THE SOUTHWORTH PLANETARIUM
207-780-4249 www.usm.maine.edu/planet
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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.
[image: 220px-Joseph_Justus_Scaliger_2.jpg]
*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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