mean solar day
noun
: 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
Jon VaraExamples of mean solar day in a Sentence
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This is actually the mean solar day, which uses the center of the sun’s disk as a reference point and is an average of every day of the year.
—Phil Plait, Scientific American, 23 Feb. 2024
For much of the twentieth century, the second was based on a nineteenth-century standard derived from astronomical calculations: 1/86,400 of a mean solar day.
—Tom Vanderbilt, Harper’s Magazine , 13 Mar. 2023
The mean solar day, to smooth out inconsistencies, is derived from the movement of an idealized, hypothetical sun.
—Tom Vanderbilt, Harper’s Magazine , 13 Mar. 2023
In the 19th century, scientific institutions worked to define the second in astronomical terms, and in the 1940s an international agreement defined the second as 1⁄86,400 of a mean solar day.
—Jay Bennett, Popular Mechanics, 24 Mar. 2017
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Phil Plait, Scientific American, 23 Feb. 2024
For much of the twentieth century, the second was based on a nineteenth-century standard derived from astronomical calculations: 1/86,400 of a mean solar day.
—Tom Vanderbilt, Harper’s Magazine , 13 Mar. 2023
The mean solar day, to smooth out inconsistencies, is derived from the movement of an idealized, hypothetical sun.
—Tom Vanderbilt, Harper’s Magazine , 13 Mar. 2023
In the 19th century, scientific institutions worked to define the second in astronomical terms, and in the 1940s an international agreement defined the second as 1⁄86,400 of a mean solar day.
—Jay Bennett, Popular Mechanics, 24 Mar. 2017
Cite this Entry
“Mean solar day.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/mean%20solar%20day. Accessed 18 Jul. 2025.
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