Calculating Easter

[Lewiy]

I have come across this code for calculating the date of Easter on Chip Pearson's site: http://www.cpearson.com/excel/holidays.htm

Public Function EasterDate(Yr As Integer) As Date

Dim d As Integer
d = (((255 - 11 * (Yr Mod 19)) - 21) Mod 30) + 21
EasterDate = DateSerial(Yr, 3, 1) + d + (d > 48) + 6 - ((Yr + Yr \ 4 + _
d + (d > 48) + 1) Mod 7)

End Function

The definition of the date for Easter is "The first Sunday after the first full moon on or after the vernal equinox".

Does anyone know how/why the above code works (cos it does!) and how on earth Excel can possibly know the phases of the moon!! This has been puzzling me for some time now.

 

[SydneyGeek]

Not a clue, but I found this as well...

Assuming the year is in A1,

=FLOOR(DAY(MINUTE(A1/38)/2+56)&"/5/"&A1,7)-34

I also saw one based on latitude and longitude which I can't locate at the moment.

Denis

 

[Andrew Fergus]

Interesting reading on this subject : http://aa.usno.navy.mil/faq/docs/easter.html

and the significance of 19 in the modular calculations:
http://www.mythicalireland.com/astronomy/moonmovements/metoniccycle.html

 

[SydneyGeek]

Thanks for that Andrew!

I did some digging through the Chip Pearson function. Easter Sunday is 1 March, offset by a calculation based on d.

d returns a sequence of 19 dates varying between 23 March and 20 April, if you convert the integer value for d to a day of the year.

I haven't looked in detail at the actual EasterSunday calculation yet...

Denis