The ‘hour angle’ is the astronomical equivalent to longitude.   The local hour angle (LHA) of a body is the difference in longitude between the observer’s longitude and the longitude of the sub-point of the body.   The Greenwich hour angle (GHA) of a body is the difference in longitude between the Greenwich meridian and the longitude of the sub-point of the body.   Both are measured West from the respective datum meridian.   If it is 12:00LMT the sun is overhead the observer’s meridian so the LHA is 000o.