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The Julian Date

In 1583, the French-born historian and astronomer Joseph Justus Scaliger proposed a continuous counting of days within a "Julia n Period", consisting of 7980 Julian years. 7980 years represent the least common multiple of the moon cycle (19 years), th e solar cycle (28 years), and the indiction (15 years). The start of this count is 1 January 4713 BCE, this year being t he first in all these three cycles. The Julian day number is marked with the letters "JD" behind the number itself.

The day starts at 12:00h UT, which is 13:00h BST or MET, or 14:00h MEST. Thus, a certain time can be expressed by fraction s of a day. As an example, 12 August 1987, 11:51h UT is represented by 2,447,019.99375 JD. This gives an easy way to compute the time between two events by simply subtracting the Julian day number of one from the other. The name "Julian" day number was probably chosen because of the use of Julian years, but there are reports that Scaliger named it after his father Julius.

The Julian day number is mainly used in astronomy. In astronautics, sometimes the so called Modified Julian day number, marked MJD, is used. This counting starts from 17 November 1858, 0h UT, which is 2,400,000.5 JD. The most significant difference to the originial JD is the beginning of the day at midnight UT. The date mentioned above would thus be written as 47,019.49375 MJD.

Another modification of the Julian Date is the Truncated Julian Date, abbreviated TJD, which is also used in astronautics. Its starting point is 2,440,000.5 JD or 24 May 1968.

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