Definition of mean solar day
: the interval between successive transits of a given meridian of longitude by the mean sun
The confusion wasn't resolved for good until 1884, when an international time conference in Washington, D.C., replaced the old-fashioned solar day with the mean solar day—a legal fiction in which the daily fluctuations are averaged out, enabling us to enjoy the blessings of a uniform 24-hour day all year long.— Jon Vara
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Merriam-Webster unabridged
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Time Traveler for mean solar day
The first known use of mean solar day was in 1810
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