[LEAPSECS] alternative to smearing

[Clive D.W. Feather]

Preben Nrager said:
> Let's say Newcomb envisions negative JDs, and astronomy thus uses the JD
> system. Astronomy then have two different "eternal" timescales, with two
> different starting points for zero: The one is the proleptic gregorian
> calendar, represented by ISO 8601, with the starting year zero, and the
> other the JD system, with the starting day zero.

Yes. So what?

> I understand the need in astronomy, and computer science, for a continuous
> timescale, and I understand that continuous days, and fractions of days, is
> better suited that need, than continuous
> years:months:days:hours:minutes:seconds. But I don't understand how
> astronomy can cope with two different starting points for zero.

Very simply. If I write "JD 12345678" I mean a specific day (pace the
issues about which definition of "day" is being used). If I write
"1234-05-26 PG" (or whatever abbreviation is used) I mean a different
specific day. Conversion between the two is a simple algorithm.

> The
> beginning of time must be a beginning in time,

We aren't talking about "the beginning of time". We're talking about the
arbitrary zero point for a date labelling system. There is absolutely no
problem with two systems having two zero points.

For that matter, the actual definition of year numbers in ISO 8601 is - or
was last time I read it - that year 1875 is the year that the Treaty of the
Metre is signed. In other words, it's not even defined by where zero is!

> and I don't see how
> astronomy can have a day zero, that is different from the year zero. The
> zero point in time must somehow be the same for both the daily, and the
> annual timescale.

The reference point of the Celsius temperature scale is the melting point
of pure H2O at an arbitrary atmospheric pressure whose value I forget. The
reference point of the Kelvin temperature scale is the thermodynamic
minimum possible temperature ("absolute zero"). Nothing stops people using
both even though they have different zero points. And that's without
mentioning Farenheit, Rankine, Romer, Newton, Delisle, Reaumur, Leiden,
Wedgwood, and Gas Marks.

Time is no different. Provided that we know the conversion algorithm, there
is absolutely no problem with having multiple systems. See the book
"Calendric Calculations" for further details.

> The way I see the JD system being used in astronomy, it is as the
> fundamental timescale. The number of JD is related to the days in either
> the julian or the gregorian calendar, but the proleptic gregorian calendar,
> with year zero, is not really being used. I don't know if that is because
> Christmas day (December 24/25), and other important days, are not the same
> JD in the julian, and the proleptic gregorian calendar,

1234-12-25 Julian and 1234-12-25 proleptic Gregorian are different days.
Just as 2017-12-25 Julian and 2017-12-25 Gregorian will be different days.
So what?

> but either way, the
> zero point in time must be the same for both the daily, and the annual
> continuous timescale.

Why?

> I propose to reform the JD system with a new system of proleptic gregorian
> days GD. The proleptic gregorian calendar, with year zero, shall be the
> fundamental timescale, and in that calendar each year has either 365 or 366
> days, in accordance with the gregorian leap year rules. Year 0 is a leap
> year.
> 
> 
> The new system of GD shall like JD count the days from noon to noon. But
> the zero day shall not be JD:0. The zero day of GD shall be the day from
> -0001-12-31T12:00 to 0000-01-01T12:00. That day is JD:17210159, so my
> reform will be the removal of 17210158 days from JD, to create GD.

So your GDs are just JD minus 17210158? That shows that you can cope with
two different zero point with no trouble.

> With the new timescale I propose, negative years and negative days will
> always be the same,

Okay.

> and that I think is important.

Why?

Anyone who's dealt with AD, AM, and AH dates knows that a date can be
positive in one system and negative in another. That's not a problem -
there's nothing magical about negative year numbers.

You might just as well argue that we should use LCDs, where the zero point
is the Mayan Long Count date 0.0.0.0.0 (-3113-08-11 PG).

-- 
Clive D.W. Feather          | If you lie to the compiler,
Email: clive at davros.org     | it will get its revenge.
Web: http://www.davros.org  |   - Henry Spencer
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