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mean noon[′mēn ′nün]
the instant of upper transit of the fictitious point on the celestial sphere known as the mean sun. At mean noon, clocks operating on mean solar time, which is in general use, register 12 o’clock. Mean noon differs from apparent noon, which is the instant the apparent, or true, sun is over the upper branch of the meridian, by a quantity known as the equation of time.