History and Biography. 



Chronological Eras. The year 1899 

 corresponds to the year 7407-8 of the Byzan- 

 tine era ; to 5659-60 of the Jewish era, the 

 year 5650 beginning at sunset on September 

 4 ; to 2652 since the foundation of Rome ac- 

 cording to Varro ; to 2675 of the Olympiads 

 (the third year of the 669th Olympiad begin- 

 ning July 1, 1899) ; to 2559 of the Japanese 

 era, and to the 32d year of the Meiji ; to 1316- 

 17 of the Mohammedan era or the era of the 

 Hegira, the year 1317 beginning on May 12, 

 1899. The 125th year of the Independence 

 of the United States of America begins on 

 July 4, 1900. 



Date of Beginning of Epochs, Eras, 

 and Periods. 



NAME. BEGAN. 



Grecian Mundane Kra B. c. 5598, Sept. 1 



Civil Era of Constantinople ' 5508, Sept. 1 



Alexandrian Era ' 5502, Aug. 29 



Ecclesiastical Era of Antioch ' 5492, Sept. 1 



Julian Period ' 4713, Jan. 1 



Mundane Era ' 4008, Oct. 1 



Jewish Mundane Era ' 3761, Oct. 1 



Era of Abraham ' 2015, Oct. 1 



Era of the Olympiads ' 776, July 1 



Roman Era (A. U. C.) ' 753, Apr. 24 



Era of Nabonassar ' 747, Feb. 26 



Metonic Cycle '. 432, July 15 



Grecian or Syro-Macedonian Era ' 312, Sept. 1 



TyrianEra ' 125, Oct. 19 



SidonianEra ' 110, Oct. 1 



Csesarean Era of Antioch ' 48, Sept. 1 



Julian Year , '. ' 45, Jan. 1 



Spanish Era ' 38, Jan. 1 



Actian Era ' 30, Jan. 1 



Augustan Era ' 27, Feb. 14 



Vulgar Christian Era A.D. l,Jan. 1 



Destruction of Jerusalem " 69, Sept. 1 



Era of Maccabees " 166, Nov. 24 



Era of Diocletian " 284, -Sept. 17 



Era of Ascension " 295, Nov. 12 



Era of the Armenians.. " 552, July 7 



Mohammedan Era " 622, July 16 



Persian Era of Yezdegird " 632, June 16 



Divisions of Time. The interval be- 

 tween two consecutive transits of a fixed star 

 over any meridian or the interval during which 

 the earth makes one absolute revolution on its 

 axis is called a Sidereal Day, and is invariable, 

 while the interval between two consecutive 

 transits of the Sun over any meridian is called 

 an Apparent Solar Day, and its length varies 

 from day to day by reason of the variable 

 motion of the earth in its orbit, and the incli- 

 nation of this orbit to the equator, on which 

 time is measured. 



A Mean Solar Day is the average or mean of 

 all the apparent solar days in a year. Mean 

 Solar Time is that shown by a well-regulated 

 clock or watch, while Apparent Solar Time is 

 that shown by a well-constructed sundial ; 

 the difference between the two at any time is 

 the Equation of 7't'me, and may amount to 16 



minutes and 21 seconds. The Astronomical 

 Day begins at noon and the Civil Day at tin 

 preceding midnight. The Sidereal and .Mean 

 Solar Days are both invariable, but one day of 

 the latter is equal to 1 day, 3 minutes, ami 

 56.555 seconds of the former. 



The interval during which the earth makes 

 one absolute revolution round the Sun is called 

 a Sidereal Year, and consists of 365 days, <i 

 hours, 9 minutes, and 9.6 seconds, which is 

 invariable. 



The Tropical Year is the interval between 

 two consecutive returns of the Sun to the Ver- 

 nal Equinox. If this were a fixed point, tlie 

 Sidereal and Tropical Years would be identic;)] : 

 but in consequence of the disturbing influence 

 of the moon and planets on the spheroidal fig- 

 ure of the earth, the Equinox has a 'slow, ret- 

 rograde mean motion of 50.26 seconds annually. 

 and the Sun returns to the Equinox sooner 

 every year than he otherwise would by 20 min- 

 utes, 23.6 seconds ; the Tropical Year, there- 

 fore, consists of 365 days, 5 hours, 48 minutes, 

 and 46 seconds. The Tropical Year is not of 

 uniform length ; it is now slowly decreasing 

 at the rate of .595 second per century, but 

 this variation will not alw r ayc continue. 



Julius Ctesar, in B. C. 45, was the first to re- 

 form the calendar by ordering that every year 

 whose date number is exactly divisible by 4 

 contain 366 days, and all other years 365 days. 

 The intercalary day was introduced by count- 

 ing the sixth day before the Kalends of Marcli 

 twice; hence the name bissextile, from bis, 

 twice, and sex, six. He also changed the be- 

 ginning of the year from first of March, to tin- 

 first of January, and. also changed the name ot' 

 the fifth month (Quintilis) to July, after him- 

 self. The average length of the Julian year is 

 therefore 365^ days, which, however, is too 

 long by 11 minutes and 14 seconds, and this 

 would accumulate in 400 years to about three 

 days. The Julian Calendar continued in use 

 until A. D. 1582, when the date of the begin- 

 ning of the seasons occurred 10 days later than 

 in B. C. 45, when this mode of reckoning time 

 was introduced. 



The Gregorian Year was introduced by Pope 

 Gregory XIII. with the view of keeping the 

 Equinox to the same day of the month. It 

 consists of 365 days, but every year exactly di- 

 visible by 4 and the centurial years which are 

 exactly divisible by 400 contain 366 days : and 

 if in addition to this arbitrary arrangement 

 the centurial years exactly divisible by 4,000 

 contain 36 days, the error in the Gregorian 



