static AstroVector DirectionVector(AstroTime time, Observer observer, double altitude, double azimuth)
{
// Convert horizontal angles to a horizontal unit vector.
var hor = new Spherical(altitude, azimuth, 1.0);
AstroVector hvec = Astronomy.VectorFromHorizon(hor, time, Refraction.None);
// Find the rotation matrix that converts horizontal vectors to equatorial vectors.
RotationMatrix rot = Astronomy.Rotation_HOR_EQD(time, observer);
// Rotate the horizontal (HOR) vector to an equator-of-date (EQD) vector.
AstroVector evec = Astronomy.RotateVector(rot, hvec);
return(evec);
}