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Julian Day System
Astronomers simplify their timekeeping by merely counting the days,
and not months and years. Each date has a Julian Day number (JD),
beginning at noon, which is the number of elapsed days since January
1st, 4713 B.C. For instance, January 1st, 1993, was JD 2448989; January
2nd, 1993, was JD 2448990; and January 1st, 2000, was JD 2451545.
Why the year 4713 B.C.? The Julian Day system of numbering the days
is a continuous count of days elapsed since the beginning of the Julian
Period. This period was devised by Joseph Justus Scaliger, a French
classical scholar in the 16th century. Scaliger calculated the Julian
Period by multiplying three important chronological cycles: the 28-year
solar cycle, the 19-year lunar cycle, and the 15-year cycle of tax
assessment called the Roman Indiction. To establish a beginning point
for his Julian Day system, Scaliger calculated the closest date before
1 B.C. which marked the first day for the beginning of all three
cycles. This day is January 1, 4713 B.C., which is Julian Day number
1.
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