RIGHT ASCENSION AND DECLINATION,

69 will suppose, or some other time-measurer, such as a watch, or ordinary clock, is set going, showing, also, at that instant, Oh, Om, Os. Now the astronomer's clock is, like the other time-keepers, divided into 24 hours, only he reckons straight forward from 1 to 24 hours, while in the ordinary time-piece, the hours are numbered twice in a day, from 1 to 12. We ought to say, however, that the astronomer begins his day at noon the 14th of April, while the civil day, April 14th, began at midnight, 12 hours before, but both clocks now show Oh, Om, Qs. The astronomer's clock has a pendulum a trifle shorter than the common clock, which makes it oscillate somewhat faster, so that the gain, on the ordinary clock, may be about 3m, 56s, in a day. After an interval of 24 hours, by his clock, the astronomer again looks into the transit telescope and sees the supposed star, or equinoctial point, which is always called the first point of Aries, just on his meridian, that is, if his clock is truly adjusted, but it is not yet a day, or 24 hours, by the civil time, but lacks 3m, 56s. The next day the clocks will be still farther apart, and in about 15 days there will be 1 hour's difference, the siderial clock showing 1h, when the ordinary clock shows 12h, or noon; the latter shows the time when the sun is on the meridian, or very nearly so, but the former indicates that the first point of Aries, or the equinoctial point, crossed the meridian an hour before. Now the great convenience to the astronomer is this: As the whole heavens appear to revolve around the earth in a siderial day, he imagines a circle traced out in the heavens, which corresponds to our equator, and, commencing at the vernal equinoctial point, or first point of Aries, he divides this celestial equator, into 24 equal portions, or hours, and these he subdivides into 60 minutes, and each minute into 60 seconds, and he calls the distance of any body from this first point of Aries, measured on the celestial equator, just as we measure longitude on a globe, or map, by ascertaining how far east or west the place is from Greenwich, measured on the terrestrial equator; this he calls the Right Ascension of that body, designated by the initials R. A., and the distance of the body north or south of the equator, he calls Declination, north or south, designated thus: N. D., or S. D., corresponding with our geographical terms, north and south

D*

latitude. The only difference between longitude as reckoned on the earth, and right ascension as measured in the heavens, is, the former is reckoned east or west from any arbitrary point, Greenwich, or Washington, for example, but the latter is reckoned eastward, or in the order of the signs, completely around, and always from the first point of Aries, which is a determined point in the sky, being the position of the vernal equinox, and which turns around, apparently, with the whole celestial concave, in its diurnal revolution.

When a new comet appears, and is announced as having a R. A. of 6h, and 10m, and N. D, of 2° 15', the astronomer places his transit telescope, or other similar instrument, so as to point 2° 15' north of the imaginary celestial equator, for he knows just how high above the horizon this is situated, and when his clock points out 6h and 10m, he looks into the telescope and sees the newly discovered object. Thus the precise position occupied by any star, or planet, in the heavens, can be mapped down, using right ascensions and declinations in the same manner as terrestrial longitudes and latitudes. We should like to say a great deal more on this subject, but the nature of our work forbids.

Our ordinary clocks and watches, are adjusted to keep mean solar time.. It would, at first, be supposed, that the interval from noon to noon, although longer than a Siderial day, would, nevertheless, be an equal period, so that if a clock was adjusted to show 24 hours during the interval of the sun's leaving the meridian at any particular season of the year, to his return to it the next day, it would always indicate an interval of 24h, for any similar revolution. This is not the case, and we think we can show, very plainly, why it is not. The instant when the sun is actually on the meridian, is called the time of apparent noon, or 12 o'clock apparent time, although, a 'clock regulated to keep what is called mean time, or mean solar time, may then show but 11h, 45m. The difference between apparent time and mean solar time, is called the equation of time, i, e. the correction which must be applied in order to determine true time, from the time indicated by the sun. It is evident that Sun-Dials will indicate apparent .time, and we will, therefore, devote the remainder of this chapter

