mean solar time

chronology

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June 30, 2026

Quick Summary

Mean solar time is a way of measuring time based on the Earth's rotation relative to the Sun, but with a crucial adjustment for consistency. While apparent solar time is what a sundial measures and varies slightly due to the Sun's uneven apparent speed throughout the year, mean solar time uses a theoretical "mean Sun" that moves at a uniform rate.123

This uniformity is achieved because the Earth's orbit isn't a perfect circle, and its axis is tilted, causing the real Sun's apparent speed to fluctuate. The difference between apparent and mean solar time is called the equation of time and can be up to 16 minutes. Modern clocks and watches keep mean solar time, which is essential for standardized timekeeping. Since 1972, common usage time is based on a blend of solar and atomic time, known as Coordinated Universal Time.132

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solar time
Mean solar time, kept by most clocks and watches, is the solar time that would be measured by observation if the Sun traveled at a uniform apparent speed throughout the year rather than, as it actually does, at a slightly varying apparent speed that depends…
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Mean solar time, kept by most clocks and watches, is the solar time that would be measured by observation if the Sun traveled at a uniform apparent speed throughout the year rather than, as it actually does, at a slightly varying apparent speed that depends…
Read More