mean solar day

noun

: the interval between successive transits of a given meridian of longitude by the mean sun

… the mean solar day, the average length of a day, based on how long it takes Earth to rotate, is about 86,400.002 seconds long.

—Angela Hutti

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

Examples of mean solar day in a Sentence

Recent Examples on the Web

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

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

Word History

First Known Use

1810, in the meaning defined above

Rhymes for mean solar day

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