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"Crown-Horned Snorkack" <chornedsnorkack@[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) 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.
|
