Page images

the celestial equator, and thus ascertain its place on the surface of an imaginary sphere, which may be conceived to revolve with it, and on which it may be considered as projected.

(243.) The right ascension and declination of a point in the heavens correspond to the longitude and latitude of a station on the earth ; and the place of a star on a celestial sphere is determined, when the former elements are known, just as that of a town on a map, by knowing the latter. The great advantages which the method of meridian observation possesses over that of triangulation from star to star, are, then, Ist, That in it every star is observed in that point of its diurnal course, when it is best seen and least displaced by refraction. 2dly, That the instruments required the transit and mural circle) are the simplest and least liable to error or derangement of any used by astronomers. 3dly. That all the observations can be made systematically, in regular succession, and with equal advantages; there being here no question about advantageous or disadvantageous triangles, &c. And, lastly, That, by adopting this course, the very quantities which we should otherwise have to calculate by long and tedious operations of spherical trigonometry, and which are essential to the formation of a catalogue, are made the objects of immediate measurement. It is almost needless to state, then, that this is the course adopted by astronomers.

(244.) To determine the right ascension of a celestial object, all that is necessary is to observe the moment of its meridian passage with a transit instrument, by a clock regulated to exact sidereal time, or reduced to such by applying its known error and rate. The rate may be obtained by repeated observations of the same star at its successive meridian passages. The error, however, requires a knowledge of the equinox, or initial point from which all right ascensions in the heavens reckon, as longitudes do on the earth from a first meri, dian.

(245.) The nature of this point will be explained presently; but for the purposes of uranography, in so far as they concern only the actual configurations of the stars inter se, a knowledge of the equinox is not necessary. The choice of the equinox, as a zero point of right ascensions, is purely artificial, and a matter of convenience : but as on the earth, any station (as a national observatory) may be chosen for an origin of lon.. gitudes ; so in uranography, any conspicuous star may be selected as an initial point from which hour angles may be reckoned, and from which, by merely observing differences or intervals of time, the situation of all others may be deduced. In practice, these intervals are affected by certain minute causes of inequality, which must be allowed for, and which will be explained in their proper places.

(246.) The declinations of celestial objects are obtained, 1st, By observation of their meridian altitudes, with the mural circle, or other proper instruments. This requires a knowledge of the geographical latitude of the station of observation, which itself is only to be obtained by celestial observation. 2dly, And more directly by observation of their polar distances on the mural circle, as explained in art. 136., which is independent of any previous determination of the latitude of the station ; neither, however, in this case, does observation give directly and immediately the exact declinations. The observations require to be corrected, first for refraction, and moreover for those minute causes of inequality which have been just alluded to in the case of right ascensions.

(247.) In this manner, then, may the places, one among the other, of all celestial objects' be ascertained, and maps and globes constructed. Now here arises a very important question. How far are these places permanent ? Do these stars and the greater luminaries of heaven preserve for ever one invariable connection and relation of place inter se, as if they formed part of a solid though invisible firmament; and, like the great

natural land-marks on the earth, preserve immutably the same distances and bearings each from the other? If so, the most rational idea we could form of the universe would be that of an earth at absolute rest in the centre, and a hollow crystalline sphere circulating round it, and carrying sun, moon, and stars along in its diurnal motion. If not, we must dismiss all such notions, and enquire individually into the distinct history of each object, with a view to discovering the laws of its peculiar motions, and whether any and what other con. nection subsists between them.

