Subject: Modified Julian Day

From: thompson at orpheus.gsfc.nasa.gov (William Thompson)
Date: 22 Sep 1994 19:40:50 GMT

I've been hearing lately that the IAU either has, or is about to, "outlaw" the
use of Modified Julian Day numbers.  Can anyone shed some light on this?

The SoHO/CDS software library contains IDL software to convert between various
time formats (URL ftp://idlastro.gsfc.nasa.gov/contrib/thompson/time).  The
so-called "internal" uses MJD numbers to store the date information.  This is
convenient, because one can easily convert this into various civilian date
formats, using equations from the Astronomical Almanac Supplement.

Is there such a ban in place now, or is it simply being considered?  Would it
apply to internal formats within software, or only to external use such as
publications and (e.g. FITS) data files?

Thank you,

William Thompson

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From: dwells at nrao.edu (Don Wells)
Date: Tue, 27 Sep 1994 21:30:21 GMT

[this followup was posted to sci.astro.research:]
From: aty at ucssun1.sdsu.edu (young a t)
Newsgroups: sci.astro.research
Subject: Re: Modified Julian Day
Date: 26 Sep 1994 09:35:45 -0500
Organization: San Diego State University

In article <35t02e$5v at pecos.msfc.nasa.gov>,
>use of Modified Julian Day numbers.  Can anyone shed some light on this?

At the recent General Assembly in The Hague, the IAU adopted a resolution
that effectively cancelled an earlier one that recommended the use of MJD.
The basic problem was that the previously adopted MJD (which was itself an
attempt to clear up the confusion among several modifications of the full
Julian Day count then in use) had led to further confusion.

The long and the short of it seems to be that one should stick to the full
JD number, instead of leaving off 2,400,000 or 2,400,000.5 or any other
quantity.  "Modified" Julian Day has turned out to be so prone to confusion
and error that we are all better off to use plain vanilla JD.  If you
absolutely *must* truncate the first 2 digits, explain that what you are using
is JD - 2,400,000 and *don't* try to call it MJD.
-- 
A.T.Young                      aty at mintaka.sdsu.edu
Astronomy Department
San Diego State University
San Diego CA 92182-0540
--
  Donald C. Wells         Associate Scientist         dwells at nrao.edu
                    http://fits.cv.nrao.edu/~dwells
  National Radio Astronomy Observatory                +1-804-296-0277
  520 Edgemont Road,   Charlottesville, Virginia       22903-2475 USA

---

From: dwells at nrao.edu (Don Wells)
Date: Thu, 29 Sep 1994 21:48:46 GMT

[Here is yet another of Andy Young's followups in sci.astro.research -Don]

From: aty at ucssun1.sdsu.edu (young a t)
Newsgroups: sci.astro.research
Subject: Re: Modified Julian Day
Date: 28 Sep 1994 09:52:11 -0500
Organization: San Diego State University
Message-ID: <36bvur$2p6 at pecos.msfc.nasa.gov>
References: <35t02e$5v at pecos.msfc.nasa.gov> <366m81$1oo at pecos.msfc.nasa.gov> <367ekq$1vr at pecos.msfc.nasa.gov>

In article <367ekq$1vr at pecos.msfc.nasa.gov>,
Joe Cain <cain at geomag.gly.fsu.edu> wrote:
>In article <366m81$1oo at pecos.msfc.nasa.gov> aty at ucssun1.sdsu.edu writes:
...
>We have been using MJD = JD - 2400000.5 for about 30 years in
>geophysics and don't see any reason to change so long as the
>definition is clear. We picked it up from SAO when first starting to
>put times on data and found that one CDC word in binary was good to a
>millisecond. Now we have to use some other means to store in the
>shorter words of "modern" platforms. It is better than the "Julian
>Day" generally used by NASA orbital mechanics which only means the day
                        ^^^^^^^^^^^^^^^^^^^^^^
(and UNIX programmers...)

>of that year. 

> How about instead just defining MJD as such and keep it as a useful
>concept for many purposes? Most people don't like the idea of a day

That was what the IAU tried some years ago.  The main problem was caused
by the half day, because the variable-star folks had been using JD - 2400000
(no half day) for a long time.  The result was that people started calling
(JD - 2400000) "MJD" and thus not following the IAU-approved definition.
This year, the IAU gave it up as a bad job; not enough users have the necessary
discipline to keep to the definition.

>... Most people don't like the idea of a day
>starting at noon except for astronomers ....

Well, most people don't like time tied to Greenwich unless they live near
England.  The half day was for the convenience of European astronomers
back in the last century; I'm not sure when that half-day offset from
Greenwich time was introduced.  It was officially abandoned by astronomers
in 1925, when we went from GMAT to UT as the standard.  But, to avoid
confusion, the JD count continued to change at Greenwich Mean Noon.

You are, of course, welcome to continue using MJD *if* you keep your
readers informed of what you mean by it.  It's just no longer approved
by the IAU as an astronomical standard.  Like Prohibition, it looked
like a good idea at one time, but did not work out well in practice....
-- 
A.T.Young                      aty at mintaka.sdsu.edu
Astronomy Department
San Diego State University
San Diego CA 92182-0540

---

From: Arnold Rots <arots at xebec.gsfc.nasa.gov>
Date: Fri, 30 Sep 1994 09:02:46 -0400

> discipline to keep to the definition.

It looks very much like the tail wagging the dog.  To me it appears as
though a tiny segment of the community couldn't muster the discipline
to call their deviant definition something else and, after having
caused confusion in their own circles (there are variable star people
who do use the proper definition of MJD), possibly out of spite, force
everbody to abandon a useful concept.

This decision to recant is even worse, since most of the community is
not going to change what they have been doing for years, acting as if
it still is an IAU-blessed unit, while in reality it isn't (speaking
of confusion!).

There are too many FITS files out with MJD, there is too much software
that uses it.  The variable star folks will have to face reality and
acknowledge that even an IAU decision can't keep the community from
using the former IAU-condoned definition of MJD.

> confusion, the JD count continued to change at Greenwich Mean Noon.

I rather suspect that the half day offset originated from nautical
practice to start the day at noon: that was the moment in the day that
all important things happened, with noon sightings of the sun and
taking the nautical almanac from the shelf - determining the elevation
of the sun at midnight is too problematic.

> like a good idea at one time, but did not work out well in practice....

Unlike prohibition, it _was_ a good idea ...

I'm sure this was pretty much a storm in a teacup.  FITS files will
continue to carry MJD - perhaps with an extra comment - and most
astronomical software will continue to use it.

What may need to be stressed, though, is that using MJD (or JD, for
that matter) for defining a moment in absolute time requires not only
an MJD value that includes a fractional part, but also an indication
what time system is being used.  The Julian Day is not tied to any
particular time system (UTC, TAI, TT, ...), it only provides a
formalism for counting calendar days within such a time system.  For
instance, the epoch 1994.0 (UTC) (January 1, 1994 at 00:00:00 UTC, or
MJD=49352.0 (UTC)), is at MJD=49352.000696574074 (TT).  Very seldom
does one see a time system identified when MJD is given.

> -- > A.T.Young aty at mintaka.sdsu.edu 
> Astronomy Department 

  - Arnold Rots