Re: Calendar systems?

On Sat, Nov 06, 2004 at 02:42:13PM +0100, Carsten Becker wrote:
> So, if I got that right, lunar calendars base on cycles of
> the moon (ca. 29,5 days for our moon). Solar calendars base
> on how long the earth needs to get one time around the sun
> (ca. 365,25 days?). So, a lunisolar calendar bases on both
> rhythms?
Right.  Bearing in mind that the exact length of a year depends on what
you're measuring against and where in the orbit you start measuring.
Most (luni)solar calendars use the "tropical" year, which is what
determines the seasons - but even then, the year from spring to spring
has a slightly different length than the year from fall to fall, etc.

> Is our calendar a lunisolar calendar since we have
> months that approximately fit the moon's cycle and the year
> that approximately fits how long the earth needs to get
> around the sun one time?
Nope; our calendar is purely solar.  The idea of "month" is inherited
from lunar systems, but our months' durations are in no way tied to the
moon; all of them except February are far too long.  In lunar/lunisolar
calendars, the correlation between month and phase is precise - the
month starts on the new moon, has a midpoint at the full moon, and ends
at the next new moon.  Even in arithmetic calendars like the Jewish -
that is, which are based on average durations and not precise astronomical
calculations - the calendar month hews very closely to the lunar month.

Lunisolar calendars (e.g. the Jewish and Chinese) have common years of
12 lunar months (about 354 days); every 2 or 3 years there is a leap
year of 13 lunar months (about 384 days), so on average the year stays
in synch with the tropical year.

The Islamic calendar, by contrast, is purely lunar and ignores the
seasons; every year is only 12 lunar months, which means that a given
month (such as Ramadan) moves backwards through the seasons.  This year
Ramadan fell in fall, but in a few years it'll be in summer, then
spring, then winter, then back to fall again - the full cycle takes
about 33 years.

> And how did the Mayas came to use their cycles?
That's a rather different type of question; I'm not sure the answer is
even known to any degree of certainty.

> ObCon: Is it true that it rains everyday at the same time in
> the equatorial regions of the Earth?
As with most generalizations, it's not completely true, but it is a good
approximation to the truth.  I remember the daily rain in Guam when I
was there one summer . . . I think it was at 3PM.

> position in the sky? I've heard the moon's orbit is to some
> extent irregular. But how can you tell then when the moon
> occurs at which time? Or has the moon, say, cycles of some
> days/months/years where it is at the same position again
> where it started? That way you could very well base a
> calendar on the moon's orbit with easy tools I guess (cf.
> Stonehenge, but this one is based on the sun AFAIK).
All astronomical cycles are irregular to some degree.  Precise
calculations require adding up hundreds, in some case thousands, of
terms that factor in the time, the square of the time, the cube of the
time, etc etc etc.   Some of the irregularities are periodic, while
others are the result of constant change.  The length of the solar day
is very slowly decreasing, for instance - but the length of the year is
also changing, so the net effect on the number of days in the year is
not obvious...

-Marcos

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