Peter Bunclark wrote in message ...
Nick Maclaren wrote:
In article ,
"David J Taylor" -this-bit writes:
| "Nick Maclaren" wrote in message
| ...
|
| Believe it or not, this is a computing question! I am interested
| in finding out whether there is a generally accepted formula for
| converting TAI to solar time and, much more importantly, whether
| there are generally accepted bounds for its uncertainty. I have
| a suspicion that there isn't :-)
|
| Nick, could you point to a reference as to _which_ solar time you mean?
| Do you mean the 20minute or so variation between (for example) local noon
| and highest solar elevation? The so called "Equation of the Sun", if I
| recall correctly?
Oops. Mea Culpa. I posted before doing enough research to check
that all of the terms I used were well-defined :-(
The problem is this:
I want to be able to convert TAI to UTC/GMT/civil time at Greenwich,
for every date from the origin to the heat death of the universe.
Now, obviously, doing so to the nearest second is a failing known
as Delusion of Accuracy, so I am not making that mistake. But,
equally, simple numerical accuracy is not enough, as a historical
record may well be a 'precise' time of day.
Now, the conversion between TAI and UTC (or, rather, the one that
computer people always refer to as UTC) is defined from its start
to the present. GMT is a bit messier, but not too bad. But I
should like to know error bounds for civil solar time before GMT
and into the future.
Note that, because of the way that I am thinking of doing this,
I don't need the ACTUAL correction - what I need to define my
interface is some approximate BOUNDS on the correction. This is
to know how many bits to allow for it rather than to specify a
value. And, yes, of course I am thinking of using a sort of
floating-point format for far-flung times :-)
Regards,
Nick Maclaren.
The short answer is, no one knows by how much the earth will spin up or spin
down in the future (long before the heat death of the Universe, the Sun
will become a red giant, which is expected to have an impact on the
length of
the day). For a much more comprehensive answer, dig deep into
http://hpiers.obspm.fr
especially at the leap seconds section.
Pete.
PS, GMT is historic.
The term 'leap' day or second denotes the less geometrical calendar
system based on the equable 24 hour day.To determine the annual
cyclical motion of the Earth as 365 days 5 hours 49 minutes,the
equable 24 hour day must of neccesity be defined and determined first.
Originally,the Equation of Time adjustment was employed by astronomers
and later by navigators to reduce the natural unequal day to the
equable 24 hour day to facilitate the seamless transition from one 24
hour day to the next 24 hour day.With this method there is no need to
take into account fraction of days as with the calendar system for it
is based on the rotation of the Earth wrt the Sun,the addition and
subtraction of minutes and seconds which is the mathematical bridge
between the observed natural unequal day and equable 24 hour clock day
equalises the variation in orbital motion (Kepler's second law) to
facilitate the isolation of axial rotation to the 24 hour/360 degree
equivalency.The following graphic should be adequate for presenting
where equable variable orbital motion is equalised by the Equation of
Time by addition and subtraction of minutes and seconds even though
there is a natural variation causing the asymmetry between one noon
and the next.
http://ircamera.as.arizona.edu/NatSc...res/kepler.htm
The following Equation of Time tables in conjunction with the above
graphic express the annual cyclical loop system which precedes the
calendar system and Flamsteeds isochronos sidereal method,again,there
are no 'leap' factor involved.
http://www.burnley.gov.uk/towneley/tryall/eot3.htm
Perhaps priority in dealing with this matter and the unwarranted
assertion that GMT or the 24 hour/360 degree equivalency for the axial
rotation of the Earth is merely historical exists in noting that to
derive the sidereal value based on the annual orbital cycle as 365
days 5 hours 49 min approx,the equable 24 hour days must of necessity
be determined first.
Apply the sidereal value to heliocentric modelling of the Earth and it
generates circular orbits and constant axial rotation wrt to the
Sun,this is something that does not occur and is in direct conflict
with Kepler's second law and subsequently Newton's gravitation laws.
http://www.absolutebeginnersastronomy.com/sidereal.gif
http://ircamera.as.arizona.edu/NatSc...res/kepler.htm