Just to clarify--
The changing length of the solar day affects the number of days in
a year but not the actual length of the year.
The earth is not near resonance with any other major solar system
bodies so variations in the actual length of the year will be small
and of long period. This remains to be quantified from references I
don't have handy. For the next several million years there should be
no overall trend for lengthening/shortening of the year.
By 1e9 years solar mass loss may have been sufficient to produce a
noticeable increase in the length of year. It's not out of the
question that spin-orbit coupling between earth-moon system and sun
should be included in attempts to model the future over that time scale.
Richard
On Wed, 11 Jan 2017, Robert Jones wrote:
Having followed some of the arguments in this thread:
For the very long time as the length of the day increases, leap seconds are
imperative as they will become progressively more common and archaelogists
may curse those that don't use them. There might be a case for temporarily
discontinuing them until computer systems become sufficiently advanced to
handle all the changes automatically in perhaps another 50 years.
In a similar vein, leap days will become progressively less frequent as the
length of the day shortens,
There was never an AD 0 in either the Julian or Gregorian calendars, the
concept of using zero in counting scales was developed in Asia and adopted by
the West many centuries later.
(I hope the formatting comes across OK - if not don't bother to read further
- Robert)
I have developed a formula to determine the effect of combined increase in
day length and decrease in year length. It calculates tropical year lengths
measured in units of the mean solar day of the date under investigation. (I
hope the formatting comes across OK - if not don't bother to read further -
Robert)
0.0000061 days per century decrease in year length. This is mean solar days
of 86,400 SI seconds per Julian year. For these purposes, the Julian year and
tropical year can be assumed to be the same, because the current discrepancy
is about one part in a million, so the effect on the factor 0.0000061 is well
below the level of accuracy in its specification. The effect of the
difference between the current mean solar day and 86,400 seconds is also well
below the level of accuracy of the factor. These assumptions are true
provided that the equation is used to work out projections from dates close
to the present, otherwise the factor must be adjusted appropriately.
units are tropical years, mean solar days and SI second
a = no. of years ahead (positive) or past (negative)
b = current no. of days per year (365.242193)
c = decrease in year length per 100 years (0.0000061 days)
d = current day length in seconds (86,400.003 seconds in 1984)
e = increase in day length per 1000 years (0.020 seconds) - this varies
according to various factors such as core/mantle changs, ice ages and
tectonic plate rearrangemens, plus major meteor impacts
days/year = (b - (c/100 * a)) * (d /(d + e/1000 * a))
days/year = (b - (c * a/100)) * (d /(d + e * a/1000))
(The second form suits my spreadsheet better, as it reduces the roundings
problem with the divisions.)
A more comprehensive formula from the more mathematically capable would
contain factors for changes in the rates of change if any, and would also
accommodate factors for the Milankovitch and other possible cycles.
Incorporating these cycles would require a reference to a specific base date
for their synchronisation.
A list of date related future events follows:
(I have left a few dates of past events in the list for interest and to
illustrate the types of problem that can arise or need consideration.)
A.D. 1999 August - first GPS rollover, repeated nearly every nineteen years,
the next will be in A.D. 2019, I don't know whether GLONASS has similar
peculiarities.
A.D. 1999 - various dates that may cause problems with nines in date fields
A.D. 2000 1st January, etc - main Y2K rollover problem
A.D. 2000 28th February, etc - first leap year of new millennium
A.D. 2000 31st December, etc - first year end of new millennium, some may
argue that it should be 2001
A.D. 2001 28th February, etc - first non-leap year of new millennium
A.D. 2004 28th February, etc - second leap year of new millennium
A.D. 2019 6th April - next GPS rollover, they occur every 1024 weeks, as GPS
is a "true" timescale it does not have leap seconds, so there is a slight
drift from UTC. (Probably fixed by now)
A.D. 2038 - older UNIX systems may have a roll over problem, while current
redevelopments are in progress to prevent this occurring, there may still be
residual problems
A.D. 2100 - about two leap seconds per year
A.D. 2100 - second century of new millennium (not a leap year)
A.D. 2132 - count of Modified Julian Days exceeds 99,999.
A.D. 3268 - first Julian period of 7980 years ends
A.D. 4000 - at current rates of change this should not be a leap year
A.D. 4595 - count of Modified Julian Days exceeds 999,999.
A.D. 10000 - probable rollover problem similar to Y2K, if the expected
automated computer systems don't resolve it in good time
A.D. 10000 - about sixty leap seconds per year, by now it would probably be
inadvisable for non-scientific applications to pretend they don't exist.
A.D. 22,378 - Julian days exceed 9,999,999.
A.D. 50,000 - about one leap second per day
A.D. 1,900,000 - 365 days per year
A.D. 7,800,000 - 364.25 days per year
A.D. 70,000,000 - sixty one seconds per minute
A.D. 1,000,000,000 - seventy five seconds per minute
A.D. 1,000,000,000 - 250 days per year
A.D. 1 billion to 6 billion - Sun may be a red giant, rendering Earth
uninhabitable, sources vary as to when this may occur, but agree that it
will.
************************************************************************************************
Robert
On 09/01/2017 12:41, Preben Nørager wrote:
On Tue Jan 3 14:18:52 EST 2017, John Sauter wrote:
"I regard leap seconds as a reasonable compromise between the needs of
civil time and of science. Civil time needs a clock that tracks the days
and the seasons. Science requires a clock that measures time in precise
intervals. UTC provides both, using leap seconds to keep atomic time
synchronized with the rotation of the Earth."
I think there is something wrong with that argument. Civil timekeeping
holds both a clock, and a calendar. The calendar track the seasons, while
the clock track the time. If it is to be said, that the clock track the
days, it is important to notice the difference between apparent time, and
mean time. The clock track either the sun (apparent time), or the seconds
(mean time).
When the Nautical Almanac in 1833 substituted mean for apparent solar time,
an important step was taken. From now on chronometry was to rely on
mechanical clocks, and with the invention of atomic clocks, the tracking of
the 24-hour day (86400 international seconds) can now happen without any
daily tracking of the sun.
The question really is whether the calendar needs the daily tracking of the
sun or not. And the answer to that question obviously depend upon which
calendar we want!
I think the disagreement about leap seconds, really is a disagreement about
which calendar to use for civil timekeeping. If we agree to use the
proleptic gregorian calendar (ISO 8601) there is really no need for leap
seconds. That calendar track the seasons well, and with the slight
modification, to add the additional rule that years evenly divisible by
4000 are not leap years, it would track them even better.
Leap seconds are really only a need for those who do not want to see the
proleptic gregorian calendar become universal. For instance for those who
want to use the julian period, as the basis for one calendar or another,
because they must somehow rely on apparent time, and not mean time, because
the julian period count apparent solar days.
Let us use ISO 8601, and abolish leap seconds all together.
_______________________________________________
LEAPSECS mailing list
LEAPSECS@leapsecond.com
https://pairlist6.pair.net/mailman/listinfo/leapsecs
_______________________________________________
LEAPSECS mailing list
LEAPSECS@leapsecond.com
https://pairlist6.pair.net/mailman/listinfo/leapsecs
