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ing of time, corresponds to January 1 day 23 hours In the astronomical reckoning; and 1 o'clock in the afternoon of the former, to January 2 days 1 hour of the latter reckoning. This usage has its advantages and disadvantages, but the latter seem to preponderate; and it would be well if, in consequence, it could be broken through, and the civil reckoning substituted. Uniformity in nomenclature and modes of reckoning in all matters relating to time, space, weight, measure, $fc, is of suck vast and paramount importance in every relation of life as to outweigh every consideration of technical convenience or custom. *
(148.) Both astronomers and civilians, however, who inhabit different points of the earth's surface, differ from each other in their reckoning of time; as it is obvious they must, if we consider that, when it is noon at one place, it is midnight at a place diametrically opposite; sunrise at another; and sunset, again, at a fourth. Hence arises considerable inconvenience, especially as respects places differing very widely in situation, and which may even in some critical cases involve the mistake of a whole day. To obviate this inconvenience, there has lately been introduced a system of reckoning time by mean solar days and parts of a day counted from a fixed instant, common to all the world, and determined by no local circumstance, such as noon or midnight, but by the motion of the sun among the stars. Time, so reckoned, is called equinoctial time; and is numerically the same, at the same instant, in every part of the globe. Its origin will be explained more fully at a more advanced stage of our work.
(149.) Time is an essential element in astronomical observation, in a twofold point of view: — 1st, As the representative of angular motion. The earth's diurnal motion being uniform, every star describes its diurnal circle uniformly; and the time elapsing between the passage of the stars in succession across the meridian of any observer becomes, therefore, a direct measure of their differences of right ascension. 2dly, As the fundamental element (or natural independent variable, to use the language of geometers) in all dynamical theories. The great object of astronomy is the determination of the laws of the celestial motions, and their reference to their proximate or remote causes. Now, the statement of the law of any observed motion in a celestial object can be no other than a proposition declaring what has been, is, and will be, the real or apparent situation of that object at any time, past, present, or future. To compare such laws, therefore, with observation, we must possess a register of the observed situations of the object in question, and of the times when they were observed.
* The only disadvantage to astronomers of using the civil reckoning is this — that their observations being chiefly carried on during the night, the day of their date will, in this reckoning, always have to be changed at midnight, and the former and latter portion of every night's observations will belong to two differently numbered civil days of the month. There is no denying this to be an inconvenience. Habit, however, would alleviate it; and tome inconveniences must be cheerfully submitted to by all who resolve to act on general principles. All other classes of men, whose occupation extends to the night as well as day, submit to it, and find their advantage in doing so.
(150.) The measurement of time is performed by clocks, chronometers, clepsydras, and hour-glasses. The two former arc alone used in modern astronomy. The hour-glass is a coarse and rude contrivance for measuring, or rather counting out, fixed portions of time, and is entirely disused. The clepsydra, which measured time by the gradual emptying of a large vessel of water through a determinate orifice, is susceptible of considerable exactness, and was the only dependence of astronomers before the invention of clocks and watches. At present it is abandoned, owing to the greater convenience and exactness of the latter instruments. In one case only has the revival of its use been proposed; viz. for the accurate measurement of very small portions of time, by the flowing out of mercury from a small orifice in the bottom of a vessel, kept constantly full to a fixed height. The stream is intercepted at the moment of noting any event, and directed aside into a receiver, into which it continues to run, till the moment of noting any other event, when the intercepting cause is suddenly removed, the stream flows in its original course, and ceases to run into the receiver. The weight of morcury received, compared with the weight received in an interval of time observed by the clock, gives the interval between the events observed. This ingenious and simple method of resolving, with all possible precision, a problem of much importance in many physical inquiries, is due to the late Captain Kater.
