Definition of Terms
Greenwich mean time (GMT) is the basis or origin of longitude
measurement. It is mean solar time measured with reference to the 0°
meridian of longitude located at Greenwich, England. GMT is of prime
importance to you because much of the time referenced in almanacs
relates to GMT.
When we discussed mean solar time we said that it was based on a
fictional Sun that is considered to move at a constant rate of 360° in 24
hours along the celestial meridian. This solved the problem of unequal
day length, but even with mean time there are slight variations. The most
precise time yet developed by man is kept by atomic "clocks," which
operate using cesium beam oscillators. This steady, internationally
adjusted time is called Coordinated Universal Time (UTC). For our
purposes, it is the same as GMT and is the time signal broadcast as radio
Local mean time is mean solar time measured with reference to your
meridian; that is, the meridian where you are located.
Zone time is the time you use to set your watch and clocks. Zone time
uses the standard (central) meridians of the various time zones as
The zone description of a time zone is the adjustment to be applied to that
time zone to determine GMT.
Although most of the above terms may be new to you, they will all make
sense when we cover the rest of the material in this chapter. In carrying
out the daily routines you will often be required to convert time zones and
work with time calculations. After a bit of practice, the procedures
contained in this chapter will become second nature. In fact, in most
cases you will learn to quickly do time calculation in your head.
Lets move on to how time is broken down into standard meridians
(zones) and then how that relates to longitude.