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Imagine a fictitious Sun, which moves in the Celestial Equator at a uniform rate and completes its passage around the equator in exactly the same time that the true Sun takes to pass around the ecliptic. Then the time given by this fictitious Sun will be such that every day is of exactly the same length and equal to the average length of the apparent solar day.
Mean solar time is the hour angle of the mean position of the Sun, plus 12 hours. This 12 hour offset comes from the decision to make each day start at midnight for civil purposes, whereas the hour angle or the mean sun is measured from the local meridian.[3] As of 2009, this is realized with the UT1 time scale, constructed mathematically from very-long-baseline interferometry observations of the diurnal motions of radio sources located in other galaxies, and other observations.[4] The duration of daylight varies during the year but the length of a mean solar day is nearly constant, unlike that of an apparent solar day.
The length of the mean solar day is slowly increasing due to the tidal acceleration of the Moon by Earth and the corresponding slowing of Earth's rotation by the Moon.
- This article is a derivative work of the creative commons share alike with attribution [1] and the public domain book [2]
[1] Wikipedia contributors, "Solar time," Wikipedia, The Free Encyclopedia (accessed March 30, 2025).
[2] Jones. General Astronomy. Longmans, Green and Company, New York, 1922.
[3] Hilton, James L; McCarthy, Dennis D. (2013). "Precession, Nutation, Polar Motion, and Earth Rotation". In Urban, Sean E.; Seidelmann, P. Kenneth (eds.). Explanatory Supplement to the Astronomical Almanac (3rd ed.). Mill Valley, CA: University Science Books. ISBN 978-1-891389-85-6.
[4] McCarthy, D. D.; Seidelmann, P. K. (2009). TIME From Earth Rotation to Atomic Physics. Weinheim: Wiley-VCH Verlag GmbH and Co. KGa. ISBN 978-3-527-40780-4.
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