(248.) So far is this, however, from being the case, that observations, even of the most cursory nature, are sufficient to show that some, at least, of the celestial bodies, and those the most conspicuous, are in a state of continual change of place among the rest. In the case of the moon, indeed, the change is so rapid and remarkable, that its alteration of situation with respect to such bright stars as may happen to be near it may be noticed any fine night in a few hours; and if noticed on two successive nights, cannot fail to strike the most careless observer. With the sun, too, the change of place among the stars is constant and rapid; though, from the invisibility of stars to the naked eye in the day-time, it is not so readily recognized, and requires either the use of telescopes and angular instruments to measure it, or a longer continuance of observation to be struck with it. Nevertheless, it is only necessary to call to mind its greater meridian altitude in summer than in winter, and the fact that the stars which come into view at night vary with the season of the year, to perceive that a great change must have taken place in that interval in its relative situation with respect to all the stars. Besides the sun and moon, too, there are several other bodies, called planets, which, for the most part, appear to the naked eye only as the largest and most brilliant stars, and which offer the same phenomenon of a constant change of place among the stars ; now approaching, and now receding from, such of them as we may refer them to as marks; and, some in longer, some in shorter periods, making, like the sun and moon, the complete tour of the heavens.

(249.) These, however, are exceptions to the general rule. The innumerable multitude of the stars which are distributed over the vault of the heavens form a constellation, which preserves, not only to the eye of the casual observer, but to the nice examination of the astronomer, a uniformity of aspect which, when con. trasted with the perpetual change in the configurations of the sun, moon, and planets, may well be termed in. variable. It is not, indeed, that, by the refinement of exact measurements prosecuted from age to age, some small changes of apparent place, attributable to no illusion and to no terrestrial cause, cannot be detected in some of them ; such are called, in astronomy, the proper motions of the stars ;- but these are so excessively slow, that their accumulated amount (even in those stars for which they are greatest) has been insufficient, in the whole duration of astronomical history, to produce any obvious or material alteration in the appearance of the starry heavens.

(250.) This circumstance, then, establishes a broad distinction of the heavenly bodies into two great classes ;

the fixed, among which (unless in a course of observations continued for many years) no change of mutual situation can be detected; and the erratic, or wandering-(which is implied in the word planet *)including the sun, moon, and planets, as well as the singular class of bodies termed comets, in whose apparent places among the stars, and among each other, the observation of a few days, or even hours, is sufficient to exhibit an indisputable alteration.

(251.) Uranography, then, as it concerns the fixed celestial bodies (or, as they are usually called, the fixed stars), is reduced to a simple marking down of their relative places on a globe or on maps ; to the insertion on that globe, in its due place in the great constellation

* Ireontas, a wanderer.

[ocr errors]

of the stars, of the pole of the heavens, or the vanishing point of parallels to the earth's axis; and of the equator and place of the equinox : points and circles these, which, though artificial, and having reference entirely to our earth, and therefore subject to all changes (if any) to which the earth's axis may be liable, are yet so convenient in practice, that they have obtained an admission (with some other circles and lines), sanc. tioned by usage, in all globes and planispheres. The reader, however, will take care to keep them separate in his mind, and to familiarize himself with the idea rather of two or more celestial globes, superposed and fitting on each other, on one of which a real oneare inscribed the stars; on the others those imaginary points, lines, and circles which astronomers have devised for their own uses, and to aid their calculations; and to accustom himself to conceive in the latter, or artificial, spheres a capability of being shifted in any manner upon the surface of the other; so that, should experience demonstrate (as it does) that these artificial points and lines are brought, by a slow motion of the earth's axis, or by other secular variations (as they are called), to coincide, at very distant intervals of time, with dif. ferent stars, he may not be unprepared for the change, and have no confusion to correct in his notions.

(252.) Of course we do not here speak of those uncouth figures and outlines of men and monsters, which are usually scribbled over celestial globes and maps, and serve, in a rude and barbarous way, to enable us to talk of groups of stars, or districts in the heavens, by names which, though absurd or puerile in their origin, have obtained a currency from which it would be difficult, and perhaps wrong, to dislodge them. In so far as they have really (as some have) any slight resemblance to the figures called up in imagination by a view of the more splendid " constellations," they have a certain convenience; but as they are otherwise entirely arbitrary, and correspond to no natural subdivisions or groupings of the stars, astronomers

« PreviousContinue »