Subject: Re: Modified Julian Day

From: arots at heasfs.gsfc.nasa.gov (Arnold Rots)
Date: Sat, 1 Oct 1994 22:23:06 GMT

[this is yet another followup copied from newsgroup sci.astro.research]

From: arots at heasfs.gsfc.nasa.gov (Arnold Rots)
Newsgroups: sci.astro.research
Subject: Re: Modified Julian Day
Date: 30 Sep 1994 15:12:20 -0500
Organization: NASA Goddard Space Flight Center -- Greenbelt, Maryland USA

In <36bvur$2p6 at pecos.msfc.nasa.gov> aty at ucssun1.sdsu.edu (young a t) writes:

>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:
>>>In article <35t02e$5v at pecos.msfc.nasa.gov>,
>>>William Thompson <thompson at orpheus.gsfc.nasa.gov> wrote:
>...
>>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.

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.

>>... 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.

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.

>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....

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.

--
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