Base Date - 17-Nov-1858

Created by: Lester Caine, Last modification: Thu 07 of Aug, 2014 (19:49 UTC)

Brief History of the Gregorian Calendar

As decreed by Pope Gregory XIII, October 4, 1582, was followed by October 15, 1582. Thus ended the 1600-year reign of the Julian calendar upon which the Gregorian calendar is based, and thus began the calendar which DECwindows Calendar uses to measure time.

Calendars based on sun and moon movement were used even by the ancients, but the first reasonably accurate one was the 365-day cycle calculated by the Greek Sosigenes. This was the calendar authorized by Julius Caesar in 46 B.C. The Julian calendar (not to be confused with the Julian period; see below) had 3 years of 365 days each, followed by a fourth year of 366 days.

The 365-day cycle was more accurately defined in 730 AD by the Venerable Bede, an Anglo-Saxon monk, who shortened the time by 11 minutes, 14 seconds. This accumulates to a whole day's error every 128 years, or a little more than 3 days every 400 years. This being the Dark Ages, nothing was done to adjust the Calendar, despite Roger Bacon sending a note to Pope Clement IV, informing him of the drifting of the date for the vernal equinox. Later, Pope Sixtus IV did become convinced that another reform was needed and called the German astronomer Regiomontanus to Rome to advise him. Unfortunately, Regiomontanus died of the plague shortly thereafter and the plans died with him.

Thursday, October 4, 1582 was the next time the calendar was adjusted. This last day of the Julian calendar was followed by Friday, October 15. So began the Gregorian calendar that we use today, named after Pope Gregory XIII. He commissioned the mathematician Father Christopher Clavius, S.J., to do the necessary calculations, having been authorized to reform the calendar by the Council of Trent in 1545.

The Vatican librarian Aloysius Giglio provided a formula for long-range accuracy. He suggested that every fourth year be a leap year, except for century years that are not divisible by 400. Thus 1700, 1800, and 1900 would not be leap years, but 2000 would be, because 2000 is divisible by 400. This rule eliminated 3 leap years every 4 centuries, making the calendar sufficiently correct for most ordinary purposes.

Julian Period

Astronomers use the Julian period because it is convenient to express long time intervals in days rather than months, weeks and years. It was devised by Joseph Scaliger, in 1582, who named it after his father Julius, thus creating the confusion between the Julian (Caesar) calendar and the Julian (Scaliger) period.

Julian Day 1 began at 12:00 noon, January 1, 4713 B.C. This date was thought by some to correspond approximately to the beginning of the universe. Certainly it predated any known astronomical events known in the 16th century without resorting to negative times. Scaliger decided on the actual date on the grounds that it was the most recent coincidence of three major chronological cycles:

  • The 28-year solar cycle, after which dates in the Julian calendar (for example September 27) return to the same days of the week (for example Tuesday).
  • The 19-year lunar cycle, after which phases of the moon return to the same dates of the year.
  • The 15-year indiction cycle, used in ancient Rome for tax regulation.

It takes 7980 years to complete the cycle. Noon of January 1, 1988, marks the beginning of Julian Day 2447161.
The Julian period is also of interest because of its use as a time base by the OpenVMS operating system.

ULTRIX and Digital UNIX Time Origins

The beginning of time for ULTRIX and Digital UNIX systems is: Thursday January 1 00:00:00 1970 The reason for this date being chosen is that this was the year that UNIX, the "parent'' of ULTRIX, was first released.
Thus dates prior to 1970 are BU; 1970 and later dates are AU.

OpenVMS and the Julian Period

Why OpenVMS regards November 17, 1858 as the beginning of time...

The modified Julian date adopted by SAO (Smithsonian Astrophysical Observatory) for satellite tracking is Julian Day 2400000, which turns out to be November 17, 1858.

SAO started tracking satellites with an 8K (nonvirtual) 36-bit IBM 704 in 1957 when Sputnik went into orbit. The Julian day was 2435839 on January 1, 1957. This is 11225377 octal, which was too big to fit into an 18-bit field. With only 8K of memory, the 14 bits left over by keeping the Julian date in its own 36-bit word would have been wasted. They also needed the fraction of the current day (for which 18 bits gave enough accuracy), so it was decided to keep the number of days in the left 18 bits and the fraction of a day in the right 18 bits of one word.

Eighteen bits allows the truncated Julian day (the SAO day) to grow as large as 262143, which from November 17, 1858, allowed for 7 centuries. Possibly, the date could only grow as large as 131071 (using 17 bits), but this still covers 3 centuries and leaves the possibility of representing negative time. The 1858 date preceded the oldest star catalogue in use at SAO, which also avoided having to use negative time in any of the satellite tracking calculations.