(151.) The pendulum clock, however, and the balance watch, with those improvements and refinements in its structure which constitute it emphatically a chronometer*, are the instruments on which the astronomer depends for his knowledge of the lapse of time. These instruments are now brought to such perfection, that an habitual irregularity in the rate of going, to the extent of a single second in twentyfour hours in two consecutive days, is not tolerated in one of good character ; so that any interval of time less than twentyfour hours may be certainly ascertained within a few tenths of a second, by their use. In proportion as intervals are longer, the risk of error, as well as the amount of error risked, becomes greater, because the accidental errors of many days may accumulate; and causes producing a slow progressive change in the rate of going may subsist unperceived. It is not safe, therefore, to trust the determination of time to clocks, or watches, for many days in succession, without checking them, and ascertaining their errors by reference to natural events which we know to happen, day after day, at equal intervals. But if this be done, the longest intervals may be fixed with the same precision as the shortest; since, in fact, it is then only the times intervening between the first and the last moments of such long intervals, and such of those periodically recurring events adopted for our points of reckoning, as occur within twenty-four hours respectively of either, that we measure by artificial means. The whole days arc counted out for us by nature; the fractional parts only, at either end, are measured by our clocks. To keep the reckoning of the integer days correct, so that none shall be lost or counted twice, is the object of the calendar. Chronology marks out the order of succession of events, and refers them to their proper years and days; while chronometry, grounding its determinations on the precise observation of such regularly periodical events as can be conveniently and exactly subdivided, enables us to fix the moments in which phenomena occur, with the last degree of precision.
* Xporos, time; ptrptw, to measure.
(152.) In the culmination or transit (t. e. the passage across the meridian of an observer,) of every star in the heavens, he is furnished with such a regularly periodical natural event as we allude to. Accordingly, it is to the transits of the brightest and most conveniently situated fixed stars that astronomers resort to ascertain their exact time, or, which comes to the same thing, to determine the exact amount of error of their clocks.
(153.) Before we describe the instrument destined for the purpose of observing such culminations, however, or those intended for the measurement of angular intervals in the sphere, it is requisite to place clearly before the reader the principle on which the telescope is applied in astronomy to the precise determination of a direction in space, — that, namely of the visual ray by which we see a star or any other distant object.
(154.) The telescope most commonly used in astronomy for these purposes is the refracting telescope, which consists of an object-glass (either single, or as is now almost universal, double, forming what is called in optics, an achromatic combination) A; a tube AB, into which the brass cell of the
object-glass is firmly screwed, and an eye-lens C, for which is often substituted a combination of glasses designed to increase the magnifying power of the telescope, or otherwise give more distinctness of vision according to optical principles which we have no occasion here to refer to. This also is fitted into a cell, which is screwed firmly into the end B of the tube, so that object-glass, tube, and eye-glass may be considered as forming one piece, invariable iu the relative position of its parts.
(155.) The line P Q joining the centres of the object and eye-glasses and produced, is called the axis or line of collimation of the telescope. And it is evident, that the situation of this line holds a fixed relation to the tube and its appendages, so long as the object and eye-glasses maintain their fixity in this respect.
(156.) Whatever distant object E, this line is directed to, an inverted picture or image of that object F is formed (according to the principles of optics), in the focus of the objectglass, and may there be viewed as if it were a real object, through the eye-lens C, which (if of short focus) enables us to magnify it just as such a lena would magnify a material object in the same place.
(157.) Now as this image is formed and viewed in the air, being itself immaterial and impalpable — nothing prevents our placing in that very place F in the axis of the telescope, a real, substantial object of very definite form and delicate make, such as a fine metallic point, as of a needle — or better still, a cross formed by two very fine threads (spider-lines), thin metallic wires, or fines drawn on glass intersecting each other at right angles — and whose intersection is all but a mathematical point. If such a point, wire, or cross be carefully placed and firmly fixed in the exact focus F, both of the object and eye-glass, it will be seen through the latter at the same time, and occupying the same precise place as the image of the distant star E. The magnifying power of the lens renders perceptible the smallest deviation from perfect coincidence, which, should it exist, is a proof, that the axis Q P is not directed rigorously towards E. In that case, a fine motion (by means of a screw duly applied), communicated to the telescope, will be necessary to vary the direction of the axis till the coincidence is rendered perfect. So precise is this mode of pointing found in practice, that the axis of a telescope may be directed towards a star or other definite celestial object without an error of more than a few tenths of a second of angular measure.
(158.) This application of the telescope may be considered as completely annihilating that part of the error of observa