THE SOUTHWORTH PLANETARIUM

207-780-4249   www.usm.maine.edu/planet

70 Falmouth Street   Portland, Maine 04103

43.6667° N                   70.2667° W 

Altitude:  10 feet below sea level

Founded January 1970

Julian date: 2458772.16

2019-2020:  XXXI

                     "What does that Julian date mean?  Is it akin to a 'star date?'"

THE DAILY ASTRONOMER
Tuesday, October 15, 2019

Out on a Julian Date

The Julian date, always included in the header, refers to a dating system used primarily, but not exclusively, by astronomers.     It tabulates the days that have elapsed since  noon time on January 1, 4713 BC* (-4712).       The noon, in this instance, refers to Universal time, designated as the time along the Prime Meridian, which intersects Greenwich, England.     This system, as one can see above, uses whole numbers and decimals to denote a specific instance in time.   Such a scheme serves astronomers well as the calendar is absolute and therefore one doesn't need to take time zone differences into account.  For instance, at noon eastern time, the Julian date will be 2458772.16 because the time in Greenwich will be 4 p.m, four hours after noon.   Another Julian date started four hours earlier, hence the .16     Let's imagine that a significant celestial event (apart from the posting of the DA) occurred at that precise instant.     Astronomers around the world would know precisely the time it happened because the Julian date is universal.  

Joseph Justus Scaliger

(1540-1609)

Medieval scholar and religious leader Joseph Justus Scaliger developed this calendar system in 1583, one year after the Gregorian calendar reform.**     Intended to make every date unique (at least throughout a 7980 year period), Scalinger chose 4713 BCE as it was the date at which three independent cycles converged.  These three cycles were

Solar - 28 years

Lunar - 19 years

Indiction - 15

28 x 19 x 15  = 7980 years

The 28 year solar cycle is based on the Julian calendar.   Every 28 years, the placement of days of the week relative to the numerical days repeats.     The twenty-eight year periodicity occurred because of the leap day included every four years.   This 28-year periodicity does not occur in the more complex Gregorian calendar

The 19 year lunar cycle refers to the repeat of lunar phases at the same time of year every nineteen years.   This cycle occurs because 19 years is almost equal to 235 synodic months, defined as the period of 29.5 days  at which the lunar phases repeat.      For instance, between 1711 - 2300, the full moon occurs on or next Christmas Day every 19 years.  We last experienced a Christmas full moon on 2015.  The next Christmas full moon occurs in 2034.

The 15 year indiction cycle, far more legal than astronomical, was a dating system used for many medieval documents. This system, which I tried almost earnestly to learn more about, was still in use during Scalinger's life.  

One could say that every single moment in time as well as every single point in space is unique and should be treated as such.      We know that we will be able to designate a unique number for each moment at least until AD 3268, when a new Julian date cycle begins anew.

*We recognize that BCE is more often used that BC.  To be deferential to all parties concerned, we will use alternately use BC and BCE.   People might be a bit chagrined to read the BC. After all, we small planetarium astronomy bloggers are notorious for our religious fanaticism.   However, we just want to be fair to all.  

**When Pope Gregory XII altered the calendric system so as to realign the vernal equinox date around March 21st.   The previous Julian calendar, instituted by Roman dictator Julius Caesar, 

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