Origin of Julian Days

(This article originally appeared in the fourth quarter 1990 STAR Newsletter. Also references to BC and AD changed to the modern versions BCE/CE)

Origin of Julian Days
by David Williams

One of the most ancient uses of astronomy was timekeeping. In every civilization, the motions of the Moon and Earth were used to calculate the time and the date. In ancient Greece, each community kept a separate calendar, and all were based on the lunar cycle. The earliest Roman calendars were derived from the Greek calendars, and until the first century BCE, had only 10 months. From the middle of the second century BCE, January 1 officially marked the beginning of the year. By 70 BCE the Roman calendar had 12 months. To give the months an average length of 29.5 days, the months had 29 and 30 days alternately. Thus, while the tropical year has approximately 365.25 days, the Roman calendar had only 354 days. In three years, this 11-day difference accumulated to approximately one month. Every third year, the Romans inserted a 13th month between February 23 and February 24, to keep their lunar year approximately in step with the seasons.

In 46 BCE, Julius Caesar adopted a calendar based on the tropical year, whose length his astronomers had determined to be 365.25 days. To account for the fractional day, three common years each contained 365 days, and every fourth year, called a leap year, had 366 days. To bring the date of the vernal equinox back to its traditional date of March 25, Caesar added three months to the year 46 BCE, giving it a length of 445 days. Caesar's new calendar went into effect on January 1, 45 BCE

By 325 CE, the date of the vernal equinox had slipped to March 21. The Julian year (365 days and 6 hours long) is 11 minutes and 14 seconds longer than the tropical-year length of 365 days, 5 hours, 48 minutes, and 46 seconds. By 1582, the accumulation of 11.25 minutes per year added up to another 10 days, and the first day of spring fell on March 11.

To prevent Easter (which, since 325 CE, is observed on the Sunday after the first full moon of spring) from being celebrated in winter, Pope Gregory XIII made further revisions to the Julian calendar. First, to restore the date of the vernal equinox to March 21, he dropped 10 days from the calendar. By proclamation, the following October 4, 1582 became October 15. To compensate for the accumulation of 11 minutes and 14 seconds per year (i.e., one full day every 128 years), he decreed that only century years that are divisible by 400 should be leap years. Thus, year 2000 will be a leap year, while 1700, 1800, and 1900 were not. His Gregorian calendar thus was made accurate to one day in 3,300 years. A modern modification to the Gregorian calendar demoted the years 4000, 8000, 12000, ..., to common years, making the modern calendar accurate to one day in 20,000 years.

A year after the Gregorian calendar was defined, a French historian and chronologist named Joseph Justus Scaliger introduced a timekeeping system that counts days. Scaliger's goal was to generate an outline of history in which all historical events could be listed in one well-defined order. His study of Greek and Roman chronology, the biblical record of the Jews, and the histories of Egypt, Assyria, Persia, and several barbarian nations revealed the existence of at least 50 different calendars. He tried to reconcile the calendars, one against the others, so that he could convert the time of any historical event, according to any calendar, into the equivalent date in any other calendar.

The process of converting days and weeks to months, months to years, and years to centuries was both tedious and confusing. Scaliger finally realized that he could establish a one-to-one mapping of dates from two or more calendars only if the calendars had a common reference point. Then he would be able to count forward in all calendars, in one-day increments, until he reached a particular day of historical interest. Scaliger concluded that if he could define an epoch (i.e., a common reference date), he could collapse all the calendars into one time-ordered list.

Scaliger wanted the epoch date to have calendrical significance. His fundamental criterion was that Day 1 should fall on January 1, so that Day 1 would begin a year. His second criterion was that the year should be distant enough in the past that it predated recorded history. Then all historical events would be associated with a positive date. Negative dates, such as March 15, 44 BCE would be avoided.

Scaliger also wanted Day 1 to fall on the day of a full moon, so that Day 1 would simultaneously be the start of a solar year and a lunar year. As early as 500 BCE, Babylonian astronomers had observed that 19 solar years exactly equalled 235 lunar months. He also wanted the year to be a leap year in which January 1 fell on a Sunday. He found that this event is also cyclic, repeating every 28 years. Combining his criteria, Scaliger looked for years in a cycle that is 28x19=532 years long. Those are years in which the solar cycle and the lunar cycle coincide, and in which January 1 begins the week, the lunar year, and the solar year. More briefly, he looked for leap years that began on a Sunday when the moon was new. Because there were many, 532 years apart, he needed one more selection criterion.

As his final criterion, Scaliger chose a historical, rather than astronomical metric. He selected the period of the indiction. In indiction years, Romans took a census of individuals and property to support a system of taxation. In 300 CE, the period of indiction was set at 15 years. The custom of indiction was still practiced during Scaliger's lifetime.

Because 15 is mutually prime with 19 and 28, it produces a cycle of 15x19x28 or 7980 years. Scaliger searched for a leap year that was also an indiction year that began on a Sunday with a new moon. He found the year 4713 BCE, and January 1 of that year became Day 1 of the Julian Period, so named in honor of his father, Julius Caesar Scaliger.

The next Julian cycle will begin on January 1, 3268, which is 7980 years after January 1, 4713 BCE.