#!/usr/bin/perl -w
#
# To calculate the local siderial time for a given julian day
# and longitude of the observatory site
#
# (c) Vlad Kondratiev, 2006
#

if ($#ARGV == -1) {
 print "Usage: $0 jd lambda [deg]\n";
 exit 0;
}
if ($#ARGV < 1) { 
 print "Not all parameters\n";
 exit 1;
} 

$sid = &calc($ARGV[0], $ARGV[1]);
$h = sprintf ("%02d", int ($sid));
$m = sprintf ("%02d", int (($sid - $h) * 60.));
$s = ($sid - $h - $m/60.) * 3600.;
if ($s < 10.) { $s = "0" . $s; }
print "Sidereal time: $h:$m:$s\n";

sub calc {
 my ($jd, $lambda) = @_;
 $part = $jd - int ($jd);
 if ($part >= 0.5) { $jd0 = int($jd) + 0.5; $H = ($part - 0.5) * 24.; }
  else { $jd0 = int($jd) - 0.5; $H = ($part + 0.5) * 24.; }

 $D  = $jd  - 2451545.0;
 $D0 = $jd0 - 2451545.0; 
 $T = $D / 36525; 

 $gmst = 6.697374558 + 0.06570982441908 * $D0 + 1.00273790935 * $H + 0.000026 * $T * $T + $lambda/15.;
# $gmst = 18.697374558 + 24.06570982441908 * $D + $lambda/15.;

 return $gmst - (int($gmst/24.)) * 24.;
}