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(Modified from The AAVSO web pages, and the Encylcopedia Brittanica) Dating of historical events and the determination of how many days elapse between events can be difficult because of the need to keep track of leap years, varying month lengths, etc. So astronomers simplify their timekeeping by simply counting the days. All days are numbered consecutively from Julian Day 0, which began at noon on January 1, 4713 B. C. January 1st, 1993, was JD 2448989; January 1st, 2000 will be JD 2451545. The Julian Day begins at noon, UT,where is the standard time kept in London, England. Clock time after noon is converted to a fraction of a day. Thus midnight GMT (12 hours beyond noon) is 0.5 day, while 1:00 P.M in London is 0.041666667 day (1/24th of a day). This fraction of a day beyond noon is added to the whole-number JD to compute the JD of any event or observation. Hence 1:00 PM in London on January 1st, 2000 will be JD 2451545.041666667. Most astronical observations are recorded to the nearest 0.0001 day (8.64 seconds) or better. The Julian Date, JD, has nothing directly to do with Julius Caesar (who was known, nevertheless for his work on calendars). In fact the origin of the Julian date has little to do with anything that is considered serious science nowadays. Rather, the Julian Date system was derived from the rather cockeyed theories of the 16th Century French classical and literary scholar Joseph Justus Scaliger (1540-1609). Born in Agen, France, on August 5, 1540, Joseph Justus Scaliger was the son of Julius Caesar Scaliger (1484-1558), an Italian physicist and philosopher who had emigrated to Agen in 1525. Scaliger'’s early education was obtained in Bordeaux, and in 1555, he traveled to Paris to study Greek and Latin. To round things out, he began teaching himself Hebrew, Arabic, Syrian, Persian and modern languages. After converting to Protestantism in 1562, he toured French and German universities, and visited Italy to study its ancient relics. The mid-16th Century saw bitter religious strife in France and, following the St. Bartholomew’'s Day massacre in August, 1575, and to avoid the persecution being meted out to Protestants, Scaliger went to Geneva and taught at an academy there, returning to France in 1574. Eventually he was called to the University of Leiden, Holland, in 1593, and came to be regarded as one of the most erudite scholars of his time. He died there in exile on January 21, 1609. Scaliger'’s greatest work, Opus de emendiatane tempore or Study on the improvement of Time, was published in 1583. In this masterpiece, he discussed and compared previous calendars, correcting errors and bringing order into the chronological system. To help in this respect, he devised a consecutive numbering system based on a cyclic period of great length, which he named the Julian Period in honor of his father. The Julian Period was taken to be a cycle of 7980 years, and based on the product of the Metonic year, a cycle over which lunar postions repeat, the solar cycle, which is the period after which the days of the seven-day week repeat on the same dates. and the indiction cycle, which is a fiscal cycle of 15 years related to Egyptian tax receipts, and has no astronomical connection. With the 7980 year period, Scaliger believed he had a period of sufficient length to encompass most historical and future events. Tracing the Metonic, solar and indiction cycles backwards in time, Scaliger found that all three coincided sometime in the year 4713 B.C. On the basis of information available to him, Scaliger believed that date to be January 1, 4713 B.C. Although the Julian Date system has no real historical or astronomical significance, it is so useful that it is still used both by historians and astronomers, for keeping a running count of time.
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