Title: Good Friday & Easter Sunday.
Question: How can I find the date of a the Good Friday & Easter Sunday
Answer:
This is my implementation of an algorithm, which calculates the date of Good Friday & Easter Sunday for a given year. The algorithm also returns other information: the Epact, the dominical letter and the Golden number. All these were used to help calculate the date.
The procedure CalculateEaster does all the calculation and return the date of Easter Sunday and the epact, the dominical letter and the golden number.
I use the procedure Output to encode the information into strings, the important point here is calculating the date of Good Friday when Easter Sunday is the first or the second of April(e.g. 6 AD).
The source and a demo application is attached to the article
Source:
unit unit1;
interface
uses
Classes;
procedure CalculateEaster(year : Integer; Var day, month, GoldenNum, Epact,
Dominical : Integer);
procedure Output(OutputString : TStrings; year, day, month,
GoldenNum, Epact, Dominical : Integer);
implementation
uses
SysUtils;
const
YearsString = 'In the year %d AD:';
FridayString = ' Good Friday was on the %d of %s';
DateString = ' Easter Sunday was on the %d of %s';
GoldenNumString = ' The Golden number was %d';
EpactString = ' The Epact was %d';
DominicalString = ' The Dominical letter was %s';
CutString = '*-----------------------------------------------------*';
April = 'April';
March = 'March';
///////////////////////////////////////////////////////
{
CalculateEaster : calculates the date of Easter Sunday, also
the year's Golden Number, Epact and Dominical
letter.
}
procedure CalculateEaster(year : Integer; Var day, month, GoldenNum, Epact,
Dominical : Integer);
var
A, B, C, D, E, G,
H, M, J, K, L, N,
P : Integer;
begin
A := year Mod 19;
B := year Div 100;
C := year Mod 100;
D := B Div 4;
E := B Mod 4;
G := (8*B + 13) Div 25;
H := (19*A + B - D - G + 15) Mod 30;
M := (A + 11*H) Div 319;
J := C Div 4;
K := C Mod 4;
L := (2*E + 2*J - K - H + M + 32) Mod 7;
N := (H - M + L + 90) Div 25;
P := (H - M + L + N + 19) Mod 32;
day := P;
month := N;
GoldenNum := A + 1;
if H Epact := 23 - H
Else
Epact := 53 - H;
Dominical := (2*E + 2*J - K) Mod 7;
if Dominical Inc(Dominical,7);
end;{ CalculateEaster }
///////////////////////////////////////////////////////
{
Output : writes the output of CalculateEaster to a
string list.
}
procedure Output(OutputString : TStrings; year, day, month,
GoldenNum, Epact, Dominical : Integer);
var
S, S2 : String;
Fday : Integer;
begin
OutputString.Add(Format(YearsString,[year]));
// convert month
if month = 3 then
S := March
Else
S := April;
//finde when was the good friday
Fday := day - 2;
S2 := S;
if Fday = 0 then
begin
Fday := 31;
S2 := March;
end;
if Fday = -1 then
begin
Fday := 30;
S2 := March;
end;
OutputString.Add(Format(FridayString,[Fday,S2]));
OutputString.Add(Format(DateString,[day,S]));
OutputString.Add(Format(GoldenNumString,[GoldenNum]));
OutputString.Add(Format(EpactString,[Epact]));
OutputString.Add(Format(DominicalString,[chr(Dominical + 65)]));
OutputString.Add(CutString);
end;{ Output }
end.