/**
* Finds the sidereal time of this intsance
*
* @param type $mode Type of sidereal time... (a = apparent, m = mean)
* @param Angle $lon If a longitude is supplied, finds local sidereal time,
* otherwise returns sidereal time at Greenwich
*/
public function sidereal($mode = 'a', Angle $lon = null)
{
// Get UT1 time
$ut = $this->copy()->toUT1();
$uta = $ut->jd;
$utb = $ut->dayFrac;
$ut = null;
// Get TT time
$tt = $this->copy()->toTT();
$tta = $tt->jd;
$ttb = $tt->dayFrac;
$tt = null;
// Compute either GMST or GAST
$st;
if ($mode == 'a') {
$strad = IAU::Gst06a($uta, $utb, $tta, $ttb);
} else {
$strad = IAU::Gmst06($uta, $utb, $tta, $ttb);
}
// Add longitude if relevant
if ($lon) {
$st = Angle::rad($strad)->add($lon)->norm()->toTime();
} else {
$st = Angle::rad($strad)->toTime();
}
// Return as hours
return $st->setUnit('hours');
}
