This function converts a date in the Gregorian calendar
to a Julian day number (i.e., a method of consecutive
numbering of days---does not have anything to do with
the Julian calendar!)
Usage
julian_day(y, m, i)
Value
double, Julian day.
Arguments
y
double, year.
m
double, month.
i
double, day of month.
Details
valid for dates after -4712 January 1 (i.e., jde >= 0)
References
Meeus, J. 1991. Chapter 7 "Julian Day". Astronomical Algorithms.
Willmann-Bell.