the horizon, as the index had been moved, and as shown at a, it

would appear in the quadrant, as in the figure preceding,brought to the line of the horizon. Now just before noon, on ship-board, the sailor sets the index of his quadrant to about the altitude of the sun, and defending the eye by a set of dark glasses, shown at G, page 82 he looks through the eye-hole D, and the unsilvered portion of the horizon glass, and sees a distinct image of the sun,' almost touching the horizon, thus :

It is true, he cannot see the horizon in the silvered portion of the horizon glass, but he can bring the image close to the line where the silvering is removed from the glass, and then by inclining his quadrant a little, as in figure 2, page 83, he can make the sun, apparently, describe the dotted arc c d, just touching the horizon. We will suppose he is looking just before noon, i. e. before the sun comes to the meridian, or reaches his highest altitude in the heavens, and that an assistant stands near, ready to note the time when this highest point is reached. As he looks through his quadrant, the image of the sun, which a moment before described the arc c d, and appeared to touch the horizon in its course, will seem to rise a little, he therefore moves the index, and brings it down again, all the time sweeping backward and forwards; if it rises a little more, he again brings it down, very soon he perceives

APPARENT TIME.

85 it to be changing its position scarcely at all, and gives notice to the person with the watch, or chronometer, to be ready; in a moment, instead of rising, as before, it begins to dip below the horizon, and he calls out, and the time is accurately noted. This is the exact instant of 12 o'clock, apparent time, or the instant when the sun, having reached its highest point, begins to decline. Now the chronometer, with which he has been observing, does not say 12 o'clock, but perhaps, 3h. 5m. 10s. in the afternoon. We will suppose the observation to be made on the 27th day of August. On this day, as will appear from a table showing the equation of time, a clock adjusted to keep true solar time, should show 12h, 1m. 10s. at apparent noon, and this is the time which the clock would show at Greenwich, at apparent noon there upon this day; but when it is apparent noon at the place where we have just supposed an observation made, the Greenwich clock shows 3h. 5m. 10s., the difference is 3h. 4m., which, allowing 150 for each hour, indicates that the observation is made in a place 46° west of Greenwich. It is west, because the sun comes to the meridian later than at Greenwich. Now if the latitude was known by observing the altitude of the polar star, then, by referring to a chart, the position, either on ocean or land, where the observation was made, could be indicated; for all charts, or globes, which represent the earth's surface, have lines drawn upon them, through the poles, called meridians, showing every degree east or west of Greenwich, and also every degree north or south of the equator.

We will close this somewhat tedious chapter, with an allusion to a circumstance which has sometimes puzzled the uninitiated, viz: two ships may meet at sea and vary in their reckoning a dayor two. Suppose a traveler, leaving New York on a certain day, to travel continually east, until after a certain time, one year, or perhaps twenty, he arrives at the place from which he started; and farther, suppose he has kept an accurate note of the number of days which has intervened. For every 15° he has traveled east, the sun has risen one hour earlier to him than to those left behind. This gain, by the time he has traveled 360°, amounts to a whole day, and when he arrives home he finds his reckoning one day in advance of his neighbors, or in other words, he has

E

seen the sun rise once more than they have. The year to him has consisted of 366 days, but to his neighbors of only 365. Now, what is not at all an improbable case, we will suppose him arriving home on a leap year, on the 28th day of February, and which he calls Sunday, the 29th, but those who have remained at home call it Saturday. The next day, February 29th, is, according to them, Sunday; here is another Sunday in February, but there have already been four others, viz: the 1st, the 8th, the 15th, and the 22d, making six Sundays in this shortest month. It is said ' that this case has actually occurred; that a ship left New York on Sunday, February 1st, and sailing eastward continually, arrived home, according to her log-book, on Sunday, the last day in the same month, but really on Saturday, according to the reckoning at home. The next day, being the intercalary day, made the 28th, and 29th both, Sundays to the voyagers; thus giving six Sundays to the month. If, on the contrary, a voyage had been made westward, one day would have been lost in the reckoning, as the sun would rise one hour later for each 150, and if two travelers should leave the same place, say on Tuesday, and each, after passing completely around the globe, the one east, and the other west, should again meet at the same place, there would be a difference of two days in their account, the one calling the day Monday and the other Wednesday, when, in reality, it would be Tuesday.

CHRONOLOGY.

87

CHAPTER VII.

Chronology.

66

Brightly ye burn on heaven's brow;

Ye shot a ray as bright as now,

When mirrored on the unruffled wave

That whelmed earth's millions to one grave."

E. P. Mason.

WE have more than once mentioned the importance of the movements of the heavenly bodies, in determining certain chronological questions, and will now give some farther illustrations of this subject. The precession of the equinoxes, and the occurrence of solar and lunar eclipses, are the two astronomical events which have been of most essential service. We have, in the preceding pages, illustrated the precession of the equinoxes, showing that the places of vernal and autumnal equinox, or the points where the ecliptic intersects the plane of the equator, moved westward at the rate of 50 seconds of arc in one year. The phenomena of solar and lunar eclipses, we have not explained, nor does it fall within the limits we have prescribed to our little volume, to embrace them. We shall, therefore, only refer at present, to the service which chronology has received from the knowledge of the retrogradation of the nodes of the earth's orbit, on the ecliptic. As already shown, the path of the ecliptic in the heavens, is divided into 12 equal parts, of 30° each, called signs, and these signs formerly gave the names to the constellations, or groups of stars near which they were located, when the ecliptic was thus first divided or portioned out. That point in the ecliptic where the vernal equinox is located, was then, and has been always, designated as the first point of Aries, but as this equinoctial point changes its position, moving contrary to the order of the signs in the ecliptic, at the rate of 50.2 seconds a year, the first point of the sign Aries no longer corresponds with that group of

stars to which it formerly gave a name, for the shifting of the equinox cannot carry forward the stars with it. The vernal equinoctial point is now situated in the constellation Pisces, having altered its position about 30° since the constellations were grouped and named in their present order. As we know the annual amount of the precession, we can determine how long ago the present zodiac was formed, viz:

50.2:1 year:: 30° (=108,000'): 2155.6 years,

that is, about 300 years before the Christian era, when the most celebrated astronomical school of antiquity, flourished under the auspices of the Ptolemies, and the labors of the astronomers of that school, the most celebrated of whom was Hipparchus, who formed a catalogue of the stars, were recorded in the Almagest of Ptolemy, and constituted the chief knowledge upon this subject, until the times of Kepler, Tycho Brahe and Copernicus. The conclusions which we may come to, from ancient astronomical observations, are necessarily liable to some error, from the imperfect manner in which their observations were made, most of them having been but approximations, and not very close ones, to the truth. We have illustrated, (page 60), in what manner the precession of the equinoxes causes the pole of the heavens to revolve around the pole of the ecliptic, the effect of which is, that successive stars, which lie in the circumference of the circle which the pole of the heavens thus describes, will, in succession, become the pole star. The present polar star was not always the pole star, nor is it as near the true pole of the heavens now, as it will be. In about 240 years, it will be but 29′ 55′′ distant from the pole. At the time of the earliest catalogues, it was 12° distant, and now, 1848, its distance is about 1° 25'. About 2900 years before the commencement of the Christian era, the bright star in the tail of Draco, called Alpha, was the polar star, and was then only 10' from the pole; and in 11,600 years, the bright star Lyra, will become the polar star, and will then be but 50 from the pole, whereas, its distance now is upwards of 51°. We give on the next page, a representation of that part of the heavens where the north pole of the ecliptic is situated.

Here we have the pole of the ecliptic in the centre, and the