Local mean time (LMT) is a form of solar time that corrects the variations of local apparent time, forming a uniform time scale at a specific longitude. This measurement of time was used for everyday use during the 19th century before time zones were introduced beginning in the late 19th century; it still has some uses in astronomy and navigation.
The difference between local mean time and local apparent time is the equation of time.
Standard time is the synchronization of clocks within a geographical region to a single time standard, rather than a local mean time standard. The term is also used to contrast with daylight saving time, a period of the year when clocks are shifted ahead one hour, supposedly to make better use of daily sunlight from spring to fall. Applied globally in the 20th century, the geographical regions became time zones. The standard time in each time zone has come to be defined as an offset from Universal Time. A further offset is applied for part of the year in regions with daylight saving time. Generally, standard time agrees with the local mean time at some meridian that passes through the region, often near the centre of the region. Historically, standard time was established during the 19th century to aid weather forecasting and train travel.
The adoption of standard time, because of the inseparable correspondence between longitude and time, solidified the concept of halving the globe into the Eastern Hemisphere and the Western Hemisphere, with one Prime Meridian replacing the various prime meridians that had previously been used.
Greenwich Mean Time (GMT) is the local mean time at the Royal Observatory in Greenwich, London, counted from midnight. At different times in the past, it has been calculated in different ways, including being calculated from noon; as a consequence, it cannot be used to specify a particular time unless a context is given. The term "GMT" is also used as one of the names for the time zone UTC+00:00 and, in UK law, is the basis for civil time in the United Kingdom.
Because of Earth's uneven angular velocity in its elliptical orbit and its axial tilt, noon (12:00:00) GMT is rarely the exact moment the Sun crosses the Greenwich Meridian and reaches its highest point in the sky there. This event may occur up to 16 minutes before or after noon GMT, a discrepancy described by the equation of time. Noon GMT is the annual average (the arithmetic mean) moment of this event, which accounts for the word "mean" in "Greenwich Mean Time".
Local time is the time observed in a specific locality. There is no canonical definition. Originally it was mean solar time, but since the introduction of time zones it is generally the time as determined by the time zone in effect, with daylight saving time where and when applicable. In some places this is known as standard time.
Some sources continue to use the term local time to mean solar time as opposed to standard time, but they are in the minority. Terms such as local mean time also relate to solar time.
Africa, the world's second-largest and second-most populous continent, spans across six different time zone offsets from Coordinated Universal Time (UTC): UTC−01:00 to UTC+04:00. As Africa straddles the equator and tropics, there is little change in daylight hours throughout the year and as such daylight saving time is currently observed in only two countries, Morocco (year-round except Ramadan) and Egypt. However, it was previously observed in several other countries.
Before the wide adoption of standard time zones, local mean time was widely used in railway time for train timetables and telegraphic time for telegraphy. Local mean time is a solar time that corrects the variations of local apparent time by forming a uniform time scale at a specific longitude; for instance, Liberia observed UTC−0:44:30 instead of an approximate offset such as UTC−01:00. British Rhodesia (at the time administered by the private British South Africa Company) was the first area in Africa to adopt standard time, switching to UTC+02:30 on 1 August 1899 as the previous time standards proved problematic for the railway system. Other countries followed suit, and by 1912, most Portuguese, French and British territories had adopted a standard offset. Liberia was the last country in Africa to adopt a standard offset, doing so on 7 January 1972.