On 2 mai, 00:35, "Mike Dworetsky" <platinum...@[EMAIL PROTECTED]
>
wrote:
> "Crown-Horned Snorkack" <chornedsnork...@[EMAIL PROTECTED]
> wrote in message
>
> news:7c123121-f96a-4fa4-9e14-c9bb54a88e3b@[EMAIL PROTECTED]
>
>
>
> >On 1 mai, 19:53, "Mike Dworetsky" <platinum...@[EMAIL PROTECTED]
>
> >wrote:
> >> "Crown-Horned Snorkack" <chornedsnork...@[EMAIL PROTECTED]
> wrote in message
>
>
>>news:5b40436f-cb79-4966-807b-7b83b9eaacd8@[EMAIL PROTECTED]
>
> >> >Well, when was it?
>
> >> >The actual length of sideric day, and of tropical day, increases
> >> >slowly, as tidal friction irreversibly slows down the rotation of
> >> >Earth.
> > >>The actual length of sideric year can change - the energy of
Earth=B4s=
> > >>orbit may change as energy is exchanged between orbital movement of
> > >>Earth and other planets. But those changes are said to be minor. The
> > >>changes in tropical year are likewise minor.
>
> >> >The result is that the number of tropical days in a tropical year
> > >>decreases.
>
> >> >Sometime in palaeozoic, tidal rhytmites allegedly show that there
were=
> >> >400 days in a year.
>
> >> >But the rate of tidal slowing is not constant. It changes with
changes=
> >> >in the configuration of shelf seas and ocean basins. There would be
> >> >major changes during ice ages, for example.
>
> >> >The true duration of tropical year is around 365,2423 to 365,2424
> >> >tropical days. Gregorian calendar requires 365,2425 days.
>
> >> >The ***ulative error of Gregorian calendar through recent is thus
less=
> >> >than two days.
>
> >> >But how valid was Gregorian calendar in ice age?
>
> >> The question is, in the everyday sense, meaningless, because the
> >> Gregorian
> >> calendar is not used for years prior to 1582. Not for any sensible
> >> purpose,
> >> because the main event it was designed to regulate thenceforward--the
> >> date
> >> of Easter--had already taken place in each of those previous years.
> >> (Later
> >> in England--Newton was born on Christmas Day in the Julian calendar,
so=
> >> we
> >> still call that his birthday.) Looking backwards for dates is what
you=
> >> get
> >> using a "proleptic" calendar and usually the Julian calendar is used
fo=
r
> >> this.
>
> >This causes problems. Julian calendar is considerably farther from
> >true length of tropical year than the Gregorian.
>
> Yes, but why is it of any im****tance to be able to put "modern" (Julian
or=
> Gregorian)
> dates to ancient events that took place in now-dead civilisations with
the=
ir
> own calendars, which were the only ones of im****tance for their
purposes?
> At the time of those events the Julian calendar had not even been
invented=
..
> (And of course the Gregorian neither.)
>
>
>
> >> Aside from the removal of 10 days in 1582, the difference between
Juli=
an
> >> and Gregorian is in the number of days in an average year: 365.25 or
> >> 365.2425. So if you tried to decide the date in some very distant
past
> >> year
> >> (e.g., 10,000 BC) you would have to account for that difference, or
abo=
ut
> >>. .75 d per century or 7.5 days per millennium.
>
> >And 75 days over 10 millennia. Which means that a proleptic Julian
> >calendar is plainly out of alignment with seasons in early recent.
>
> Absolutely. But unless you are organising Christian or other modern
> religious festivals
> for long dead ancient stone age peoples this does not really matter.
>
>
>
> >In which calendar month did Laach lake eruption take place?
>
> I'm not familiar with this specific event, but presumably one could
judge
> the season from fossil pollens, etc trapped in sediments. Though you
woul=
d
> need to take climatic differences into account. For example, the
northern=
> summer solstice currently coincides with aphelion.
> bout about 13,000 years ago it was near the perihelion, so northern
summer=
s
> would have been considerably hotter, other things being equal.
