Mean solar time conceptually is the hour angle of the fictitious mean Sun. Currently (2009) this is realized with the UT1 time scale, which is constructed mathematically from very long baseline interferometry observations of the diurnal motions of radio sources located in other galaxies, and other observations. Though the duration of daylight varies during the year the length of a mean solar day is nearly constant, unlike that of an apparent solar day. An apparent solar day can be up to 20 seconds shorter or 30 seconds longer than a mean solar day. Because many of these long or short days occur in succession, the difference builds up so that mean time is greater than apparent time by about 14 minutes near February 6 and mean time is less than apparent time by about 16 minutes near November 3. An analemma is a graph of this relationship. Since these periods are cyclical, they do not accumulate from year to year.
The length of the mean solar day is increasing due to the tidal acceleration of the Moon by the Earth, and the corresponding deceleration of the Earth rotation rate by the Moon.
Read more about this topic: Solar Time
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