Converting Julian dates

[Bob Stockler]

On Fri, Nov 12, 2004 at 08:34:31PM -0500, Ron Kracht wrote:
| >
| Technically a Julian date is the number of days since noon on January 1, 
| 4713 BC <http://scienceworld.wolfram.com/astronomy/BC.html>.  It's 
| called a Julian date because that method of dating was first proposed by 
| Julius Scaliger in 1583.  In the computer world it seems to mean the 
| number of days since any commonly agreed upon epoch date - which is why 
| Ken asked what kind of Julian date.  Since it is originally an 
| astronomical calculation the time of day matters although for normal 
| date calculations the time of day is generally assumed to be noon - 
| resulting in whole number values when calculating the Julian date.

Just for the purpose of supplying more information than
anyone on this list wants, I didn't have to google for
what follows - it's extracted from the Calendar FAQ by:

        This document is Copyright (C) 1996 by Claus Tondering.
        E-mail: ct at login.dknet.dk.

a copy of which resides on my computer.  The first section
is quoted only because it's referred to in the second one,
in a discussion of why the year 4713 BC selected.

============================== extract =================================
2.11. What is the Indiction?
----------------------------

The Indiction was used in the middle ages to specify the position of a
year in a 15 year taxation cycle. It was introduced by emperor
Constantine the Great on 1 September 312 and abolished [whatever that
means] in 1806.

The Indiction may be calculated thus:
        Indiction = (year + 2) % 15 + 1

The Indiction has no astronomical significance.

The Indiction did not always follow the calendar year. Three different
Indictions may be identified:

1) The Pontifical or Roman Indiction, which started on New Year's Day
   (being either 25 December, 1 January, or 25 March).
2) The Greek or Constantinopolitan Indiction, which started on 1 September.
3) The Imperial Indiction or Indiction of Constantine, which started
   on 24 September.

2.12. What is the Julian period?
--------------------------------

The Julian period (and the Julian day number) must not be confused
with the Julian calendar. The Julian calendar is named after the Roman
leader Julius Caesar (c. 100-44 BC), whereas the Julian period is
named after the Italian scholar Julius Caesar Scaliger (1484-1558).

Scaliger's son, the French scholar Joseph Justus Scaliger (1540-1609),
introduced the Julian period and named it after his father. His idea
was to assign a positive number to every year without having to worry
about BC/AD.

Scaliger's Julian period starts on 1 January 4713 BC (Julian calendar)
and lasts for 7980 years. AD 1996 is thus year 6709 in the Julian
period. After 7980 years the number starts from 1 again.

Why 4713 BC and why 7980 years? Well, in 4713 BC the Indiction (see
section 2.11), the Golden Number (see section 2.9.3) and the Solar
Number (see section 2.4) were all 1. The next times this happens is
15*19*28=7980 years later, in AD 3268.

Astronomers have used the Julian period to assign a unique number to
every day since 1 January 4713 BC. This is the so-called Julian Day
(JD). JD 0 designates the 24 hours from noon UTC on 1 January 4713 BC
to noon UTC on 2 January 4713 BC.

This means that at noon UTC on 1 January AD 2000, JD 2,451,545 will
start.

This can be calculated thus:
        From 4713 BC to AD 2000 there are 6712 years.
        In the Julian calendar, years have 365.25 days, so 6712 years
        correspond to 6712*365.25=2,451,558 days. Subtract from this
        the 13 days that the Gregorian calendar is ahead of the Julian
        calendar, and you get 2,451,545.

Often fractions of Julian day numbers are used, so that 1 January AD
2000 at 15:00 UTC is referred to as JD 2,451,545.125.

Note that some people use the term "Julian day number" to refer to any
numbering of days. NASA, for example, use the term to denote the
number of days since 1 January of the current year.
============================ end extract ===============================

Anyone who has read this far is either a masocist or finds
stuff like this to be as interesting as I do.

Bob

-- 
Bob Stockler - bob at trebor.iglou.com
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