Local mean time (LMT) is a form of

solar time Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day, based on the synodic rotation period. Traditionally, there are three types of time reckoning based ...

that corrects the variations of local apparent time, forming a uniform time scale at a specific

longitude Longitude (, ) is a geographic coordinate that specifies the east- west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek lett ...

. This measurement of time was used for everyday use during the 19th century before

time zone A time zone is an area which observes a uniform standard time for legal, Commerce, commercial and social purposes. Time zones tend to follow the boundaries between Country, countries and their Administrative division, subdivisions instead of ...

s 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.

Past use

Local mean time was used from the early 19th century, when local

sundial A sundial is a horology, horological device that tells the time of day (referred to as civil time in modern usage) when direct sunlight shines by the position of the Sun, apparent position of the Sun in the sky. In the narrowest sense of the ...

time was last used until

standard time Standard time is the synchronization of clocks within a geographical region to a single time standard, rather than a local mean time standard. Generally, standard time agrees with the local mean time at some meridian that passes through the r ...

was adopted on various dates in the several countries. Each town or city kept its own meridian, so locations one degree of longitude apart had times four minutes apart. This became a problem in the mid 19th century when railways needed clocks for railway time that were synchronized between stations, while local people needed to match their clock (or the church clock) to the time tables.

Standard time Standard time is the synchronization of clocks within a geographical region to a single time standard, rather than a local mean time standard. Generally, standard time agrees with the local mean time at some meridian that passes through the r ...

means that the same

time Time is the continuous progression of existence that occurs in an apparently irreversible process, irreversible succession from the past, through the present, and into the future. It is a component quantity of various measurements used to sequ ...

is used throughout some regional time zone—usually, it is at an offset from

Greenwich Mean Time Greenwich Mean Time (GMT) is the local mean time at the Royal Observatory, Greenwich, Royal Observatory in Greenwich, London, counted from midnight. At different times in the past, it has been calculated in different ways, including being ...

or the local mean time of the capital of the region.

References

{{DEFAULTSORT:Local Mean Time Time scales Time in astronomy de:Mittlere Ortszeit

Past use

See also

References