Calendar Basics

The Babylonian Calendar

The Babylonian calendar was made up of 12 lunar months. The year starts at Nisanu in the Spring; within about 30 days of the vernal equinox [1, p. 131]. However 12 lunar months are about 11 days short of a year, so in order to prevent the months from drifting through the seasons, an intercalary 13th month, either a second Ululu (VI₂) or a second Addaru (XII₂), was occasionally inserted by decree of the King. Prior to approx. 475 BC this was done at irregular intervals, a list of known intercalary months is contained in Babylonian Chronology 626 B.C.-A.D. 75 [2, p. 4].

The Babylonian day began at sunset. Each month starts 'when the thin crescent of the new moon was first visible in the sky at sunset' [2, p. 1]. The moon becomes visible between about 15 to 48 hours after conjunction (at Mediterranean latitudes), although first sighting may also be delayed by weather conditions, but not by more than one day [1, p. 131].

Julian and Gregorian Calendars

The Julian calendar was introduced by Julius Caesar in 45 BC [3]. However, the primary calendar used today is the Gregorian calendar, named after Pope Gregory XII who introduced it in October 1582. It is a minor refinement to the Julian calendar with the aim of bringing the length of an average year closer to an actual solar year, it does so by modifying the rules used for choosing leap years.

The Gregorian calendar uses the following rule: "Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400." the mean length is therefore 365.2425 days. Whereas the Julian calendar has a leap year every 4 years and therefore has a 365.25 day mean length. The actual solar year is approx. 365.2422 days.

In addition to changing the leap years, the Gregorian calendar was offset by a 10 days to restore the date of Easter to the time which it would have been celebrated by early church. In the 20th and 21st century the Julian calendar would be about 13 days ahead of the Gregorian calendar. Note that whilst the Gregorian calendar was quickly adopted in Roman Catholic Europe, it took some time before the rest of the world followed. It was adopted in Great Britain in 1752 and was not adopted in most of Eastern Europe until the early 20th century.

Mapping Babylonian dates to the Julian calendar

For the purposes of astronomy and historical dating it is the standard practice to use the Julian calendar when dealing with dates prior to 15 October 1582, and the Gregorian calendar for later dates. All calendar dates that are used in this book when discussing the Neo-Babylonian era will therefore be using the Julian calendar.

Also note that the year 0 BC / AD did not exist - rather the years progress from 1 BC directly to 1 AD. Therefore, when using negative years there will be a 1-year difference; for example the year -100 is the same year as 101 BC.

References