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 [4] and the
public domain book [1].
References
[1] Jones, General Astronomy, Longmans, Green and Company, New York, 1922.
[2] Hilton, James L; McCarthy, Dennis D., “Precession, Nutation, Polar Motion, and
Earth Rotation” Mill Valley, CA: University Science Books. ISBN 978-1-891389-85-6.
[3] McCarthy, D. D.; Seidelmann, P. K., TIME From Earth Rotation to Atomic Physics.
Weinheim: Wiley-VCH Verlag GmbH and Co. KGa. ISBN 978-3-527-40780-4.
[4] Wikipedia contributors, Solar time. The Free Encyclopedia (accessed March 30,
2025).
