Lord Bracknell
On fire
The date of Easter is one of those great issues that have divided Christianity over the centuries. The First Council of Nicaea tried to settle the issue in 325. But there are still differences of opinion.
How do you determine exactly when the first full moon occurs after the spring equinox?
In a congress held in 1923, the eastern churches adopted a modified Gregorian Calendar and decided to set the date of Easter according to the astronomical Full Moon for the meridian of Jerusalem. However, a variety of practices remain among the eastern churches.
There are three major differences between the ecclesiastical system and the astronomical system.
The times of the ecclesiastical full moons are not necessarily identical to the times of astronomical Full Moons. The ecclesiastical tables did not account for the full complexity of the lunar motion.
The vernal equinox has a precise astronomical definition determined by the actual apparent motion of the Sun as seen from the Earth. It is the precise time at which the apparent ecliptic longitude of the Sun is zero. (Yes, the Sun's ecliptic longitude, not its declination, is used for the astronomical definition.) This precise time shifts within the civil calendar very slightly from year to year. In the ecclesiastical system the vernal equinox does not shift; it is fixed at March 21 regardless of the actual motion of the Sun.
The date of Easter is a specific calendar date. Easter starts when that date starts for your local time zone. The vernal equinox occurs at a specific date and time all over the Earth at once.
Inevitably, then, the date of Easter occasionally differs from a date that depends on the astronomical Full Moon and vernal equinox. In some cases this difference may occur in some parts of the world and not in others because two dates separated by the International Date Line are always simultaneously in progress on the Earth.
That's why, in some years, the Eastern churches celebrate Easter on a different day from Cadbury's.
Easter in 2007 is universally accepted as being on Sunday April 8. Next year it will be celebrated by the eastern churches on April 27. We will be stuffing our faces with chocolate on March 23.
How do you determine exactly when the first full moon occurs after the spring equinox?
In a congress held in 1923, the eastern churches adopted a modified Gregorian Calendar and decided to set the date of Easter according to the astronomical Full Moon for the meridian of Jerusalem. However, a variety of practices remain among the eastern churches.
There are three major differences between the ecclesiastical system and the astronomical system.
The times of the ecclesiastical full moons are not necessarily identical to the times of astronomical Full Moons. The ecclesiastical tables did not account for the full complexity of the lunar motion.
The vernal equinox has a precise astronomical definition determined by the actual apparent motion of the Sun as seen from the Earth. It is the precise time at which the apparent ecliptic longitude of the Sun is zero. (Yes, the Sun's ecliptic longitude, not its declination, is used for the astronomical definition.) This precise time shifts within the civil calendar very slightly from year to year. In the ecclesiastical system the vernal equinox does not shift; it is fixed at March 21 regardless of the actual motion of the Sun.
The date of Easter is a specific calendar date. Easter starts when that date starts for your local time zone. The vernal equinox occurs at a specific date and time all over the Earth at once.
Inevitably, then, the date of Easter occasionally differs from a date that depends on the astronomical Full Moon and vernal equinox. In some cases this difference may occur in some parts of the world and not in others because two dates separated by the International Date Line are always simultaneously in progress on the Earth.
That's why, in some years, the Eastern churches celebrate Easter on a different day from Cadbury's.
Easter in 2007 is universally accepted as being on Sunday April 8. Next year it will be celebrated by the eastern churches on April 27. We will be stuffing our faces with chocolate on March 23.
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