mean sun

Astron. a hypothetical sun thought of as moving uniformly along the celestial equator at a speed equal to the actual sun's average speed along the ecliptic: hypothesized as a basis for keeping time, because the actual sun does not travel along the ecliptic at a uniform rate

See mean sun in American Heritage Dictionary 4

noun
A hypothetical sun defined as moving at a uniform rate along the celestial equator at the mean speed with which the real sun apparently moves along the ecliptic, used in computing the mean solar day.

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