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.