The calendar is very important for a society so that
it can determine when things should be done. In the ancient world, knowing
the time
to plant crops was vital; and in modern times the calendar enters
every aspect of our lives, telling when to celebrate, when to work, and when to
pay our bills.
Our calendar is based on the sun with the fundamental units being the
day (determined by the rotation of the earth with respect to the direction
of the sun) and the year (determined by the rotation of the earth
around the sun).
Not all calendars have been determined by the sun. For example the
Moslem calendar is moon-based; whilst the Babylonians based their
calendar on a combination of the sun and moon.
In our calendar, a hint of a moon-based system remains with the "month",
although the average month is a few days longer than the rotation
of the moon around the earth.
A further remanent of a moon-based system
is the determination of the date of Easter. Many
people know the definition of Easter as "Easter
Sunday falls on the first Sunday after the Full Moon on, or after,
the March Equinox".
But this definition hides a number of problems and can require
quite accurate knowledge of the orbit of the moon. The first
problem is the need to define a longitude on earth for which
Sunday is determined. For example, at the instant of Full
Moon it may be Sunday in Sydney but still Saturday in London
giving rise in certain circumstances to a
different date for Easter. A second problem arises
from the variabilities in the orbit of the moon which is perturbed
by the sun and the major planets. The exact orbit of the moon
was not known precisely until recent times and it would be difficult
if the date of Easter changed as our knowledge of the moon's orbit
changed.
To obtain consistency in the date of Easter, the Church at the Council
of Nicea decided to define Easter with respect to an imaginary
moon - known as the "ecclesiastic moon". Also, the date of equinox
was fixed at March 21 even though it can vary slightly from this
date. With this definition, the date of Easter can be determined
in advance without further astronomical knowledge. But the sequence
of dates
varies significantly from year-to-year with Easter Sunday being as early
as March 22 and as late as April 25. In fact, the exact sequence
of Easter dates repeats
itself approximately every 5,700,000 years in our Gregorian calendar.
The Eastern Orthodox churches decided not to follow the above definition
and they determine Easter in another manner, sometimes leading to
a difference in the date
of Easter between churches.
For those interested, the following table lists the date of Easter Sunday
up to the year 2010.
March 31, 2002 April 20, 2003 April 11, 2004
March 27, 2005 April 16, 2006 April 08, 2007
March 23, 2008 April 12, 2009 April 04, 2010
The following is an algorithm by which the date of Easter may be
calculated.
Sources: Astronomical Algorithms, Jean Meeus; Man and the Stars, Robert
Hanbury Brown.
Educational