MEASURES OF TIME.
Time is best defined as the succession of motion and phenomena, independent of relative human perceptions. Time is measured by man by the impressions of successive ideas, and these diminish in a ratio of their own increase; conse quently, time appears less as men advance in age, or are variously employed. At ten, a year seems to be twice as long as at twenty; three times as at thirty ; four times less at forty ; five times at fifty ; and six times at sixty; circumstances of employment and position being the same. Hence a month employed in traveling seems equal to three of usual pursuits. Time being mentally measured by the impressions of new ideas. Absolute time, independent of the feelings of individuals, is measured by certain regular motions, as the rotation of the earth, the swing of a pendulum, the fall of a body, the revolu tions of the moon round the earth, or the earth round the sun. The tropical year is 365 days 5 hours 48 minutes 51.6 seconds; but the sidereal year, or return to the same star, is 365 days 6 hours 9 minutes 11 seconds; but as the line of apsides or aphelion point advances 655 seconds, the orbit is completed in 365 days .6 hours 15 minutes 20 seconds, and this is called the anoma listic year. The Chaldean made the sidereal year 365 days 6 hours 14 minutes, or one minute 49 seconds more than our present year, and the tropical year 365 days 5 hours 49 minutes 30 seconds, being 38 minutes 4 seconds more than ours. If Hypparchus was right in his measure of the tropical year, it is 11.2 seconds legs than in his time. The Brahmins made it 1 minute 43 seconds more than now.
The precession of the equinoxes is performed in 25,868 years; and the revolution of the line of apsides is 20,931, or 1 degree, 43 minutes, 10 seconds in a century. The precession of the equinoxes Is 50 minutes 25 seconds per annum, or 1 degree 52 minutes 45 seconds in a century, or the 360 degrees in 25,868 years. Leap year Is the year which divides evenly by four; but the year 1900 will not be leap year, to make ap for the odd minutes gained between the astronomical and computed year, as 365 days. The astronomical aqninoxes are on the 21st of March and the 21st of September, and the sun is in the tropics on the 21st of December or June. Quarter days in civil reckoniug are March 25, June 24, September 29, and December 25, being festivals of the Catholic Church. The Synodical Inner montn of her departure from the sun's center to the return, or from fall to fall, is 29 days 12 hours 44 minutes 12 seconds, and was the universal month of the ancients, twelve being accounted a year. The difference between the solar and lunar year is nearly eleven days. The sun and moon return invariably to the Same relative positions every 223 lunations; according to the ancient Chaideaue in 6585 days 8 hours. Our modern tables make it 17.29 seconds less. The Eastern nations, where the day varies little, reckon the day from sunrise. The Romans reckoned as we de, from midnight. Christian nations assign thirty days to April, June, September, and November, thirty-one to other months, and twenty-eight to February, making three hundred and sixty-five ; but three hundred and sixty-six in Leap-year, when February is twenty-nine. The Romans added the day in Leap-year on the sixth of the calends of Mardi, making two sixths or tris sextua, and hence the word Bissextile. The astronomical day Is the time which elapses from the sun's being on the meridian of a place till his return, divided into twenty-four hours of sixty minutes. And astrono mers begin the day at the departure of the sun from the meridian of the place, counting twelve hours till midnight, p.m. or after; and twelve hours from midnight till noon a.m. or before; their day after twelve at night being a day later than civil reckoning, which begins a new day at twelve at night, and reckons from twelve at night to twelve the next night. As the earth advances in its orbit di minutes 9.9 seconds, when in Its perihelion, and only 57 minutes 10.7 seconds, when In its aphelion, while it returns the same meridian to the sun, that meridian arrives at the same fixed star in twenty-three hours 56 minutes 41 seconds or 3 minutes 6.59 seconds less; others make it twenty three hours 56 minutes '34 seconds or 3 minutes 56.6. seconds less. Sidereal days are always the same, and accord with a true clock ; but owing to the uneqnsl velocity, as expressed, of the earth in its orbit in the perihelion and aphelion, the sun's return to the meridian varies; and also• as the earth's path is inclined to the axis of rotation, the solar days vary. Owing to the first cause, the extreme• difference is 7 minutes 39 seconds on March 21, and 6 minutes 5 seconds on May 6- 0 on July 1, making the sun slower than the clock; but owing to the oblicinity It is 0 on March 21; 9 minutes 53 seconds on May 6th, and O June 22, making the sun faster than the clock. The cam-. bination of both causes produces the table of equation of time by which true clocks ought to he kept faster or slower than the meridian sun or a sun dial, called appar ent time, and the other true or mean time. As the earth moves forward in•its orbit 59 minutes 8.3 seconds, while• it turns on its axis, any place arrives at a fixed star 3 min utes 56.6 seconds, before it arrives again at the center of the sun, called twenty-four Hours; hence the sidereal day is but 24 hours 56 minutes 3.4 seconds, which is the real
