This message is brought to you by Androcles
http://www.androcles01.pwp.blueyonder.co.uk/
"Crown-Horned Snorkack" <chornedsnorkack@[EMAIL PROTECTED]
> wrote in message
news:aca40425-08a7-445e-9fdc-e33a0847bfe0@[EMAIL PROTECTED]
1 mai, 15:03, "Androcles" <Headmas...@[EMAIL PROTECTED]
> wrote:
> This message is brought to you by Androcles
> http://www.androcles01.pwp.blueyonder.co.uk/
>
> "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´s
> | 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?
>
> It was never "right", it is an approximation as you've pointed out.
> For time spans in the order of 2000 years it is good enough to have
> a leap day every four years (but not every century) simply because
> society finds it convenient to use integers. In a human life span
> of 70-80 years that's good enough, but if you want to be exact
> you'll have a difficult task. Besides which nobody really cares
> if the last ice cube melted at 3:05:45pm on July 23rd, 9,356 BC
> by my watch, currently about 8 seconds slow, and I won't argue
> if I'm proven wrong - which you cannot do anyway because you
> don't know exactly when it was any more than I do.
> The Gregorian calendar was adopted in favour of the Julian (still
> used by astronomers)
| How do astronomers define their Julian calendar?
It's a mythical date startiing at zero since the world supposedly began,
counted in days, according to some ancient religions. The reason for
retaining it is that there are records of astronomical im****tance
(particularly
planetary data) which nobody wanted to throw away, and so astronomers
retained the older system of counting days rather than convert to day,
month, year system which is awkward - if only because some months have 31
days and some have 30 (except February).
http://en.wikipedia.org/wiki/Julian_day
> because some leap days were missing and the
> effect of the error is ***ulative. With 25 leap days each century
> we ac***ulate 20 * 25 days in 2 millennia and we do better with
> 20 * 24, but that undershoots so we add a leap day every 400
> years. It is still not exact but it is close enough for everyday
> purposes.
| There are plenty of recent things which can be dated to a specific
| year. Tree rings. Layered sediments in various water bodies. Snow
| layers in Greenland and Antarctic ice sheets.
When Pope Gregory was alive those things didn't matter. The still
don't if you get a speeding ticket on Friday the 13th and can prove
you were in the Scottish Highlands on that date. Calendars can be
useful for all manner of things including alibis.
| The year by year records of Greenland and Antarctic ice sheet clearly
| go back to ice age.
So it may, but you are still locked into integer years. You cannot look
at one tree ring and pick out any particular day in that ring, the growth
rate is not linear. Some rings are thicker than others which may tell us
something about the climate that year when the growth was poor because
it was a dry summer and a wet spring or vice versa. What a good
scientist will do is look at both the tree ring and the ice record and
make some *****sment based on all the data he can gather, but there
is no guarantee he's right.
| Now, tidal sediments should clearly indicate phase of moon, neap/
| spring tide. Right?
Not necessarily. A single tsunami can wipe the record clean, a glacier
can gouge a grove in sedimentary rock which in turn can be covered
by a volcanic eruption... which epoch are you trying to pin down?
|
