Re: CHAT: Passover

> Date:         Wed, 26 Apr 2000 06:21:52 +0100
> From: Raymond Brown <ray.brown@...>
> With the calculation of Easter & the Passover, there's always the exact
> date of the vernal equinox to consider.  This must surely also differ by
> one day for communities in the Pacific around the international date-line.
> IIRC a proposed reform among Orthodox Christians earlier in the (20th)
> cent., was to use the meridian of Jerusalem for determining the date of
> Easter; this seems a pointless exercise to me if the meridian of anywhere
> would always give exactly the same result.
The definition I was taught is that Easter Sunday is the first Sunday
after the first full moon after the vernal equinox. The interesting
point here is what 'after' really means.

In the astronomical sense both the equinox and the full moon are
events which can be timed to split seconds, and as such the first full
moon after the vernal equinox is well-defined and does not depend on
the observer's meridian.

However, even if everyone agrees on when a Sunday should begin --- at
local sunset, at midnight local mean sun time, at 0:00 local zone
time, whatever --- that astronomical full moon can occur when it's
still Saturday west of some line, and Sunday east of it (with the date
line separating the areas 'on the other side'). So Easter will be a
week later in the east.

If we read the rule in terms of the _dates_ of vernal equinox and full
moon, even more bizarre things can happen. Suppose that one specific
year, an astronomical full moon falls 3 hours after the astronomical
vernal equinox. There will be a zone of 45 degrees of longitude where
that full moon is on a later date than the equinox. (And for added
fun, if the date line is in that zone the full moon can be on Saturday
east of it and on Sunday to the west).

The same can happen with any observation-based methods of determining
the dates of equinox and full moon, of course. So it really does make
sense to fix a meridian, if only to know when 'Sunday' starts.

Lars Mathiesen (U of Copenhagen CS Dep) <thorinn@...> (Humour NOT marked)