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the stars; or in other words that the axis of the equator did not uniformly maintain its position relatively to that of the ecliptic, but that the poles of the equator performed a sort of revolution round those of the ecliptic, by which the places of the intersection of these two planes were continually changing. These observations were made so far back as in the time of Hipparchus, about 150 years before Christ, and even by the Chaldean and Indian astronomers at a much earlier period, Hipparchus discovered that in bis time the point where the equator crossed the ecliptic at the autumnal equinox was situated about 6 degrees to the eastward of the bright star called Spica Virginis in the constellation Virgo : he drew, also, iom observations made 150 years before bis ou n time that the point of the same equinox was then about s degrees to the eastward of that star: from these and other discoveries he concluded that the equinoctial points in the heavens were not fixed, but that they moved to the westward about 2 degrees in 150 years, or 1 degree in 75 years, or 48 seconds in one year. Subsequent observations, however, have shown that this motion of the equinoxes amounts to about 50 seconds of a degree
every year: so that if the heavenly equator cut the ecliptic at any given point or any given day of one year, ir will on the same day of the following year cut in 50 seconds 10 the wesiward of the former point; and the carth will come to the equinox 20 minutt's 23 seconds before she has completed her circuit round the heavens from one star to the same again. Ilence, we see the reason for the difference between the tropical or equinoctial year and the sider al year, as stated in the table formerly given of the motions of the solar planets where the tropical year is marked 305 days, 5 hours, 48 minutes, 49 seconds, and the sidercal year 365 days, 6 hours, 9 minutes, 12 seconds, their disference being as above 20 minutes, 23 seconds. This motion of the equinoctial points is called the
precession precession of the equinoxos, because the earth's position at those points precedes the calculations.
The signs of the zodiac formerly coincided with the constellations from which they draw their names, but by the precession of the equinoctial points of the earth's orbit, the stars of the constellation Aries at this time, in fact, occupy the sign or space of 30 degrees in the ecliptic, which we call Taurus, the constellation Taurus occupies the sign Gemini, and so on to the constellation Pisces, which now fills the sign Aries.
The limits of this work preclude the possibility of enumerating and illustrating all the valuable purposes to which astronomy is applicable in the occurrences of ordinary life; all therefore that can be attempted is to point out some of its uses in two very important departments of human knowledge, Chronology, or the art of measuring time, and by it July arranging the succession of events from the earliest periods, and Navigation, or the art of conducting vessels from one part of the globe to another, across the path. less ocean.
CHRONOLOGY is a term formed from two greek words,
The simplest and most natural mode of dividing time
The commencement of the day has, in different countries, and at different periods of history, been reckoned in very different ways. The ancient Babylonians, Syrians, Persians, and Indians, counted their days to begin at sunrise ; the Jews began their civil day at the same time, but the sacred
day YOL, Ile
day began at sunset; the Romans counted from midnight to midnight, the mode now adopted all over Europe, with the exception of the Venetian territory, the Papal dominions, and some other districts of Italy where it was lately the practise to begin the day at sunset, the first hour after which was one o'clock, the second hour two o'clock, and so on through the four and twenty hours to the following sunset, by which mode the hours of midnight and midday were continually varying according to the time of sunset in the different seasons of the year. The astronomical and nautical day in Europe is reckoned from the noon of one day to the noon of another, and begins 12 hours later than the civil or ordinary day: hence 8 o'clock in the morning of the 10th of February will in astronomical and nautical language be counted the 20th hour of the 9th of February, and 5 o'clock in the afternoon of 22d of November, which is the 17th hour of the common day, will be counted only the 5th hour of the astronomical day.
Among the Jews and the Romans, the day was divided into four vigils or watches, the first extending from sunrise half way to noon, the second from that intermediate point, abuut 8 or 9 o'clock, to noon when the third watch began and continued to 3 or 4 in the afecrnoon, whence the fourth watch reached to sunset : in the same way was the night also divided into other four watches. In modern Europe the day is divided into 24 equal parts calle: hours, each hour into 60 equal parts called minutes, and each minute into 60 equal parts called seconds. It has been found by experiment at London, in a spot elevated 113 feet above the level of the sea, wben Fabrenheit's thermometer in dicated a temperature of 60 degrees, and the mercury in the barometer stood at 30 inches, that a pendulum in length 39,1196 English standard inches (not 39.2 inches, as is commonly supposed) would perform its vibrations precisely in one second of time, that is vibrate 60 times in one
minute, 3,600 times in one hour, and 85,400 times in one day. This length of the pendulum, from the figure of ine earth an oblate spheroid of which the equatorial diameter is greater than the polar axis, will not beat seconds in all parts of the earth's surface: fur as the gravitation by which the pendulum is kept in motion acts the more powerfully the nearer any body is to the center of the earth, the vibrations of a pendulum regulated for the latitude of London would be too quick at the poles, and two slow at the equator; consequently at the former station, the pendulum would require to be lengthened, and at the latter station to be shortened, in order at each to vibrate 60 times in one minute, or once in a second of time.
By observing the regular returns of the various phases of the moon in her course round the earih, was formed the notion of that extent of time called a month : and as twelve revolutions of the moon brought round the several seasons nearly to their proper place, so many months were reckoned to compose the year. The moon's revolution being divided into four equal parts, by the first quarter the full moon and the third quarter, the period of that revolution, reckoned to be 28 or 29 days; was also divided into four equal portions of seven days each, called weeks.
The ancient Greeks adopted a mode of calculating time, in which they differed not more from other people than amongst themselves. The Athenians divided the year into 12 months consisting alternately of 30 and 29 days, by this compensation agreeing with the monthly circuit of the moon; and each month was divided into three portions of 10 days each, called dechemera or decades, and in those of 29 days, one day was suppressed in the computation, so that the last was always reckoned the 30th day of the month.
In imitation of this mode of reckoning time, the late republican government of France distributed the year into 2 K 2