On 2016-04-25 11:11 AM, John Sauter wrote:
On Mon, 2016-04-25 at 09:40 -0400, Brooks Harris wrote:
Hi John,
"understood and widely used ", yes. Standardized by an international
standards organization, I'm not sure. Anyone know of one? There's a
lot of things in timekeeping that are done on a "common practice" or
"de facto standard" basis. In some cases these are not as commonly
understood as one might wish.
I also don't know of an ISO standard for the Julian Day Number, but it
has been used by astronomers for about 400 years, and everybody seems
to agree on its definition.
Yes, its well known and well established. But it drives me a little nuts
when there's not a real standard, and timekeeping is full of such
things. Gregorian calendar is well defined in ISO 8601 for example, and
that is something you can hang your hat on. UTC similarly, but the
specifications are scattered throughout BIPM, IERS, and ITU-R so its not
so easy to understand. It holds water in the end, but its pretty fragmented.
Doing something non-standard just to create a unique time scale
doesn't seem like a good enough reason.
It can avoid any ambiguity of interpretation if its clearly defined
especially its alignment to 1972-01-01
00:00:10 (TAI) = 1972-01-01T00:00:00 (UTC).
To be sure, but it is also possible to avoid any ambiguity of
interpretation by using a well-understood and widely-used method for
specifying days.
Sure. But a negative 86400 day number is simpler to explain, understand,
and implement - no conversion required until you need to align it to
something else - Julian, MJD, POSIX, PTP, GPS, etc. Just a thought.
Julian Day has an epoch of "12 noon 1 JAN -4712 (4713 BC)". Beyond
that you've got to go to a "proleptic Julian Date" which is not
exactly "standard". A negative 86400 second day number extends to the
arbitrarily distant past depending on how many bits you decide to
carry.
Julian Day may be OK. But somebody might ask when, exactly, did the
Chicxulub meteor impact? I know that's beyond your scope but your
timescale extended further as need arose.
I suspect negative Julian Day Numbers isn't "a standard" because there
is little need for them. I myself don't have any problem with negative
Julian Day Numbers. The meteor hit at approximately Julian Day Number
-24,105,840,000.
OK. Wow. Fun to think about :-)
Maybe someday we will know when it hit to the day, or
even the second.
A really good time scale would start with the Big Bang and count time
using a fundamental unit something like Planck time, about 10 to the
-43 seconds.
Yup. But "proleptic UTC" as you are doing it is a useful engineering
approximation for "civil time" purposes on Earth. It gets a little dicey
if you ask what proleptic UTC time it was when the impact that created
the moon occurred. Were there Leap Seconds before that?
-Brooks
I am happy for programs which read the data file to compress it to
suit
their needs, but TAI-UTC won't fit in 11 bits if you want to go
back to
the year -1000, which has a DTAI over 25,000.
Right. Depends on how far back you want to go. 11-bits TAI-UTC gives
you 2048 Leap Seconds, so, by your table 1, back to year 1000 or
there abouts. That would be good enough for a lot of historical
events. Who uses it for what would drive the implementation choices.
32-bits is very lightweight. Its just an observation - your target
range is bigger.
-Brooks
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