to a description of the principles of dialing, and then proceed to illustrate the causes, which make the discrepancy observed between the times indicated by a clock supposed to run with an uniform motion, and a good sun-dial. We do this the more willingly, for we intend our book to be of some advantage to the reader, and we trust that after its attentive perusal, he will feel sufficiently interested to either erect a good dial, or a meridian mark, in order to determine his local time with something more of accuracy than suffices for the ordinary wants of life. We mean by local time, the correct solar time for the place, in distinction from Greenwich time, or Siderial time. Chronometers, which are accurate, but portable, time-keepers, are often set to Greenwich time, i. e. they are adjusted so as to show, wherever they are carried, the actual time then indicated by the clock at Greenwich, the difference between this and the time indicated by the clock at any other place, or the local time will give, by simple inspection, the difference of longitude.

Let PA B C, be the earth, and E the position of a spectator upon it, and let F G be the horizon, or a horizontal circle, and let CH A be the plane of a great circle parallel to the small circle F G, and let P B be the axis of the earth inclined to the diameter

CA of the great circle C H A, according to the latitude of the spectator E. Now as the earth turns once on its axis in 24 hours, it is evident that the several meridians P, PI, PII, PIII, &c., will come successively under the sun at exact intervals of 1 hour, if they are all 15° apart, for 24 multiplied into 15 gives 360, the whole number of degrees to the circle. Suppose, for a moment, that instead of the earth turning up on its axis, once in 24 hours, that the sun moves around the earth in this time, the effect will be the same If the sphere of the earth was transparent, but its axis P D B opaque, then P D' would, as the sun passed around, cast a shadow in the directions D A, DI, DII, DIII, &c., when the sun was in the opposite direction, and the progress of this shadow would mark the hour, according to the meridian in which it should fall. It will be observed, that the intervals A-I, I-II, IIIII, are not equal intervals, but vary, because the circle C H A, cuts the meridians obliquely. Now the sun is so far distant, that if the observer at E should locate a horizontal plane, which, of course, would be parallel to the large plane C H A, and describe on it a small circle, and then divide this circle in proportion as the meridians divide the large circle C H A, and should, likewise, erect from its centre a gnomon, or shadow stick, inclined so as to point to the north star, or in other words, to be parallel to P D, the progress of this shadow would mark the hour. We have here,

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VI VIV

then, the principle, of the horizontal Sun-dial, and all that is necessary to construct one, is, to graduate it proportionally according to the latitude. This can easily be done by calculation, which, however, would involve more of mathematical skill than we shall suppose the reader to possess; we will, therefore, show how it may be done experimentally, and thus any one, with the least ingenuity, can construct a horizontal dial. Referring back

to the figure, page 71, it will not be difficult to perceive that if the circle C H A, had been the equator, then all the angles of the hour lines D A, D I, D II, &c., would have been measured by equal arcs, each 15°. The same would be true of any small circle, IK, parallel to the equator, the meridians, 15° apart, would divide it into 24 equal parts. Now, if on a globe, we should divide any parallel of latitude, such as I K, before alluded to, into 24 equal parts, and then pass a plane, a sheet of paper for example, through each of these divisions and the centre of the globe, then, wherever this plane intersected the plane of any other circle, C HA for example, it would mark out the directions of the hour lines DA, DI, D 1I, D III, &c. Take, now, a flat board, on which a sheet of paper is fastened, and describe a circle whose centre is O, as in the diagram below, and let O B be a metallic

rod, inclined to the line A C, drawn on the paper to represent a meridian line, at an angle equal to the latitude of the place, let DE be a small circle, so fixed on O B, that its plane is everywhere perpendicular to it, or in other words, so that the distance from the point B to the circumference of the circle, may be the same throughout. Let this smaller circle be graduated into 24 equal parts, and subdivided into halves, and quarters, and if desired, still smaller spaces. Take, now, a fine thread, or a straight edge, and carry it from B through each division of the little circle, successively, down to the plane of the paper below, taking care, if a thread is used, not to crook it against the edge of the little circle, but simply passing it straight down. Through the points F, G, H,