Long before any computer emerged monks already knew all about ‘computus‘, which means ‘calculating the day of Easter’. A very serious matter since the date of Easter depends on the lunar calendar, instead of our common solar calendar. And in church the whole liturgical calendar pivots on that ever changing date. Wikipedia explains the whole process in detail (but more concise in Dutch). BTW, it is great fun to have a group of 12-year olds do the full Gauss algorithm! I did.
Today we can, of course, do the computus on a computer. This will capture and hide the calculation in a software algorithm, to be applied in Perl, Python, Java, BASIC, MS-SQL or just Excel. Those who run Linux can also use a nice wee tool: Easter. Made in Spain by Antonio Luque Estepa and available under GNU GPL.
The easy way is to go to the Easter website and use the online tool. Just fill in the year and it will return the Easter date. However, the stand-alone version, running in a terminal, will provide more details and more options.
To install it, download the tarball to a directory of your choice. Unpack it and run the common installation sequence. You can do this all stylishly from a terminal.
[destination directory:] mkdir [path-to-directory-of-your-choice] cd [path-to-directory-you-just-made] [download:] wget http://woody.us.es/~aluque/proy/easter-2.2.2.tar.gz [unpack:] tar -xzvf easter-2.2.2.tar.gz [install:] ./configure make sudo make install
That’s you done. You can now remove the directory, or just leave it.
Easter does not come with a manpage, but the help tells it all. In a terminal type:
Typing just easter returns the Easter date for the current year (in MM-DD):
$ easter 04-05
That’s for now, i.e. 2015. Adding a year returns the Easter date for that year:
$ easter 2016 03-27
The -a option will add plain dates for all Sundays and holy days from Ash Wednesday to Pentecost:
$ easter -a 2016 02-10 02-14 02-21 02-28 03-06 03-13 03-20 03-27 03-28 05-05 05-15 05-16
The -A option wil show the same with full names:
$ easter -A 2016 Calendar for 2016: Ash Wednesday 02-10 I Lent 02-14 II Lent 02-21 III Lent 02-28 IV Lent 03-06 V Lent 03-13 Palm Sunday 03-20 Easter Sunday 03-27 Easter Monday 03-28 Ascension 05-05 Pentecost Sunday 05-15 Pentecost Monday 05-16
The -AW option will also add the main factors from the computus:
$ easter -AW 2016 Calculating Easter for year 2016 Year is 2016, and is leap. Century is 21 Golden Number is 3. Epacta is 21 Paschal full moon is 03-23, which is the 83th day of year 03-23 is Wednesday Easter is 87th day of year, that is 03-27 Calendar for 2016: Ash Wednesday 02-10 I Lent 02-14 II Lent 02-21 III Lent 02-28 IV Lent 03-06 V Lent 03-13 Palm Sunday 03-20 Easter Sunday 03-27 Easter Monday 03-28 Ascension 05-05 Pentecost Sunday 05-15 Pentecost Monday 05-16