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George wrote

> Motion of Sun and stars is very regular and predictable. The only real
> problem is the awkwardness of our calendar, because a year doesn't
> correspond to a whole number of days. The Sun doesn't return to the same
> point in the sky until about 365 and-a-quarter days have passed. So after
> most calendar years of 365 days, it has lagged by a quarter-day, or so. At
> the fourth year, we put in an extra day, to put it right again.
>
> If it was exactly a quarter-day out, then choosing a four-year gap would do
> the trick rather exactly, but actually a year is more like 365.2422 days,
> not 365.25, which gives rise to the residual error.

I mostly understand the argument regarding 365.25 days per year given some
years are missing leap years to adjust. I'll accept, given the model used in
cel nav, "The Sun doesn't return to the same point in the sky until about
365 and-a-quarter days have passed" as being equivalent to the earth not
reaching the same position in its orbit around the sun until *about* 365
and-a-quarter days have passed.

Yet I am missing something subtle. To use the almanac for the next (non
leap) year the NA uses an 87 degree offset, nominally 5 hours and 48 minutes
(5.8 hours) using 15d per hour as opposed to 6 hours (if it did indeed take
the sun and extra quarter day to return to the same orbital position).
5.8/24 = an extra .2417 days (total 365.2417), while George stated 365.2422.
In either case, baring surprises, the current leap years system seems to be
good for thousands of years before it runs into trouble.

Given George's batting average, I will accept his figure and ask what
factor(s) are responsible for the difference between my calculation based on
the NA and his figure.

Bill B.




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