The angle subtended at P between Z and X is the Local Hour Angle ( LHA ) of X . If Z is on the Greenwich Meridian, then that Angle must be the Greenwich Hour Angle ( GHA ) of X . If Z is not on the Greenwich Meridian, then you have the Local Hour Angle ( LHA ) of X, and if you know the GHA of X, all you need to do is add subtract the LHA of X accordingly, to discover the Longitude of Z . As you are usually standing on the point Z, you have your Longitude.
