Listing 3 get_astro_JD function

# This function returns the Astro-Julian Date.  The Julian 
# date (JD) is a continuous count of days from 1 January 4713 BC. 
# The following algorithm is good from years 1801 to 2099.
# See URL: 
# http://aa.usno.navy.mil/faq/docs/JD_Formula.html 
# for more information
# arguments: $1 = day, $2 = month, $3 = year in format YYYY
get_astro_JD()
{
typeset -i JDD

JDD=$(($1-32075+1461*($3+4800+($2-14)/12)/4+367*($2-2-($2-14)/12*12)/12-3*(($3+4900+($2-14)/12)/100)/4))
echo $JDD
}