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. Two types of solar time are apparent solar time (sundial t ...

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 let ...

. 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, commercial and social purposes. Time zones tend to follow the boundaries between countries and their subdivisions instead of strictly following longitude, because i ...

s were introduced beginning in the late 19th century; it still has some uses in astronomy and navigation.

Past use

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

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

time was last used until

standard time Standard time is the synchronisation 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 ...

was adopted on various dates in the several countries. Each town or city kept its own

meridian Meridian or a meridian line (from Latin ''meridies'' via Old French ''meridiane'', meaning “midday”) may refer to Science * Meridian (astronomy), imaginary circle in a plane perpendicular to the planes of the celestial equator and horizon * ...

. This led to a situation where locations one degree of longitude apart had times four minutes apart. This became a problem in the mid 19th century when railways needed clocks that were synchronized between stations, at the same time as people needed to match their clock (or the church clock) to the time tables. Standard time means that the same

time Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, into the future. It is a component quantity of various measurements used to sequence events, t ...

is used throughout some region—usually, it was either offset from

Greenwich Mean Time Greenwich Mean Time (GMT) is the mean solar 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 co ...

or was the local mean time of the

capital Capital may refer to: Common uses * Capital city, a municipality of primary status ** List of national capital cities * Capital letter, an upper-case letter Economics and social sciences * Capital (economics), the durable produced goods used fo ...

of the region. The difference between local mean time and local apparent time is the equation of time.

References

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

Past use

See also

References