Message Concerning
Chronological Julian Days/Dates

From: Peter Meyer
To: John Hynes
Subject: Julian Day Number
Date: Fri, 23 Jan 2004

Hi John,


>I was wondering what your sources are for "Chronological Julian Day Number."
>I have not encountered this anywhere else, and all links in Google seem to
>use your page as their source. I know that JD started with Herschel in
>1849, MJD started with the SAO in 1957, TJD started with NASA in the 60's,
>but I'm curious about when, where and who CJD started with, and where it is
>used now.

The fact that your Google searches return only my webpages would suggest
that the term originated with me. That is correct. The basic difference
between an astronomical JD and a chronological JD (D = date or day) is that 
in the former case a nychthemeron begins at noon and in the latter case at 
midnight. Chronological JDs are useful in calendrical studies, whereas 
astronomical JDs are inconvenient because calendrical dates usually 
(though not always) refer to nychthemerons which begin at midnight. 
The usefulness of the concept, and the need to distinguish CJDs from AJDs, 
may not be obvious to astronomers but it is to calendricists.

As for where the concept of the CJD is used now, it is used at least 
(and only?) by people who have followed my usage, particularly those 
students of calendrical science who read the CALNDR-L mailing list.

The other difference (often overlooked) between AJDs and CJDs is that the
former is tied to zero degrees of longitude whereas the latter is relative
to any longitude, e.g., 120 degrees East for Beijing. When speaking of CJDs
one may assume zero degrees longitude unless otherwise stated, e.g., for
CJDs in the study of the Chinese Calendar.

A consequence is that CJDs associated with the Chinese Calendar begin
eight hours earlier than CJDs associated with the Common Era Calendar,
since when it is midnight [BST] in Beijing it is 4 p.m. [GMT] in London. 
Thus although the most-recent Chinese new years day, 79-21-01-01 CHL,
corresponds to 2004-01-22 CE, at 6 p.m. [GMT] on 2004-01-22 CE the Chinese
date is 79-21-01-02 CHL, since the next nychthemeron has already
begun in Beijing.

Peter Meyer