>
>
>
>
>
> >> Of course we also have a ***ulative error in the sense that the
actual
> >> period length of a tropical year is 365.2422 days (to 4 decimals) so
th=
e
> >> Gregorian calendar itself will gradually need some reforms; a leap
year=
> >> will
> >> have to be omitted, perhaps the year 4000. I imagine the Catholic
Chur=
ch
> >> will give thought to this in about 1700 years or so. (Could be an
> >> interesting SciFi story theme?). The error ac***ulated is about 1
day
> >> every
> >> 3300 years rather than 4000 years, so in the very far future a
further
> >> reform might be needed.
>
> >> The effect of changes in the rotation of the Earth would be smaller
but=
> >> would ac***ulate more rapidly as you went back in time. The figure
for=
> >> c.
> >> 2000 years ago is of the order of three hours error. See:
>
>
>>http://www.blackwell-synergy.com/doi/pdf/10.1046/j.1468-4004.2003.442...=
>
> >> It is difficult to extrapolate rotational rates back accurately over
ma=
ny
> >> millennia because more than tidal effects are involved.
>
> >Indeed. However, they could be measured directly.
>
> >Misalignment over a few weeks should show up in geologic record.
>
> An interesting suggestion, but I doubt that anyone has been able to
exploi=
t
> it yet with any confidence to establish say a three or four second per
day=
> faster rotation tens or hundreds of thousands of years ago. There are
> practical difficulties...such as the assumption that plants were similar
> enough to today's plants to generate the pollen on the same seasonal
dates=
..
>
Which is actually quite a reasonable assumption, over the timescales
of tens of thousands of years.
But there are more fundamental things, like physical climate. There
are strong physical reasons why a maximum of insolation at high
latitudes is at summer solstice, by tropical years - reasons which
apply in recent, applied through pleistocene ice age and were equally
applicable in carbonian or ordovician ice ages. Sure, there are
physical reasons why warmest weather is slightly later than maximum
insolation and why the timing of warmest weather is variable year to
year, but over a number of years it could be possible to resolve
summers and winters unambiguously. And if you can unite large numbers
of treetrunks into a chronology spanning thousands of years, why not
unite tidal sediments into chronologies spanning thousands of synodic
half-months? And then you can directly tell how many spring tides have
come and gone between two epochs - and how far those markers were from
true summer solstice.
>
>
> >> Astronomers use something called the Julian Day number which is a
runni=
ng
> >> count. The Julian period starts on 1 January 4713 BC (Julian
calendar)=
> >> and
> >> lasts for 7980 years. It starts on the day when the Roman Indiction,
> >> Golden
> >> Number, and Solar number all had a value of 1. This allows them to
avo=
id
> >> having to use dates in one calendar or another for calculated events
in=
> >> the
> >> distant past (like eclipses). For accurate timings they of course
have=
> >> to
> >> use a dynamical time scale independent of variations of Earth
rotation.=
>
> >Indeed. Julian day is bound to rotation of Earth which, in long term,
> >is more variable than tropical year (bound to orbital movement of
> >Earth). Neither of which is constant flow of time as bound to inertial
> >laws.
>
> But the point is, there is no practical need to assign modern historical
> dates to ancient events predating the invention of writing and
calendrical=
> records. If you would like to do this for your own amusement, go ahead.
=
No
> historian needs to do this. The oldest dates (Ancient Egypt) are
uncertai=
n
> by decades, so why worry about the exact date as described by *our*
> calendar?
>
> >> When they define somenthing to be measured in years they usually use
a
> >> unit
> >>of Julian years of 365.25 days, e.g., if discussing the period of a
visu=
al
> >> binary star, or the orbit of the Sun around the galaxy.
>
> >Of course, it may be a question what the most im****tant year is:
> >tropical, sideric or anomalistic...
>
> Yes, it depends on what you are discussing. Orbits? sidereal or
> anomalistic. The date of the Vernal Equinox? Tropical.
>
|
