dear Roger,
The computation of Easter is a little bit complex, it follows some
canonical rules.
There is an astronomical equinox but the canonical equinox is always the
march 21.
I have notice from the ephemeris that this year the astronomic full moon
is on the march 21 (1:43 UT), that is in the same date of the canonical
equinox. Someone says that it should be calculate in Rome, others in
Jerusalem, anyway it is wrong because the reference is the canonical
full moon and it is calculated with the epact, that is the moon's age at
January 1.
Clavius worked on calendar reform and he wrote the epact for 4000 years.
This year the epact is 24.
This means I have to come back 24 days to find a canonical new moon,
that is december 8.
Now I have calculate the spring new moon adding 30, 29, 30, 29 days.
This rule needed to approximate the lunar cycle of about 29 and half.
Adding 30 + 29 + 30 days I get march 7, than I have to add 13 days to
get the canonical full moon and I get march 20, that is one day before
the canonical equinox.
For this reason the rule adds other 29 days reaching april 18, so Easter
is on april 21.
To get the full moon from the new moon the rule adds 13 days and not
14.76 because it starts from the observation of the first lunar sickle,
that is about 30 hours later the astronomical new moon.
These are the canonical rules and they differ from the astronomical
abservations because the aim was to avoid complex and contestable
observations, keeping the approximation as small as possible.
Sometimes the rules create curious arrangement like this year: the
canonical full moon of march is one day before the canonical equinox
while the astronomical full moon is one day after the astronomical equinox.
There also are ecceptions, for example if the canonical full moon is on
april 21 and it is saturday, Easter is not on sunday 22 but on sunday
29. Someone says because in the first case the Catholic Easter overlap
Passoverr but the reason is to avoid to celebrate the resurrection in
the same days of the death of Christ (Coyne, G. V., Michael A. Hoskin,
and O. Pedersen. Gregorian Reform of the Calendar Proceedings of the
Vatican Conference to Commemorate its 400th Anniversary, 1582-1982, 1983).
These infos come from the book of Piero Tempesti, Il calendario e
l'orologio (the calendar and the timepiece), 2006. Tempesti deeply
analyze this matter and I have summarized what Bepi De Donà recently
exposed on the italian mailing-list of gnomonics about this topic.
ciao Fabio
Il 20/03/2019 05:00, Roger ha scritto:
I always thought Easter Sunday was on the first Sunday after the first
full moon after the spring equinox. This year at my location, time
zone PDST, the equinox is at 2:59 pm Wed 20 March 2019. The full moon
is about 4 hours later at 6:43 pm. Why is this Sunday not Easter and
Friday not Good Friday. What about the Passover. It is also a month
later. I know setting the date of Easter was the problem that inspired
astronomy but this year the scientific data and the religious credo do
not seem to agree.
Where have I been mislead? (other than finding silly girls posing as
sundials)
Roger Bailey
Walking Shadow Designs
N 48.669°, W 123.403°
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--
Fabio Savian
[email protected]
www.nonvedolora.eu
Paderno Dugnano, Milano, Italy
45° 34' 9'' N, 9° 9' 54'' E, UTC +1 (DST +2)
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