; these he will therefore call fixed stars, or only stars. Besides these, he will find some others moving among the foregoing with different velocities, which he will call wandering stars or planets.


Having proceeded thus far, our spectator will endeavour to find out some method of distinguishing the stars from each other; concluding, that as they do not change their relative positions one to the other, he may easily make an exact description of them, and by repeated observations determine the position and order which subsist among them.

That he may avoid confusion in description, and be able to point out any particular star, without being obliged to give a name to each, he will divide them into several parcels ; to each of these he will assign a figure at pleasure ; these assemblages or groups of stars, he will call constellations. Thus, a number of stars near the north pole is called the Bear, because the stars which compose it are at such distances from each other, that they may fall within the figure of a bear. Another constellation is called the Ship, because that collection of stars which coinpose it is represented upon a celestial globe as comprized within some part of the figure of a ship.

As the fixed stars will appear to our observer of different degrees of magnitude and splendor, he will divide them into different classes. Those which seem the largest and brightest, he will call stars of the first magnitude; the smallest that we can see with the naked eye are called stars of the sixth magnitude; and the intermediate ones, according to their different apparent sizes, he will call of the second, third, fourth, or fifth magnitudes. Those stars which cannot be seen without the assistance of a telescope, are not reckoned in any of these classes, and are called telescopic stars.

By a knowledge of the fixed stars and their positions, our observer will obtain so many fixed points, by which he may observe the motions of the planets and the relation of these motions to each other; he will use them as so many land-marks, if the word may be allowed, by which the situations of other celestial bodies may be ascertained, and the varieties to which they are subject be observed. For, from the same place, the motions of the heavenly bodies can only be estimated by the angle formed at the spectator's eye by the space which the moving body passes over.

To measure the spaces, the stars must be used, and considered as so many luminous points fixed in the concavity of a sphere, whose radius is indefinite, and of which the observer's eye is the centre. We may learn from hence the necessity of forming an exact catalogue of stars, and of determining their

positions with accuracy and care. With such a catalogue the science of astronomy begins.

Although, to those who are unacquainted with the nature of celestial observation, it might at first sight appear almost impossible to number the stars; yet their relative situations have been so carefully observed by astronomers, that they have not only been numbered, but even their places in the heavens have been ascertained with greater accuracy than the relative situations of most places on the surface of the earth.

The greatest number of stars that are visible to the naked eye, are to be seen on a winter's night, when the air is clear, and no moon appears. But even then a good eye can scarce distinguish more than one thousand at a time in the visible hemisphere : for, though, on such a night, they appear to be almost innumerable, this appearance is a deception, that arises from our viewing them in a transient and confused manner; whereas, if we view them distinctly, and only consider a small portion of the heavens at a time, and, after some attention to the situation of the remarkable stars contained in that portion, begin to count, we shall be surprized at the smallness of their number and the ease with which they may be enumerated.

The number of the ancient constellations was 48; in these were included 1022 stars. Many constellations have been added by modern astronomers; $0 that the catalogue of Flamsteed and De la Caille, when added together, are found to contain near_five

[ocr errors]
« AnteriorContinuar »