Here a couple methods that go to and from modified julian day numbers to <day,
month, year> values. These algorithms are found in a number of standard sources
for astronomical computing.
These are methods from a date class that I use. They only thing necessary to
know about that class is that three of its instance variables are day, month
and year. Making these methods work as an NSDate category should be
straightforward.
Tom Wetmore
// Return the modified julian date of this day.
//---------------------------------------------------------------------------------------------
- (long) toJulian
{
NSInteger m = month;
NSInteger y = year;
if (m <= 2) {
m += 12;
--y;
}
NSInteger b;
if ((10000L*y + 100*m + day) <= 15821004L)
b = -2 + ((y + 4716)/4) - 1179; // Julian calendar
else
b = (y/400) - (y/100) + (y/4); // Gregorian calendar
return 365L*y - 679004L + b + (NSInteger)(30.6001*(m + 1)) + day;
}
// Return the day for a modified julian date.
//----------------------------------------------------------------------------------
+ (TWDay*) fromJulian: (long) mjd
{
long a = mjd + 2400001;
long b, c, d, e, f;
if (a < 2299161) {
b = 0;
c = a + 1524;
} else {
b = (long) ((a - 1867216.25)/36524.25);
c = a + b - (b/4) + 1525;
}
d = (long) ((c - 122.1)/365.25);
e = 365*d + d/4;
f = (long) ((c - e)/30.6001);
NSInteger day = (int) (c - e - (int) (30.6001*f));
NSInteger month = (int) (f - 1 - 12*(f/14));
NSInteger year = (int) (d - 4715 - (7 + month)/10);
return [[[TWDay alloc] initDay: day month: month year: year]
autorelease];
}
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