period of revolution. Hence, in 365 solar days, the earth turns 366 times on its axis, and by this exact quantity it gains in its orbit with reference to the sun and stars in every revolution, and hence the procession of the equi noxes. For as the orbit is caused by the sun, and the equinoxes have reference to the sun and not to the stars, so the equinoxes fall back 24,890 miles, or in space 50.30 seconds with reference to the stars. Every other body like the earth turns once on its axis by going round a central body, and this, therefore, is the sole cause of the equinoxial points.. The Chal deans, Egyptians, and Jews, pegan their civil year from the. autumnal equinox. The Persians, Greeks, Romans, and the English till 1752, began the year at the vernal equinox. Theuce to make dates agree with those of other nations, between January and Lady-day, writers used to put two, 1708 dates, as Feb. 10, -- The bottom date being from. 4709 January 1, and the upper that from the previous Lady-day. Sidereal days are always 23 hours 56 minutes 3.4 seconds; but as the axis of the earth is inclined to the orbit, and the. earth moves faster iu the winter than in the summer the clocks which measure equal time do not agree with the sun. A clock and a sun dial will, therefore, vary as. follows :March 21, clock too fast 7 minutes 35 seconds; April 6. do. 2.55 seconds; April 20, June 13, September 5, December 25, they are equal. When the sun Is in. Libra and Scorpio, or in October and November, the difference is from ten minutes to 16 minutes 18 seconds, and the clock ought to be so much faster than the dial. The difference of velocity makes the dial faster than the clock, while the earth is moving from its perihe lion on January 1, to the aphelion Jnly 1; and on the contrary, the dial is slower from July 1 to January 1. But the obliquity makes the dial faster from March 21 to Jane 22; and from September 22 to December 21, and slower from June 22 to September 22, and from December 21 to March 21. The union of both produces the common equa tion table, page 39, showing to the nearest minute how much a true clock should be faster or slower than the sun. The pendulum for true and exact measuring of time was it suggestion of Galileo, in consequence of his observing the oscillations of a chandelier. Every oscillation, whe ther long or short, of the same pendulum, is performed in the same time. A pendulum which vibrates seconds at London ought to be 39.139 inches nearly, and the length of pendulums for less or greater times is as the square of time. In India a day is divided into 60 Ghurries, a Ghurry into 60 Pule, a Pul into 60 Prans, and a Pran into 10 Tu. in 2-5ths of a second. The beats iu an hoar of a common second's clock are 3,600, and 17,280 a common watch ; but second watches beat 18,000 times, or 5 per second. A lumi nous point to produce a visual circle must go round seven. times In a second. Fewer than 30 vibrations in a second give no sound, and when the vibrations exceed 7,520 in a second the tones cease to he discriminated. 10 beats of a healthy pulse Is equal to 9 seconds. The Roman lustre were periods of 5 years; and the Greek olympiads periods of 4 years; and the first commenced in 776 B. C. The Metonich cycle was 19 years, intended to be equivalent te the Chaldean period. It was afterward adopted as the golden number for Easter. The Hegira, or Flight, took place July 16, 622, and is the Mahomedsn Era. Their year is 12 lunar months, or 354 days 8 hours 48 minutes; and 11 days being lost, a year must be alloWed every 33, to reconcile their dates with ours. The periodical month of the Turks and Arabs. or sidereal period of the Moon Is 2'1 days 7 hours 43 minutes 48 seconds. The synodical month, or return to the conjunction of the Sun 11129 days 12 hours 44 minutes 8 seconds 11 thirds. The Jews began the year in March, and the months were Niailn, Sivan, Tammuz, Ab, Elul, Tieri, Bul. Cialen, Tibeth, She bat, Adar. The Sabbath, or seventh day is Saturday. The days and nights, from sunrise to sunset, wore divided into twelve equal parts or hours, 1, 2, 3, etc. The night watches were three hours each, from sunset to sunrise. The months were lunar, or 80 days and 29 days, and they intro duced an extra month every two or three years. The day commences and ends at sunset. The Jewish months were 29 and 30 days, and their year of twelve lona tons 354 days. Their year commences with the vernal equinox. To recover the four daya they intercalcate a whole month after every two or three years, following their twelfth month, or Adar; and they call this extra month, ve-adar. The Jewish day commences at six in the evening, or sunset, and continuea till the same hour on the evening. Their civil year commeucea with the new moon near the vernal equinox, in the month called 'Thai, of 80 days, corresponding with part of September .and part of October. The year 1829 was the Jewish year -6589, and ended September 27, beginning September 9, 1828.
northern signs. From the autumnal to the winter solstice, 89 days 16 hours 47 minutes. From the winter to the spring equinox, 89 days 1 hour 42 minutes. South ern signs.