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In 1583, the French scholar Joseph Justus Scaliger published De emendatione temporum, in which he attempted to resolve the various historical eras in use by relating all of them to a single system. Rather than use negative numbers, he chose to use an initial epoch which predated any known historical record. |
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His approach used three different cycles which were well-known at the time the 28-year solar cycle through which weekdays and calendar dates move in the Julian calendar, the 19-year cycle of Golden Numbers over which the moon (almost) returns to the same calendar dates, and the 15-year indiction cycle used in Roman taxation. The Golden Number cycle is also referred to as the ''Metonic" cycle, in honor of the Greek astronomer who discovered it. This cycle was known, however, to the Chinese prior to this time. |
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Thus, a given combination of these three cycles occurs once every (281915 =) 7,980 years. Scaliger determined that the most recent year in which all of these cycles had the value 1 was the year 4713 B.C. This he called year 1 of the Julian Period. The name "Julian" has usually been taken as an honorific referring to Scaliger's father, but Scaliger himself states that it refers to the Julian calendar. |
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The use of the term Julian for this numbering scheme has led to a certain amount of confusion. First, it may be confused with the Julian calendar, named after Julius Caesar, who instituted a calendar reform in Rome which was used until the Gregorian reform in 1582. Second, it can also be confused with the doubtful practice, often used in commercial circles, of calling a day count within a year a "Julian date". |
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It remained, however, for the astronomer John F. Herschel to turn this idea into a complete time system, rather than a method of relating years. In 1849, Herschel published Outlines of Astronomy and explained the idea of extending Scaliger's concept to days. |
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In this system, Julian Day 0 commenced on noon of 1 January 4713 B.C., and the Julian Day number (JD#) represents the number of days that have elapsed since that epoch. |
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Herschel added several important contributions to Scaliger's idea. First, he extended the concept to days. Second, he further extended the concept to fractions of days, so that decimal fractions could be converted into hours, minutes, and seconds. In so doing, he linked both kinds of time clock time and calendar time into one system. Finally, being an astronomer, he did himself a favor. Astronomers typically work at midnight, and it is a nuisance to change units in the middle of the working day, so he designated noon, not midnight, as the zero point for the start of a day. The midnight that starts a civil day therefore begins 0.5 days earlier than the Julian Day. I'm being a bit facetious here, of course. The identification of noon is easier, using simple instruments, than is the identification of midnight. |
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Therefore, by definition, Julian Day 0 is 1 January 4713 B.C., or as astronomers would have it, 1 January 4712. |
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