julian2cal.m

球面天文学（岁差、章动）和天体力学（行星星历表）的原始计算公式、算法和程序。matlab编译通过。主要包括：公历/儒略历转换为儒略日

function cal=julian2cal(J)
%Calculate month, day, and year from Julian date

if( J < 1721423.5 ) %January 1.0, 1 A.D.
	BC = 1;
else
	BC = 0;
end

%start from Julian March 1, 4801 B.C.

if( J < 2299160.5 ) % before 1582.10.4. 24:00 is Julian calender
   
j0=floor(J+0.5);
dd=J+0.5-j0;

else  % after 1582.10.15. 00:00 is Gregorian calender

%number of certury years that are not leap year
n1=floor((J-2342031.5)/36524.25/4)+1;  %1700.3.1.0
n2=floor((J-2378555.5)/36524.25/4)+1; %1800.3.1.0
n3=floor((J-2415079.5)/36524.25/4)+1;  %1900.3.1.0

j0=n1+n2+n3+J+10;
dd=j0+0.5-floor(j0+0.5);
j0=floor(j0+0.5);
   
end

j0=j0+32083;
year0=ceil(j0/365.25)-1;
year=year0-4800;
day=j0-floor(year0*365.25);
month=floor((day-0.6)/30.6)+3;
day=day-round((month-3)*30.6);
	if month>12
   	month=month-12;
   	year=year+1;
	end

   year=year-BC;


cal=[year, month, day, dd];