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CHAPTER VI.

Dials and Dialing
" This shadow on the Dial's face,

That steals from day to day
With slow, unseen, unceasing pace,
Moments, and months, and years away,
Right onward, with resistless power,
Its stroke shall darken every hour,
Till Nature's race be run,
And Time's last shadow shall eclipse the sun."

* J. Montgomerj. In the preceding chapter, we have made frequent use of the word day, and have throughout meant what is called a mean Solar day. We have already shown that the Siderial day is the time of an exact revolution of the earth on its axis. This day is shorter than the Solar day, by about 4 minutes. We have also alluded to the apparent motion of the sun in the heavens, showing that if to-day he came to the meridian at the same time with any particular star, to-morrow the star would come to the meridian before the sun, which had apparently changed its place in the heavens. Let us consider to what the difference between Solar and Siderial time is really owing, and see how much the Siderial day should be

shorter than the Solar, to do which we will have recourse to a diagram

Let A B C D, represent the earth's annual orbit, showing the earth in four different positions, and let a be the situation of some particular meridian, that of Greenwich, for example. Now, on the supposition that the earth does not rotate on its axis at all, suppose it moving in its orbit, in the order of the letters ; it is not difficult to see that the effect will be the same, as though the earth, remaining at rest in its orbit, had turned once on its axis during the year, but in a contrary direction to its present diurnal motion. Thus, while at A, the sun would be on the meridian a, but at B, one fourth of a year after, the sun would set in the east, and at C, half a year afterwards, it would be midnight at the same meridian, a. At D the sun would just begin to rise in the west, and finally at A would come to the meridian again. It will now be understood, that although the earth does turn on its axis, during its yearly circuit, yet this day as really occurs as if the earth had not the diurnal revolution, hence the number of rotations, measured by the sun's coming to the meridian, will be less than the number as announced by a star, by one day, and therefore the Siderial day must be shorter than a Solar day, by the proportional part of a revolution, which is thus divided up among, and added to the 365 Solar days of the year. Upon the supposition that the mean Solar day is just 24 hours in length, the Siderial day will be, the one-three hundred and sixty-fifth and one-fourth, of 24 hours, shorter, i. e. 3m, 56s, very nearly, and a star, in consequence, will come to the meridian 3m, 56s, sooner than the sun, each day, or will gain so much on the sun daily.

We have more than once intimated that the time elapsed between a star's leaving the meridian, to its return to it again, viz: 23h, 56m, 4.01s, is the precise measure of a rotation of the earth, and for this reason astronomers prefer to regulate their time keepers to show what is called Siderial time. Now, suppose to-day to bo the 14th of April, which is near the time of vernal equinox, the precise point where the ecliptic intersects the equator, we will imagine to be shown by a bright star. By means of his transit instrument, the astronomer ascertains exactly when this star is on his meridian, and just then sets his clock going, the hands showing at the time Oh, Om, Os, and at the same time the town-clock, we

RIGHT ASCENSION AND DECLINATION,

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, Os. 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 lh, 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 portion's, 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

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 2015 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 ofdinary 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, deyote the remainder of this chapter

SUN-DIALS.

71 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 4 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 aro 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.

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Let P A B C, be the earth, and E the position of a spectator upon it, and let F Gebe 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

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