The equation of time ranges between about plus and minus 16 minutes. This equation can be used to calculate LMT if LAT and E are known. The times discussed so far have been local times defined in terms of an object's local hour angle. They are unique to a particular observer because hour angle is measured from an observer's meridian and observers at different longitudes have different meridians. For example, when the Sun is at upper transit on a particular observer's astronomical meridian, its LHA is 0 hours and the LAT is 12 noon.
Longitude is measured east and west of the prime meridian passing through Greenwich , England.
Since the continental United States lies to the west of Greenwich, the time everywhere in the United Sates. Nowadays it is obviously impractical for everyone to keep their own local time. The meridian passing through the center of each zone is called the standard time meridian . The corresponding time zones are Eastern Standard, Central Standard, Mountain Standard, and Pacific Standard. Standard time is defined as the local mean time of the standard time meridian.