/**
* Get obliquity of the ecliptic.
*
* @param null|DateTime $DateTime Date (optional)
* @return float In degree
*/
public static function getEclipticObliquity(DateTime $DateTime = null)
{
$jc = Time::getJC($DateTime);
$k = Math::dmsToDecimal(['d' => 23, 'm' => 26, 's' => 21.448]);
$k1 = Math::dmsToDecimal(['d' => 0, 'm' => 0, 's' => 46.815]);
$k2 = Math::dmsToDecimal(['d' => 0, 'm' => 0, 's' => 0.00059]);
$k3 = Math::dmsToDecimal(['d' => 0, 'm' => 0, 's' => 0.001813]);
$e = $k - $k1 * $jc - $k2 * $jc ** 2 + $k3 * $jc ** 3;
return $e;
}
