Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!elroy.jpl.nasa.gov!ucla-cs!valeria.cs.ucla.edu!wales From: wales@valeria.cs.ucla.edu (Rich Wales) Newsgroups: comp.binaries.ibm.pc.d Subject: Re: (Astronomical) Julian Date Source Wanted Message-ID: <1991Apr16.235756.7558@cs.ucla.edu> Date: 16 Apr 91 23:57:56 GMT References: <9873.2808dac2@ohstpy.mps.ohio-state.edu> <40790006@hpcvra.cv.hp.com.> Sender: usenet@cs.ucla.edu (Mr. News Himself) Reply-To: wales@CS.UCLA.EDU (Rich Wales) Organization: UCLA CS Department, Los Angeles Lines: 34 Nntp-Posting-Host: valeria.cs.ucla.edu In article <40790006@hpcvra.cv.hp.com.> everett@hpcvra.cv.hp.com. (Everett Kaser) writes: >Anyone have (preferably TIGHT) source for Julian (Astronomical, >not USGovn't) Dates? C would be nice, but Se Habla Fortran, Pascal, Lisp, ... Just out of curiousity, what's the difference between Astronomical and USGovernment Julian dates? The NIST (formerly NBS) uses something called the "Modified Julian Date" (MJD) in their computer time service. I assume this is what is meant, since as far as I know, there is nothing that is officially called a "US Government Julian Date". The MJD is equal to the ordinary Julian Day number minus 2,400,000.5. Example: 18 April 1990, 00:00 UTC, is JD 2,447,999.5; but MJD 47999.0. Motivations for MJD are twofold: (1) The initial "24" in the Julian Day will not change until well into the 22nd century, so for present-day applications it is superfluous. (2) Some people are irked by the half-day (.5) offset in the Julian Day. Neither the Julian Day nor the Modified Julian Date have anything at all to do with the "Julian Calendar" (the older version of our western civil calendar, which inserted leap years a bit too often, and which was modi- fied into our present-day Gregorian Calendar several centuries ago). -- Rich Wales // UCLA Computer Science Department 3531 Boelter Hall // Los Angeles, CA 90024-1596 // +1 (213) 825-5683