# Re: [LEAPSECS] MJD and leap seconds

From: Rob Seaman <seaman_at_noao.edu>
Date: Tue, 10 Jan 2006 10:54:11 -0700

On Jan 10, 2006, at 9:17 AM, Peter Bunclark wrote:
> On Tue, 10 Jan 2006, Poul-Henning Kamp wrote:
>> In message <Pine.GSO.4.58.0601101151020.1614_at_cass18>, Peter
>> Bunclark writes:
>>> Good grief. MJD is used widely in astronomy, for example in
>>> variablility studies where you want a real number to represent
>>> time rather than deal with the complications of parsing a date.
>>
>> So how do you deal with fractional days in that format ?
>
> with decimals.

I'm not one to shy away from irony (see! just proved it again...),
but I do think there is a real issue here. Was interested to read
the pages Tom pointed us to. Both the IAU position and McCarthy's
exposition of same are curiously silent about the issue of resolving
ambiguities resulting from non-denumerable SI intervals and solar days.

The IAU tells us:

> 1. Julian day number (JDN)
>
> The Julian day number associated with the solar day is the number
> assigned to a day in a continuous count of days beginning with the
> Julian day number 0 assigned to the day starting at Greenwich mean
> noon on 1 January 4713 BC, Julian proleptic calendar -4712.
>
> 2. Julian Date (JD)
>
> The Julian Date (JD) of any instant is the Julian day number for
> the preceding noon plus the fraction of the day since that instant.
> A Julian Date begins at 12h 0m 0s and is composed of 86400 seconds.
> To determine time intervals in a uniform time system it is
> necessary to express the JD in a uniform time scale. For that
> purpose it is recommended that JD be specified as SI seconds in
> Terrestrial Time (TT) where the length of day is 86,400 SI seconds.
>
Which is to say that day number is (always) a solar unit and fraction
of day (sometimes) an SI unit.

In "practical" terms, a JD(TT) expression would simply be calculated
by running a count of TT seconds since some epoch through the obvious
conversion mill, but we're then returned to the central issue of
reconciling such a JD(TT) with a JD(UT1). A calculation would simply
show a growing fractional difference between the two, of course. At
issue is the unit jump in JDN. Which day is it? This ambiguity only
holds for a bit over a minute a "day" in the current epoch. (UTC =
TAI - 33s, TT = TAI + 32.184s) The ambiguity is growing.

Perhaps the SI unit should have been called the "essen", rather than
the "second", as Steve Allen has said. But whatever it is called, it
has a clear definition. But what is the definition of a day? Am
convinced we need to reach a consensus on this before leaping (irony
again) into any changes to the current rules of civil/business/
international/legal/historical date and timekeeping.

You'll note that I omitted "technical" and "scientific" from that
list. This is not now and has never been a discussion about
resolving purely technical issues, although some of the implications
strongly affect technical people.

Rob
Received on Tue Jan 10 2006 - 09:55:20 PST

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